Hellenic J Cardiol 48: 296-299, 2007 Drug-Induced Long QT Syndrome K ONSTANTINOS P. LETSAS , MICHALIS EFREMIDIS , GERASIMOS S. FILIPPATOS , 1Second Department of Cardiology, Evangelismos General Hospital of Athens, 2Second Department of Cardiology,Atticon University Hospital of Athens, Athens, Greece. Key words: Drugs, long QT, torsades de pointes, sudden cardiac A continuously growi
Manigs.infoTheory Group, Dept. of Physics and Astronomy University Of Manchester, Manchester, M13 9PL, U.K.
2 Lamarck, Darwin and the rise of neo-Darwinism 3 Ecology, Population Biology And Evolution Of Communities 4 Theories Of Speciation And Paleontology 5 Oxygen Revolution, Thermodynamics And Complexity Evolutionary theories in biology evoke more controversy both among bi-ologists and among onlookers than theories in any other branch of sciencewith the possible exception of social and political theories. Part of this con-troversy is due to the very diverse range of approaches among evolutionarybiologists and the lack of any unifying theory in evolutionary biology. Inrecent years the experience has been more and more like Alice’s experi-ence with the Cheshire Cat. The more we go along the reductionist path theharder it becomes to recognize any synthesis in biological evolution.
It could be argued that the neo-Darwinism and the extension of it to the "modern synthesis" provide a unifying model for the evolution of the bio-logical systems. Unfortunately there exists no unique description of whatthe modern synthesis implies. The modern synthesis is a set of ideas whichis sufficiently broad and variable to accommodate a multitude of truthsand sins. The early advocates of modern synthesis, especially Dobzhan-sky (1937) and Simpson (1944), were pluralistic and expansive in their ap-proach. With the passage of time these same authors hardened their atti-tudes and had become more inflexible (Dobzhansky, 1951; Simpson, 1953).
An analysis of this transformation to a more rigid attitude by Simpson be-tween 1944 and 1953 is discussed by Gould (1980). A large number ofcritics of the neo-Darwinian model use the works of Dobzhansky (1951),Simpson (1953) and Mayr (1963) to define the neo-Darwinian theory. Thishas been unfortunate, since the deficiencies in the models proposed by theabove mentioned authors have then been highlighted by the critics as posi-tive proof that Darwinian theory is inadequate in explaining biological evo-lution. Various garbled versions of such criticisms by eminent biologistshave been used by creationists and by popular media to question whetherevolution in biological systems did at all occur. In this chapter I shall dis-cuss some of the problems encountered in the study of biological evolutionand indicate why it is not feasible at present to formulate a theory of bio-logical evolution that takes into account evolution from the molecular levelthrough the organismic level to the global ecosystem level. It may be that nosingle theory can be constructed to describe biological evolution at all thevarious levels of hierarchy. But, as we shall see in the following sections, within each level, evolution can be understood on the basis of Darwinism,provided our application of Darwinian theory takes into account the con-straints in the system arising through laws of physics and chemistry. It isour inability to incorporate such constraints into the model and to bridge thegaps between the various levels of hierarchy that has led to the present ten-dency among some biologists to reject Darwinism outright and yet provideno tangible model to replace it.
2 Lamarck, Darwin and the rise of neo-Darwinism What we define as Lamarckism today is a much modified version of theviews regarding evolution held by the French biologist Lamarck (1744 -1829). According to this theory, an organism, during its life, adapts to theenvironmental conditions to which it is exposed and such adaptations canbe passed on to its offsprings. If this could occur, then it would contributeto the evolution of new and improved adaptations. Lamarck further held theview that organisms had an inherent drive to evolve into higher and morecomplex forms.Charles Darwin in the Origin of Species (1859) aimed to es-tablish two basic axioms. First, that all existing organisms are descendantsof one or a few simple ancestral forms, thus arguing that evolution has infact occurred. Secondly, Darwin claimed that the mechanism for evolution-ary change was natural selection operating on the variations in the popula-tion and that the origin of the variations was non-adaptive. Darwin had nomechanism for the origin and maintenance of variation and he thought thatif the organisms acquired characteristics through use and disuse during theirlifetime, these would influence the nature of the offspring and thus producevariations. In this sense, Darwin was a Lamarckian. But Darwin believedthat natural selection was the primary cause of evolution , thus rejecting theLamarckian thesis that organisms had an intrinsic drive towards evolvinginto more complex forms.
The next major advance in the theory of evolution came from August Weismann (1834 - 1914). Weismann totally rejected the Lamarckian theoryof inheritance of acquired characters. Instead he proposed that the develop-ment of the fertilized egg involved two independent processes of cell divi- sion, leading to the ’soma’ cells and to the ’germ’ cells. Acquired charactersaffect only the ’soma’ cells while the germ line leads to gametes that starta new generation. The ’soma’ cells would eventually die while the ’germ’cells are potentially immortal. Interestingly, as Maynard Smith notes, Weis-mann was led to this model more through his insight into the fact that theprocess of inheritance is a process of information flow, rather than throughempirical evidence. Weismann could not conceive of a mechanism of ’re-verse translation’ whereby the hypertrophied muscles of a blacksmith couldbe translated into large muscles in the next generation (Maynard Smith,1982). As Weismann remarked ’if one came across a case of the inheritanceof acquired character, it would be as if a man sent a telegram to China, andit arrived translated into Chinese’ (Maynard Smith, 1989). Weismann’s the-ory provides a distinction between the phenotype and the genotype. Thesomatic cells, adapting to the environment are phenotypes while the germcells form the genotypes. Information flows from the germ cells to the so-matic cells defining the form and the character of the organism while thereverse is not possible. This is the ’central dogma’ of evolutionary biologyand as we shall see later, has its basis in molecular biology.
Weismann’s postulate greatly strengthened Darwin’s theory. Natural selection becomes the only process that can yield evolutionary changes.
Since natural selection can only operate on the variations in the popula-tion, one still had the problem of the origin and maintenance of variation.
This question was resolved with the rediscovery of Mendel’s laws during thefirst decade of the 20th century and with the formulation of the chromosometheory of heredity. The rediscovery of Mendel’s laws started a controversyregarding the importance of mutation that raged with greater ferocity thanequivalent controversies today such as that between the punctuationalistsand the gradualists. The Mendelians saw mutations as the origin of variationand hence as the potential starting point for the formation of new species.
The biometricians, on the other hand, considered mutations to be irrelevantandrapidly lost through selection. To the biometricians continuous variationwas the essence of evolution when acted on by the forces of natural selec-tion. To the Mendelians, mutations being discrete, evolution could not becontinuous and thus they considered themselves to be non-Darwinians! Wehave come to a full circle with the present controversy on whether evolution is continuous or jerky with the proponents of punctuationism, Eldredge andGould, considering themselves as non-Darwinians! The early controversy between the Mendelians and biometricians was resolved by the work of population geneticists, Fisher, Haldane and Wright.
The continuous variation studied by the biometricians was shown to arisefrom a number of alternative genes at many loci. The discipline of popula-tion genetics became well established with robust mathematical argumentsand a series of experiments, mainly done in the laboratory. There was afeeling of euphoria that biology, like physics, could be turned into a hardscience. Since the mathematics becomes intractable when one considerssystems with more than two loci, almost all mathematical attempts wereconcentrated on single locus models with very few alternative genes at thatlocus. Such simplistic models led to the formulation of Fisher’s so called’Fundamental Theorem’ which states that the rate of increase of the aver-age fitness of a population is equal to the population variance in fitness.
As we saw above, in the Darwinian theory of natural selection, selection isassumed to operate on the variation within a population to improve the av-erage fitness of the members of the population. Fisher’s result encapsulatesthis traditional view of natural selection and was thus taken to be a mathe-matical vindication of Darwin’s theory. Apart from the various difficultiesin the interpretation of the Fundamental Theorem which I shall not discusshere, Fisher’s theorem is a static result since it implies that natural selectionwould eventually produce a static population with no variation and henceno evolution. Thus for evolution to proceed, there must be a continuousinjection of variation through mechanisms such as mutation, migration andstochastic effects arising from finite populations.
As mentioned earlier, the characters such as height, size etc. which vary continuously within a population were explained in terms of popula-tion genetics through the action of a large number of genes at a large numberof loci, with each locus contributing a small effect to the eventual pheno-typic character. Since the mathematics of a multiloci system is not easilyamenable to analytic considerations, the theory for such continuous charac-ters, called quantitative genetics, was mainly developed at the phenotypiclevel based on assumed variation in the character space acted on by naturalselection. Thus, though there was a reconciliation between the biometric and the Mendelian approach, the theory of quantitative characters was notas easily reducible to the underlying genetic field as the theory of discretecharacters. Population genetics describes a ’bottom to top’ dynamical sys-tem connecting the genetic space to the phenotypic space. In general it doesnot consider the ecological interactions such as competition, prey-predatorinteractions and mutualism. Its dynamics is reversible and thus, by itself,it provides no mechanism for birth and extinction of species. An applica-tion of population genetics to the problem of phenotypic evolution can beseen in the rapid increase of the melanic or darker forms of the pepperedmoth in the industrial regions of Britain. This is an oft-quoted exampleof the occurrence of Darwinian evolution. The phenomenal increase in themelanic forms of this moth from the middle of the 18th century to the earlierpart of this century has been attributed to differential bird predation. Withchange in background colour in the environment due to industrial pollution,the lighter or the typical variety is more visible than the melanic form, andhence suffers a relatively high rate of predation. Population genetics mod-els have had reasonable success in explaining the change in the melanicfrequency as well as its distribution over England and Wales from 1850 to1960 (Cook and Mani, 1980; Mani, 1980, 1982, 1989). Since 1960, withthe advent of the Clean Air Acts and with the replacement of domestic coalfires by central heating, the pollution levels have steadily decreased and thefrequency of the melanic forms has also shown a stead decrease; and thischange in the peppered moth has been predicted with considerable degreeof success using population genetics models ( Cook et al, 1986; Clarke etal, 1985; Mani, 1989). Thus the population genetics model, acting uponthe variation in the population of genotypes, can produce reverse evolutionin phenotypic forms, provided the variant genotypes are not lost. In otherwords, at this level of sophistication, population genetics models have no’arrow’ in time. Long term biological evolution has a direction in timeand thus any evolutionary model incorporating the mathematical structureof classical population genetics must also have features that allow for non-reversibility in long term evolution. The work of population genetics led tothe amorphous ’theory’ called the ’modern synthesis’ of evolutionary biol-ogy developed during the period 1930 - 1950 by, among others, Dobzhan-sky, Simpson, Stebbins, Mayr, Ford and Julian Huxley. The essentials of the theory can be summarized as follows : 1. the ultimate source of variability is point mutations.
2. natural selection acts on the variability to produce evolutionary changes.
3. rates and direction of changes are controlled by natural selection alone.
4. the selective process leads to adaptation.
The modern synthesis emphasized that the neo-Darwinian mechanisms of natural selection in Mendelian populations were sufficient to explain theevolutionary process observed in nature and that it could also explain theobserved geographic variation between and within species as well as theorigin and formation of species.
In this section I have traced the origin and the evolution of the neo- Darwinian theory. This, as we saw, was based on the concept of naturalselection in Mendelian populations and on the rejection of the Lamarckianprocess through the central dogma of Weismann. Are there any cases inbiological evolution where the Darwinian model is invalid or inapplicable?There are a few cases in which the neo-Darwinian assumption appear to bedubious. For example, the members of a clone of the water flea Daphniacan have different morphologies that are adaptive in response to differentenvironmental stimuli and once occurred, are transmitted through the egg.
These are assumed to be caused by changes in gene activation rather thanthrough changes in the sequence of DNA and therefore may be the result ofa system of flexible response incorporated through natural selection. Sim-ilarly, flax plants are known to acquire morphological changes in the pres-ence of high levels of fertilizer and these are seen to persist for a number ofgenerations, though not indefinitely. The changes again are caused not bychanges in DNA base sequences but through a process of producing multi-ple copies of the same gene, called gene amplification. In ciliated protozoa,newly acquired patterns of cilia are transmitted through binary division withno apparent change in the underlying DNA. This process is still not under-stood. Finally, cultural evolution, and animal learning do get propagatedwith no hereditary mechanism. The famous example is the transmission ofthe information concerning the change in milk bottle tops from one blue tits generation to another. The process of cultural evolution, (and hence eco-nomic and technological evolution), is Lamarckian. Recently, E. J. Steele,in his book Somatic Selection and Adaptive Evolution (1979) has proposeda mechanism whereby acquired characters might be transmitted to the off-spring, which led a leading British newspaper to splash across its centrepage that Darwinism is dead ! Steele’s mechanism involves the DNA-codedinformation from selective cells to be incorporated in RNA viruses, whichare then carried to the germ cells and there incorporated into their chro-mosomes, resulting in the genetic transmission of an acquired character tothe offspring. This is quite a plausible model since one knows that thereare viruses, called retroviruses, which can transcribe their RNA informationinto DNA, thus slightly denting the ’central dogma’. The AIDS virus isone such virus. An experimental confirmation of Steele’s hypothesis wasreported ( Gorczynski and Steele, 1980, 1981) but many attempts to repro-duce the results of the experiment by others (Howard, 1982) have failed andthus Steele’s model for Lamarckian transmission of characters is yet to beconfirmed.
Why is the Lamarckian process, except for the case of cultural inheri- tance, so uncommon in biological evolution ? The answer partly lies in thefact that there exists the genetic system which prevents reverse translationand thereby reduces error propagation. Phenotypic changes, (except learntones) are non-adaptive and thus a hereditary mechanism by which these aretransmitted to the offspring would not be favoured by natural selection. Arethere any other mechanisms by which the variability in a population can bealtered without the force of natural selection ? The answer is yes. Kimura,a Japanese theoretical biologist and others have developed a robust mathe-matical model, called the neutral model (Kimura, 1984) by which they showthat the variability in a population can be maintained without natural selec-tion. In the neutral model, variability is generated through mutation and islost through effects of finite population size, called genetic drift. The rate ofloss depends upon population size and there is a dynamic balance betweenloss through genetic drift and gain through mutation. Thus the variabil-ity is maintained and altered without the intervention of a selective mecha-nism. There is ample evidence to show that at the molecular level the neutralmodel does operate, though not exclusively, but at the organismic level nat- ural selection is the dominant force. One of the observations supporting theneutral model is the existence of an approximately even ticking molecularclock, which favours the conclusion that most changes in structural genesarise from neutral substitutions rather than from a grand averaging of var-ious types of selection over time. In the past two decades, there has beenvery strong and sometimes acrimonious debates between the neutralists andthe selectionists and occasionally these arguments have gone beyond thebounds of scientific thinking into the political and social arena. Selection-ists are considered to be the product of capitalist free market philosophyevolved from the English gentry class. Neutralists, on the other hand, areproducts of Marxist thinking and socialist philosophy. Could the recent ten-dency for these two groups to converge towards each other’s point of viewbe a reflection of the corresponding convergence between Thatcherites andReaganites on one side and the socialists and Marxists on the other side?There indeed exists noise at the molecular level and adaptation at the indi-vidual level does prevail. What is needed is a unifying approach that canexhibit neutral effects and at the same time be amenable to selective forces.
The extreme points of view developed by either the neutralists camp underthe leadership of Kimura and Nei or by the selectionists such as RichardDawkins in his well publicised book The Selfish Gene contribute little to thereal understanding of evolution.
3 Ecology, Population Biology And Evolution Of Commu- The population genetics models discussed in the previous section do not in-clude selective interactions at the species level. The very complex natureof the inter- and intra- species interactions as well as the interactions withthe environment, precludes the inclusion of such effects within these mod-els. So the study of the populations at the species level has traditionallybeen divorced from the reductionist genetics models and the discipline ofevolutionary ecology had emerged. The models in evolutionary ecology arebased on the Darwinian theory that the evolution of adaptation takes placeas a result of natural selection; but the non-inclusion of genetics implies that Mendelian mechanisms as well as effects of mutations are neglected. Itwould be a great step forward in theoretical biology if a coherent and consis-tent model of evolutionary biology could be constructed through a marriageof genetics and ecology. The lack of such a "Grand Unifying Model", toborrow a phrase from physics, has led Lewontin (1979) to bemoan the factthat ecology and genetics ’remain essentially separate disciplines, travellingseparate paths while politely nodding to each other as they pass’.
Theoretical ecology has its origin in the work of Lotka and Volterra on prey-predator interactions. The simplest dynamical equations of the prey -predator system is based on 1. an exponential growth of prey in the absence of predator.
2. a rate of increase of the predator population that is proportional to the product of prey and predator densities.
3. a constant death rate for the predators.
The resulting equations closely resemble the equations for the oscil- lations of a pendulum in the absence of friction. The prey and predatordensities are analogous to the kinetic and potential energies in the case ofthe pendulum and exhibit cyclic changes with time. This basic set of equa-tions is biologically unrealistic since it assumes that the prey density is con-trolled by the predator density alone and that of the predator by prey densityalone in a linear fashion. Also the model is structurally unstable since smallperturbations could significantly alter the dynamics. A large industry hasgrown in constructing various modifications of the Volterra equations, in-troducing more realistic biological features. The Lotka-Volterra equationscan also be modified to include other types of intra- and inter- species in-teractions such as competition and mutualism. It has been shown by May(1974) and others that the stability of the ecosystem is dependent upon theseinteractions. Such stability arguments have been based on complex modelsinvolving the network of food-webs in the ecosystem. These types of eco-logical models which analyse the conditions of stability of eco-systems havebeen applied with varying degree of success in the evolution of economicsystems.
The main direction of study in recent ecological models has been to un- derstand the mechanism of coevolution of species. In the simplest version ofcompetition between two species, when restricted to a single resource, oneis led to the concept of competitive exclusion, which states that two speciescannot coexist in the same niche provided there is only a single resourceavailable. If, on the other hand, they compete for a range of resources, thenit is possible that they partition the resources between them and both willsurvive and coexist. The ecological models suggest that two similar specieswill usually evolve so as to utilize different resources, resulting in the twospecies becoming morphologically more different. This process is usuallyreferred to as character displacement . Character displacement can alsooccur in the absence of resource competition, for example, with selectionfor mating isolation. In real ecosystems, any species would interact with alarge number of competitors and parasites. In such cases, the models of co-evolution assert that evolution will cease in a physically stable environmentonce the coexisting species attain their maximum fitness. This obviously isnot borne out by observations in real ecosystems. As each species evolves,it alters the environment in which the other species coexist. This led VanValen (1973) to propose the so called ’Red Queen’ hypothesis. The namearises from the Red Queen explaining to Alice in Lewis Carroll’s Throughthe Looking Glass "here, you see, it takes all the running you can do, tokeep in the same place. If you want to go somewhere else, you must run atleast twice as fast as that!". In this Red Queen model, evolution would con-tinue indefinitely, even in a constant environment, as each species evolvesto meet the changes in others. A model involving the Red Queen hypoth-esis and based on Darwinian theory has been proposed by Stenseth andMaynard Smith (1984). An analysis of the model shows that evolution in aphysically constant environment may continue for ever at a constant rate inwhat is presumably in a species-rich system but would cease in a species-poor system. The results are important to the extent that in a species-richsystem, Darwinian evolution does not necessarily cease in a physically con-stant environment. Ecosystems are not physically stable over a period oftime and in this case both species-rich and species-poor ecosystems exhibitcontinued evolution. The long term behaviour of the ecosystem depends onthe nature of the ecological interactions and on the stability of the physi- cal environment. Thus on the basis of the Darwinian theory, an ecosystemcould be in a fixed state or slowly changing for long periods of time andthen suddenly have an accelerated evolution. Thus the observation of sta-sis followed by punctuation in fossil records need not be non-Darwinianas claimed by Eldredge and Gould. I have discussed at some length thesetypes of ecological models since I feel that they would be the ones mostuseful in any theory of evolutionary economics. It would be interesting toapply the concepts of ’character displacement’ and Red Queen behaviour toeconomic models.
There has been in biology a large amount of interest in understanding the evolution of behaviour and the evolution of sex, mostly based on thegame theory concept of ’Evolutionary Stable Strategy’ (ESS) initiated pri-marily through the work of Maynard Smith (1976, 1982). Since these mod-els discuss only short-term evolution, they will not be discussed here. TheESS models are basically devoid of genetics and hence could have usefulapplication in economic systems.
4 Theories Of Speciation And Paleontology In the previous two sections I have described genetic and ecological modelsthat explain the dynamics of the changes that occur in biological evolutionon the short or ecological time scale. A true understanding of biologicalevolution can only occur when we consider the mechanisms for formationof species and the rate at which they are formed and go extinct.
Even the most casual observer of nature could not but be fascinated by the enormous diversity of organisms which live in the world around him.
Further he would not fail to recognize that this set of diverse organismsat any one place could be classified into unique and discrete subsets. Themembers within each subset are more or less morphologically connectedwhile the individuals from different sets are more often morphologicallydistinct with, in general, no continuity among the subsets.In other words,the morphological differences from one subset to another are discrete withno subset having intermediate characteristics, and he would recognize thesesubsets to be species. He would, for example, be able to recognize two in- dividual dogs from their differences in morphology but would classify themboth as dogs since such differences are continuous while the morphologicaldifferences between cats and dogs are so discrete and noncontinuous that hewould not confuse one with the other.
Thus even at the level of the most casual observer, the various organ- isms can be classified in terms of species and his limited observation isfound to be true for a wide range of animals and plants. Formation of suchdiscrete groups is seen to be almost universal whether one looks at animalsor plants or at structurally simple or complex organisms. Any adequatetheory of biological evolution should thus be able to explain the formationof such discrete species as well as the maintenance of continuous varia-tion within each species. This leads us to the question of the definitionof species. In the case of sexually reproducing organisms, one can definespecies as being groups of organisms that are isolated from one anotherthrough mating incompatibility. Since mating and Mendelian segregationmore or less randomize the genetic structure of each individual in the popu-lation, it is not surprising that there exists a continuous distribution of char-acters within the species population.The concept of species becomes moreambiguous when we consider organisms that reproduce asexually. In thiscase one can define species in terms of their morphological as well as func-tional differences. Since these differences are also more or less reflectedat the molecular level, one could also use the study of DNA and protein se-quences in resolving some of the ambiguity. Even at the level of the viruses,which can only reproduce with the aid of DNA in the host cell, one can eas-ily distinguish the species that cause influenza from that, for example, thatcauses AIDS. In both cases it is known that at the molecular level there ex-ists large diversity and yet the morphological and functional differences aresufficient to separate them into two different species. I shall not enter hereinto the debate concerning the concept of species and I shall assume that inthe large majority of cases we can classify organisms unambiguously intospecies.
The true extent of organic diversity can only be surmised at present.
According to Mayr, there are at present around one and a quarter millionknown species in the animal and plant kingdom. Of these, one millionbelong to the animal kingdom. In the animal kingdom the largest abundance of species is found among insects, being around three quarter of a million.
These estimates are those that have been recorded and the question arisesas to how close these values are to the actual number of species existingtoday. Estimates between 10 to 80 million species has been put forward byvarious authors, based on the rate at which new species are being observed,especially in rain forests. When we consider that over 99 percent of allspecies since the origin of life are extinct now, one realizes the enormousdiversity in the living system. This leads to the interesting but unanswered(and maybe unanswerable) question of whether the diversity has increasedor remained constant over a large part of the evolutionary time, namely, 2.7billion years. I would venture the opinion that at least in the past 0.5 to1.0 billion years the species diversity has been more or less constant withsome periods when there has been episodic increase in the diversity. Thisarises from the fact that if the number of species in the past one billionyears is anywhere between 1 to 2 billion, then the species turnover mustbe very rapid yielding on the average a reasonably constant value for theindex of diversity. Thus there exists no evidence that biological evolutionhas proceeded in the direction of increasing species diversity, at least in thepast half a billion to a billion years. In the early stages of evolution thediversity was small as I shall discuss later. What then was the mechanismthat triggered the enormous diversity observed in the past billion years? Apartial answer to this question is given in the next section.
The species can be classified into higher hierarchical order by cluster- ing those species that have more common morphological, taxonomic, be-havioural and molecular characters. This process of classification can becontinued to higher orders and one obtains a phylogenetic tree showing thedirection of speciation. Such classifications are not unique and many pos-sible trees can be obtained. The ambiguity increases with the number ofspecies involved and with the complexity of the pattern. In general onechooses the most parsimonious tree. This criterion of parsimony is invokedbecause we cannot empirically have knowledge of true historical pattern butthis does not imply that the hypothesis is true and that it mirrors the true his-torical pattern. Parsimony is accepted "not because nature is parsimoniousbut because only parsimonious hypotheses can be defended by the investi-gator without resorting to authoritarianism or apriorism" (Wiley, 1975). In general one attempts to obtain consistent patterns using such varied data asmorphological characters and changes at the molecular level and this, whencoupled with paleontological study, yields an acceptable picture of the pro-cess of species evolution. Such a model need not necessarily reflect thehistorical pattern since most weight is given to the existing species, whichas we saw earlier, are only a small sample of the total number of speciesthat has evolved since the origin of life.
I shall now discuss why there exists such large diversity in organic life, and why such diversity can be clustered into converging nests of higher hi-erarchical orders ? We have already seen that species diversity has remainedmore or less constant over long periods of time, implying that species areformed and go extinct at a reasonably constant rate. This probably is whatoccurs at the lowest level with a regular turnover of species within the low-est group due to small accumulation of mutation and adaptation in agree-ment with the model of the neo-Darwinists. This is probably true, sinceat the lowest level the members within a group usually have very similarmorphology at the embryonic stage and the differences are magnified in theadult stages. The large diversity is a reflection of the enormous range ofenvironmental conditions over the earth. Species which are well adaptedto large environmental changes would then show greater diversity,with thecaveat that this diversity would decrease with increasing body size since,due to energy considerations, the number of large body-sized animals mustbe restricted. Such evolutionary changes cannot be seen in fossil recordswhich are only a extremely small sampling of the totality of species thathas existed. Thus paleontology yields a very crude average of the evolu-tionary process. In recent years, based on paleontological studies, somebiologists, primarily Gould and Eldredge have asserted that evolution pro-ceeds in jerks with long periods of stasis followed by a sudden acceleratedspeciation, claiming that such a process is non-Darwinian. The sudden ap-pearance of a new species in a geographical region need not necessarily bedue to evolution but may be due to migration. Fossil records cannot oftendistinguish between the two possibilities. It is true that there are many caseswhen such punctuationism had occurred but as we saw earlier, this could beexplained on the Darwinian basis. In general, most mutations have small ef-fects but occasionally there could be a macro-mutation that produce a large change. How one can reconcile such mutations with large effects with adap-tation to large-scale changes in the structure of the organism is still a matterof debate. Some biologists believe that the answer lies in the mechanismof development and my own sympathy is with them. This could still beadaptive and there is no reason why Darwinian theory should be discarded.
I am not claiming that all morphological changes have an adaptive reasonbut the general evolutionary trend is consistent with Darwinism. In any ge-ographical region, the fact that energy comes from solar radiation throughphotosynthetic plants to the rest of the living system in that region impliesa constraint on the amount of biomass that can be supported. Within thisconstraint, different species utilize the available resources through a foodchain network. The stability of the ecological community in the regiondepends on the stability of the food chain and evolutionary changes takeplace through changes in the food chain caused by environmental fluctu-ations. When we consider the global situation, it is self-evident that theenvironmental conditions are infinitely variable and hence could producelarge species diversity. It may be that the constancy of species diversityover evolutionary time arises from the restriction on the photosynthetic ef-ficiency in converting the constant solar energy into biomass. But why inspite of speciation and extinction, the diversity is maintained constant is notfully understood.
Up to now I have discussed the diversity of life and the various mech- anisms that are invoked to explain the maintenance of this diversity. I nowturn to the question of how species originate. Following on the views ofDarwin as expressed in the Origin of Species , the modern synthesis hasused the accepted model of adaptive geographic variation as a paradigm forthe origin of species. Populations get geographically isolated with no geneflow between them. Under this condition mutational and adaptive changesaccumulate at a slow rate, and after sufficient divergence between the pop-ulations, they become reproductively isolated. Thus successful speciationis a cumulative and sequential process driven by selection through a largenumber of generations. This mode of speciation is called allopatric speci-ation. Thus the allopatric mode is an extension of the standard populationgenetic model for micro-evolution to the species level. It is certainly truethat many species have originated this way. But recent studies show that al- lopatric speciation may not be the only, and perhaps not the most dominant,mode for the origin of species. Species occupying the same geographicalarea could be isolated by many other mechanisms, such as low fitness ofhybrids and temporal isolation through, for example, delay in emergencetime and thus provide a barrier for gene flow. These could then accumulatechanges that eventually produce new species. Such modes of speciation arecalledsympatric speciation. The inherent assumption in the allopatric modeas described by the modern synthesis is that the population within a geo-graphical region is large and randomly mating. In reality the physical spaceis divided into demes within which random mating can occur. The geneflow between the demes are often too weak to overcome selection and otherintrinsic processes within local demes (Ehrlich and Raven, 1969). And it isthus possible that local demes are sufficiently independent for potential spe-ciation. If this be the case, then the distinction between allopatric and sym-patric speciation becomes cloudy. If demes are largely independent, newspecies can originate anywhere within the geographical range of an ances-tral form. White (1978) believes that speciation could also occur betweenpopulations in continual contact, provided the gene flow can be overcomeby strong selection or through sheer rapidity of potential fixation of majorchromosomal variants. Even Mayr (1963) acknowledged that sympatric for-mation of new insect species could arise through switch in plant preferencefor host-specific forms. In recent years, many authors have pointed out thatchromosomal alterations could provide an isolating mechanism. Also therehave been suggestion that speciation may be more a matter of gene regu-lation and rearrangement than changes in structural genes that adapt localpopulations to varying environment. We thus see a multiplicity of mecha-nisms that induce new speciation. Some biologists have remarked that sincethe allopatric mode of speciation through accumulation of small changes isnot always observed and since speciation can occur on the ecological timescale, the premise of the exponents of the modern synthesis cannot be trueand thus the processes must be non-Darwinian. I cannot see the logic ofthis argument and as I have argued earlier, Darwinian theory can explainsuch sympatric speciation and one need not invoke any new mechanism.
Natural selection would still operate at the species level, though some ofthe assumptions in the formulation of population genetics and evolutionary ecology may have to be altered. But then, science proceeds through such aprocess of evolution of theory.
Another question one can ask is whether biological evolution proceeds towards greater complexity ? In many phyletic lines of animals, the generaltendency is towards larger size. This general rule works more often than itfails and is called Cope’s rule. The opposite phenomenon of gradual sizedecrease is very rare. In general when such size decrease occurs in a lineage,they lead to extinction. Gould (1983, pp.314) cites two examples when adecrease in body size was followed by extinction. (This article of Gould isa humorous essay on the phyletic size decrease in the products of humanmanufacture!) The reason why Cope’s rule operates in biological system isnot well understood. But associated with Cope’s rule is the general beliefthat evolution does proceed towards systems of greater complexity. Sinceevolution is not teleological, why it should move towards larger size andperhaps greater complexity is an open question.
5 Oxygen Revolution, Thermodynamics And Complexity The two most important steps in biological evolution has been the inno-vation of photosynthesis and respiration. Evolution would have come toa dead end but for these two evolutionary landmarks. In the primordialworld, the atmosphere had very little oxygen and the surface of the Earthwas exposed to the harmful ultraviolet radiation from the Sun. Around threebillion years ago, biochemical evolution had proceeded far enough for dis-crete heterotrophic organisms to appear. As their name implies, these organ-isms derived their energy and nourishment from externally formed organicmolecules. They were probably found in deep waters like seas and lakes toavoid the harmful effects of ultraviolet radiation. The oceans were a verydilute, virtually oxygen-free, broth of organic molecules. In the absenceof oxygen, the main energy source for these primal organisms was throughthe process of fermentation, in which energy was derived by breaking or-ganic molecules and rearranging their parts. The most familiar example ofsuch a process is fermentation of sugar by yeast to produce alcohol. In thischemical reaction, 1 gramme of sugar is converted into 0.49 g of carbon dioxide and 0.51 g of alcohol, releasing 110 calories of energy. If on theother hand, sugar can be completely oxidized to produce carbon dioxideand water, as happens in respiration, then 1 g of sugar would produce 3900calories of energy! Fermentation is a very inefficient process for energyproduction compared with oxidation. Also, in the fermentation process var-ious poisonous waste products such as alcohol, lactic acid and formic acidare produced. Thus, these early primitive, and basically inefficient formsof life consumed the organic compounds in the oceanic broth to live, growand reproduce and in so doing, created vast amount of poisonous pollutantsin the environment. Such a process of utilization of limited resources withthe consequent production of self-destructive waste products would havebrought evolution and perhaps life to a halt.
The reasons why these early primordial organisms faced an evolution- ary dead end can be summarized as follows : 1. They were consuming the dilute organic materials from the surrounding 2. They were accumulating poisonous waste products in their environment.
3. Their energy production was very inefficient.
4. They were not in equilibrium with the environment.
5. They were unable to colonize the land mass because of the harmful ef- All these points have extreme relevance to human cultural and techno- logical evolution. Fortunately, for them and for us also, the waste productcarbon dioxide saved the situation,at least partially. This gas was enter-ing the ocean and the atmosphere in ever increasing quantities. Sometimebefore these early heterotrophic organisms had exhausted the organic com-pounds in the environment, they had managed successfully to evolve neworganisms which were capable of utilizing the carbon dioxide produced bytheir predecessors. In this process, the energy of sunlight was made use ofin processing organic molecules from carbon dioxide and water and yield-ing oxygen. For example, using 3900 calories of solar energy, they were able to produce one gramme of sugar molecule. Though photosynthesismade the organisms independent of the limited external supply of organicmolecules, energy for the metabolism and reproduction was still being pro-duced through the inefficient process of fermentation creating poisonouswaste products. The next large step in evolution solved this problem. Theoxygen from the photosynthetic process was entering the atmosphere inever increasing quantities , changing the composition of Earth’s atmosphere.
Also the oxygen, rising to higher altitudes was helping in the formation ofthe ozone layer through photochemical reactions. The ozone layer grew tosufficient thickness to shield the Earth from ultraviolet radiation making itpossible for life to spread from oceans to land and air. It was during this pe-riod that a radically new evolutionary stage, namely respiration, as a methodof energy production was developed. In this process sugar was reduced tocarbon dioxide and water, releasing around 35 times as much energy froma unit mass of sugar as was possible with the fermentation process. Thewaste products were no longer harmful, and the carbon dioxide released isrecycled through photosynthesis. The oxygen - carbon dioxide cycle canbe considered as a giant wheel pumping solar energy into the living system.
The new atmosphere, composed mainly of nitrogen and oxygen, maintainedthrough the dual process of photosynthesis and respiration, possessed a sta-bility that has lasted for the past billion years. There exist in the biospheremany other cycles such as the hydrological (water) cycle, the nitrogen cy-cle, the sulphur cycle, the phosphorus cycle and others. These cycles areinterconnected through the living system and the external environment.
Apart from the energy requirement satisfied through the carbon diox- ide - oxygen cycle, living systems require various other materials such assulphur, phosphorous, nitrogen etc. as building materials. Since these mate-rials are finite resources, for evolution to be maintained they need to be con-served through recycling. As remarked above, the material cycles involveboth the physical environment and the layer of life. All material cycleshave three basic pathways in common. The first of these is growth, takingfood matter from the environment. Involved in this growth phase are allenergy-requiring processes of the living organisms, including motion, per-ception, reproduction and body maintenance. Plants and other autotrophicorganisms absorb material from the environment and convert it into usable organic compounds. Animals derive their raw materials from plants or otheranimals and after processing these raw materials, the waste products are re-leased into the physical environment.The second path in the cycle is theprocess of death of the organism and the accumulation in the environmentof waste products and dead organic matter. These are made up of complexorganic molecules that need to be degraded before they can re-enter the foodchain. In the third step the metabolic waste products and dead organic mat-ter are decomposed into simpler chemical compounds that can be reused tostart the cycle again. This decomposition is accomplished by a specializedand diverse group of micro-organisms called decomposers.
There are two points to note in this basic cyclic structure that exists in the biosphere. Firstly, matter assimilated by organisms is eventually re-turned to the environment in its original form for further usage. Thus thereis no such thing as a waste product in the broad ecological context. Wasteexists only in terms of specific species. The waste product of one speciesforms the food input of another. The self-regulatory equilibrium that existsin the biosphere depends on this recycling of waste products and thus hasdeep significance for the stability of the ecosystem. Secondly, the cycle canonly be maintained through birth and death of organisms. Thus birth anddeath form a necessary condition for the long-term stability of evolution-ary systems. Living systems are thermodynamically far from equilibrium,consuming negentropy from the sun to produce highly complex ordered sys-tems. The various interconnecting cycles discussed above are an essentialfeature for the maintenance of the system over large periods of time.Thereis a slow, small input of materials into the biosphere through the weath-ering of rocks and volcanic eruptions. The amount that is thus gained isbalanced through sedimentation in the oceans. This part of the material cy-cle is very slow compared to the recycling time involved in the biologicalcycle. The material cycle can be considered as made up of two interact-ing flywheels, a slow wheel based on geochemical mechanisms and a fastwheel turning through the biochemical process. The energy to power theslow wheel comes from the Sun as well as from the energy stored in theinterior of the Earth. The energy from the interior of the Earth is used in theuplifting of crustal rocks. Winds and rain that produce weathering of sur-face rocks derive their energy from the Sun. The biochemical cycle utilizes solar energy to maintain its motion. The solar energy is stored as poten-tial energy in molecular bonds of organic molecules and is released duringvarious metabolic processes that take place in the organism.
I shall now describe briefly the mechanism involved by which cells transform energy during respiratory metabolism since this process givessome clue to the question of large-scale conservatism at the molecular level.Surprisingly,oxygen is destructive to all forms of carbon-based life. Molecular oxygenreacts spontaneously with reduced organic compounds. There are, for ex-ample,very sensitive anaerobes that cannot tolerate even 1 - 2 % increasein the oxygen concentration above the present levels found in the atmo-sphere. It is the toxic nature of oxygen that has necessitated the evolutionof a very complex mechanism for respiratory metabolism. The evolutionaryprocess had to contend with the following criteria. The energy requirementfor higher forms of life can only be met with oxidative metabolism. Yet theburning of oxygen in the furnaces of organisms had to be at a very low tem-perature to prevent cellular damage. Nature’s solution for satisfying thesetwo criteria was to endow molecular oxygen with the role of an electronsink or equivalently, a hydrogen acceptor in biological oxidation. Biolog-ical oxidation proceeds not so much by the addition of oxygen as by theremoval of hydrogen. This process of dehydrogenation is brought aboutby the action of enzymes which remove the hydrogen atom from the sub-strate molecules that function as hydrogen carriers. The energy released inthe oxidation process is harnessed to regenerate the molecule ATP from themolecule ADP and phosphate. The ATP - ADP cycle can be thought of asa fuel cell for energy storage. This method of storage is utilized by bothphotosynthetic and respiratory processes, exhibiting the economy of usagein nature. The respiratory cycle, known as Krebs cycle, consists of a largenumber of stages, such that the free energy of the glucose molecule is trans-ferred in smaller units to thirty six ATP molecules. The hydrogen in theglucose molecule ends up as water and the carbon atoms as molecules ofcarbon dioxide. The efficiency for the conversion of the free energy storedin glucose to the ’organic fuel cells’ ATP is around 66% .When this energyfinally gets converted into muscular work, the overall efficiency drops toaround 30 %. But it is worth noting that a large part of the energy storedin ATP molecules is used in the direct biosynthesis of macromolecules like proteins without conversion first to mechanical or heat energy.
A modern steam generating plant is capable of converting around 30% of energy input into useful work and thus compares favourably with theefficiency for the production of mechanical work by biological cells. But toachieve this, the steam temperature has to be as high as 600o - 800oC. Thusevolution has been able to realize overall energy efficiency comparable to, ifnot better than, that obtained in human technology but at room temperatures.
The price that nature had to pay for such a low temperature, high efficiency,energy conversion system was to evolve an extremely complex chemicaltechnology.
As pointed out earlier, the evolution of photosynthesis and respiration made it possible for the living system to utilize the constant energy sourceof the sun, yielding a mechanism for an increase in the specific energy avail-able. This thus paved the way for evolving multicellular systems. Also thecreation of the ozone layer increased the physical environment in which lifecan exist. The main constraint on the maximum possible biomass is theamount of sunlight that is incident upon a region and the very low photo-synthetic efficiency of 1 - 2%. Thus the conditions were optimum around500 million years ago for an explosive increase in species diversity. Thisperiod, called the Paleozoic era, was also marked by large scale continentaldrifts, producing vast swampy regions. The decaying biomass of this erawas turned into bituminous coal over geological time scale. A large frac-tion of the present day coal deposits, almost 60%, is bituminous coal thatoriginated around half a billion years ago. When we indiscriminately useour fossil fuel resource, it might be worth while reflecting on the sequenceof favourable events that helped to accumulate this heritage for us.
Since the discovery of the helical structure of the DNA by Watson andCrick in 1953, there has been an enormous information explosion in molec-ular biology. The success of molecular biology in explaining many aspectsof the genetics of the organism has led to a strong and widespread beliefamong molecular biologists that biology can be reduced to the molecular level and nothing new can be gained by studying the properties at the levelof the organism. There exists at least among some molecular biologiststhe feeling that biological systems are no more than a collection of atomsand molecules oriented in specific fashion through chemical and physicallaws and that all aspects of biology can be ultimately explained in termsof the physics and chemistry of macromolecules. It is true that organismsare made up of atoms and molecules, but they are highly complex patternsof form and structure of these atoms and molecules. In short, living pro-cesses are highly improbable patterns of physical and chemical processes.
In view of the claims by some molecular biologists, it would be interestingto ask what the study of the organism at the molecular level can tell us aboutbiological evolution.
The DNA, as it exists in the chromosomes of cells, consists of two polymeric fibres wound around each other in a helical way - the double he-lix .Each fibre consists of a backbone of sugar-phosphate groups to whichare attached four types of molecular units, called nucleotides, in some spe-cific order along its length. The chromosome is composed of the long DNApolymer coiled into a complex three dimensional structure and various pro-teins needed for the basic functioning of the DNA molecule. The number ofnucleotides in the DNA can vary from a few thousands to hundreds of mil-lions depending on the organism. Along the length of the DNA moleculeare regions of finite length, called coding regions, whose beginning and endare recognized through specific sequence of nucleotides. The coding regioncontains the information that is translated into proteins through a complexmechanism involving intermediaries called RNA. The protein molecule ismade up from molecular units called amino acids. There are twenty dif-ferent types of amino acids and these are almost universally used by allliving systems in producing proteins. In the coding region of the DNA,the amino acids are recognized through specific triplet sequences of the nu-cleotides. Thus the coding regions are made up of such triplets defining theprotein they code for. The genes we have alluded to earlier are the codingregions and we could assume that each gene corresponds to a single pro-tein, though this is not universally true. The mechanism of translation isone way, namely from DNA to RNA to proteins. This then is the molecularexplanation of the Central Dogma discussed in an earlier section. Though there are a few instances in which an RNA can be translated back into DNA,the retroviruses being one of them, in no case has one found proteins to bereverse translated to DNA. It is this central dogma that prevents a Lamar-ckian mechanism for biological evolution. The proteins together with theenvironmental interaction produce phenotypes while the coding regions arethe genotypes that are passed on more or less faithfully to the offspring. TheDNA also has the ability to self replicate with the help of enzymes that arecoded by it. Thus when a cell divides, each of the resultant cells contains analmost exact copy of the original DNA. In man, for example, there are tenmillion million cells and each of these carry a complete copy of the DNA intheir chromosomes. The coding regions, producing proteins and enzymesthat determine the structure, form and functions of cells and the organisms,are called structural genes. There are also coded into the DNA regulatorygenes that determine the expression of structural genes both spatially (indifferent cells) and temporally.
At the molecular level one sees an enormous conservatism in biologi- cal evolution. Many of the chemical elements such as oxygen, carbon andhydrogen are found in very similar proportion in almost all living organ-isms. The DNA structure is universal and the DNA code is also almostuniversal. There exist some differences in what are called the mitochon-drial DNA that regulates energy production. All organisms are based oncellular structure with the cells falling into two general patterns, namely,the prokaryotes and the eukaryotes. All organisms utilize the same twentyamino acids for constructing proteins. It is even more interesting that mostof the proteins are found in an extremely large range of organisms. Thenumber of novel types of proteins in the organisms of any species is verysmall indeed. The initiator and terminator sequences in the coding regionsof the DNA are almost universal. Quite often, instead of producing newand novel types of proteins, the organisms use homologous proteins to per-form different functions. Finally almost all macromolecules involved theliving system have a specific chiral structure. Many chemical substancesexist in two dissymmetric molecular forms, the right (D) and the left (L)isomers, whose structures are the mirror reflection of one another. In mostorganisms, the principal constituents of protoplasm, particularly the aminoacids, are represented exclusively by L isomers. Certain bacteria do have D type amino acids but these are mostly components of antimetabolites, toxicto competing organisms. The essential proteins of these bacteria containexclusively L isomers. The D isomers are also found among some earth-worms and insects. Before stepping through the mirror into the nonsenseworld behind the looking glass, Alice said to her kitten "How would youlike to live in Looking-Glass House ? I wonder if they’d give you milk inthere ? Perhaps Looking-Glass milk isn’t good to drink". In fact it followsfrom what has be said above regarding the asymmetry in the molecules ofliving beings that to drink mirror-image milk is, at best, useless.
What is the cause for such a high degree of conservatism and how can we reconcile this with the extreme diversity observed at the phenotypic andspecies level? A partial answer to this question can be found in the ear-lier discussion on thermodynamics. We have already seen the type of con-straints required for maintaining respiration. The ATP and ADP moleculesmay have been present from the early stages of pre-biotic evolution. Theneed for the mechanism of electron transport in converting the free energyin glucose into the energy-rich ATP molecules puts a severe constraint onthe chemical mechanism. Once the Krebs cycle has been established, itwould be difficult to alter the process without seriously affecting the wholestability of the system. The extraction of energy from the ATP moleculesfor both the construction of macromolecules as well as for mechanical en-ergy imposes restrictions on the type of enzymes needed and the variousmetabolic pathways become more or less fixed. The various other cyclesthat pass through the living strata produce similar constraints. Thus for alarge class of proteins, mutational changes that do not alter the form andhence the function of the proteins are the only ones that can, in general, sur-vive. Thus evolutionary changes at the molecular level are, for large part,neutral and the claims by the neutralists are justified. The conservatismand hence the neutrality must have occurred during the period when thephotosynthetic-respiratory mechanism got established. Thus the long termevolutionary stability through recycling of materials imposes conservatismat the molecular level. I shall discuss the significance of this to technologi-cal evolution later.
If molecular evolution, at least of the coding regions and hence of en- zymes and proteins, is neutral and conservative, how can we understand the large diversity seen at the phenotypic and species level? The answer to thislies in the fact that the eventual structure and form of the organism dependon the spatial and temporal distribution of proteins controlled by regula-tory genes especially in the early stages of development. Different cellsproduce different amounts and types of proteins at different stages in thedevelopment. In spite of the vast strides made by molecular biologists, themechanism for the control and production of proteins at the right time and atthe right place is still a mystery. The adaptive ability of the phenotype thusformed would determine which would survive. Thus the Darwinian processis more applicable at the organismic level and at the molecular level the var-ious constraints have established an almost conservative system. If this betrue, then one should see the effects of selection more on regulatory genes,especially at the early stages of development than in structural genes. Devel-opmental pathways controlled by the protein production in space and time,thus determine the structure and form of the adult experiencing Darwinianselection. Thus it is not unreasonable to predict that mutational changes thataffect the spatial and temporal expression of structural genes could producelarge scale evolutionary change, provided the pre-adult and the adult stagescan adapt to the environment. Most of these mutational changes would beharmful but favourable changes could occur at infrequent intervals. A sur-prising feature of the DNA molecule in higher organisms is the occurrenceof a large amount of repeated sequences, occupying between 30 and 90%of the polymer length, with apparently no function. Many biologists re-gard these "junk" regions of DNA as selfish DNA replicating selfishly withno advantage and presumably, no disadvantage to the organism. This, theyclaim, is a clear example of non-Darwinian evolution with no adaptive sig-nificance. I believe that the highly repeated, apparently functionless regionsdo have an important function. The replication time for a DNA moleculeis related to its length. In complex organisms, the intrinsic time scales,for example of metabolism, heart beat etc., must all be tuned such that thesystem can function efficiently. This tuning process need not be very criti-cal. The situation has close analogy with complex manufacturing systems,where the timing of various inputs and outputs are very important for theefficient functioning of the system. Interestingly, in mammals it is well es-tablished that the time required to complete 50% of its growth is about 3% of its lifespan, independent of body size. Similarly, it is found that 1.5% ofa mammal’s life span is required for gestation no matter how big it is. Bothrespiratory cycle and heart beat cycle occupy nearly size-independent frac-tions of its lifespan. As a consequence, every mammal can expect to live forone third of a billion breath cycles and for 1.5 billion heartbeats (McMahonand Bonner, 1983). These examples clearly indicate that there are con-straints on the various intrinsic time scales in the system. DNA replicationis associated with cell division which occurs in order to increase the numberof cells during the developmental stage or to partially replace dead cells indeveloped adults. Since both these processes are related to the various timescales in the system, it should not be surprising that the length of the DNAis adjusted so that its replication time is in conformity with the rest of thesystem. I do not believe that this is a very severe constraint. The replicationtime scale can probably vary by a fair amount. Also, the constraint is on thewhole of the genome in a statistical way. But this constraint still provides anadaptive significance for the existence of junk DNA. How could such junkDNA been introduced into the organism’s genome ? The answer lies in thefact that there are entities called plasmids and transposons that are bits ofDNA which can attach themselves at specific points in the organism’s DNA.
Multiple copies of this transposed DNA material can be produced throughvarious mechanisms present in the genome. Such mechanisms would beadaptively suppressed or would be entirely absent in lower organisms suchas viruses and bacteria. In the case of the higher organisms multiple copieswould be produced. The number of copies would depend upon what hap-pens in other regions of the genome. This will not be a static process, but adynamic one with copy loss at some regions and copy gain at other regions.
Thus one would expect a large variability in the copy number of any specificrepeat unit. If the transposed element lodges itself in the middle of a codingregion, then it has to be removed in the process of translation to proteins.
This could be a time and energy consuming process and natural selectionwould tend to keep the copy number down. Such regions are known to occurin the coding sections, especially in higher organisms, and are called introns. Since introns, probably caused by transposable elements, do occur fromtime to time in a random way, the protein manufacturing process has devel-oped mechanisms to recognize such regions and remove them at the stage of protein formation. The translation of the DNA coding region into proteinspasses through a large number of intricate intermediate steps and hence itmay be more efficient to excise these introns towards the end of the manu-facturing process. This also may be due to the fact that the DNA with junkand introns is passed on to the germ line and thus the incisions at the DNAlevel during translation may not be efficient. If the mechanism describedabove for the occurrence of junk DNA is valid, then this is compatible withDarwinian mechanism at a statistical level over the whole genome. There isno need to invoke non-Darwinian, non-adaptive ’selfish genes’.
I have already remarked on the conservatism that exists at the molecular level. The form and structure of proteins are very important in determiningtheir function and this implies that some amino acids in the protein moleculecan be replaced by other amino acids such that the protein function is notinhibited. Proteins are composed of a few hundred to a few thousand aminoacids. Thus within the constraints of form and function, there could be con-siderable variability in the amino acid structure of proteins caused throughmutations at the DNA level. Since a triplet code is used to define an aminoacid, one has a mapping of 64 triplets onto 20 amino acids. Thus the codeis degenerate in the sense that some amino acids are defined by more thanone triplet sequence. Thus one would expect even greater variability at theDNA level. Both these types of variability have been observed and it is notsurprising that the variability can be well explained on the basis of the neu-tral model. Since mutations at the DNA level occur, in general, randomly,the variation in the DNA coding region or in the amino acid sequence of theproteins yield a time direction for the evolution. One can construct phylo-genies using this data and such phylogenies are not incompatible with thosearrived at through phenotypic characters and fossil data. The discrepanciesthat one does observe may partly be explained by the fact that there is nounique one to one mapping between proteins and phenotypes. Given thetwo sets of phylogenies, one arrived at through macroscopic analysis andthe other from analysis at the molecular level, provided the macroscopicdata is reasonably complete, biological evolution would be better describedby the macroscopic model. Unfortunately, the data at the macroscopic levelare often inconsistent and incomplete. On the other hand, at least for thepresent living organisms, there is an explosive increase of data regarding DNA and proteins. This gives one the misplaced feeling that the study ofvariation at the molecular level is all that is necessary for describing themode of biological evolution.
As we have seen, the most fascinating aspect of a living system is the strong conservatism at the molecular level and the extreme diversity at thespecies level. I have argued earlier that this molecular conservatism impliesthat molecular biology alone cannot provide all the answers to biologicalevolution. On the other hand, such a strongly conserved system could pro-vide the clues to the origin of the primordial molecular self-organizing sys-tem. With our present knowledge regarding molecular organization in livingcells, we could conjecture the following physico-chemical steps in the tran-sition from non-living to the living molecular complex. We could recognizethree major phases in the early development of life. The first phase can betermed the period of chemical evolution. It is fairly well understood howunder the conditions of the primitive earth, the precursors and monomers ofbiological macromolecules, such as amino acids, nucleotides, phosphates,saccharides etc. could have formed. There must have existed a large diver-sity in the chemical species, since all conceivable chemical substances thatconform to the laws of chemical thermodynamics and kinetics could, andprobably did, form spontaneously. There is experimental evidence to indi-cate that these early monomers could condense spontaneously to biologicalmacromolecules leading to almost random sequences and many of the re-sultant proteins thus formed are seen to possess some catalytic activity at alow level. It could be thought that the nucleation of life could occur once asufficiently large number of these primordial catalytic proteins were present.
But as shown by Eigen and Schuster (1979), Küppers (1983) and others, thetransition from such random sequences to self-organizing biological macro-molecules which formed the precursors to living organisms could only haveoccured through a process of selective optimization. The next phase in themarch towards biological evolution could be called the phase of molecularself- organization. In the early prebiotic condition, proteins were more eas-ily formed than sequences of nucleic acids that are needed for informationstorage and transmission. Molecular selection and evolution presupposea capacity for self-replication, a property possessed by nucleic acids andnot by proteins. Since RNA molecules fold themselves into specific three- dimensional structures, unlike DNA molecules, the RNA molecules are ca-pable of expressing themselves phenotypically and thus interact selectivelywith the environment. The evolution of such molecules in this early stagesof life on earth was severely limited by the rate at which error propagates.
It can be shown that such molecules could not have been much more than100 bases (nucleotides) long, which is approximately the size of the presentday tRNA molecules. For further evolution to take place, one would re-quire highly optimized proteins for highly accurate copying and informationtransmission. But the RNA were too short to carry information for such pro-teins. This appeared as the classic chicken and egg example. The problemwas solved by Eigen through postulating a catalytic hypercycle in whichthe proteins and the nucleic acids were cyclically coupled. A hypercyclehas information capacity much higher than that which existed in the earlyshortlength RNA’s and is thus well equipped to further the evolution of thecomplex machinery of replication and translation. Eigen showed that thehypercycles were optimized through natural selection. They had nonlineargrowth properties and hence led to the "once-for-all-time" decision in theirstructure and function when a particular species had reached macroscopicpopulation numbers. Thereafter they could not be dislodged by the few se-lectively advantageous competitors that were present in the environment. Ithas been shown that even if several code schemes are equally likely to befound in the environment, the nonlinear growth will reinforce fluctuationsand thus ensure that only one hypercycle becomes established with all theothers dying out. Even though this model is very attractive, it is not clearwhy different hypercycles could not have occurred at different geographi-cally separated regions. The final phase in the early biological evolution canbe called compartmentalization. Since compartmentalization provides a se-lective advantage to hypercycles, the formation of protocells was inevitable.
This saw the end of nonlinear selection and the era of divergent evolutionhad begun. Further evolution of differentiated cells could only occur whenthe information-carrying capacity is increased and this led to the evolutionof the DNA molecule. The process through which this transition took placeis still not understood.
In this brief review I have omitted many interesting problems in evo- lution, the most outstanding of these being the evolution of sex. It is well known that sexually reproducing organisms have a two-fold disadvantagecompared to non-sexual systems. Why then is sex so successful and howcan we understand it within the Darwinian frame work ? There are manytheories attempting to explain the evolution and maintenance of sex but atpresent there is no consensus among biologists. Though most biologistswould agree that sex evolved and became universal through a process ofnatural selection, there is little agreement on what this process may be. Theother major aspect in evolutionary biology that I have only alluded to is de-velopmental biology. Though both these subjects have important relevanceto biological evolution, I have not discussed them since they are still in theearly stages of understanding.
In this review I have argued that the extremely long term dynamic stabilityin biological evolution is a consequence of the evolution of resource recy-cling. The only resource that is consumed is solar energy which eventuallyreturns to the environment as low grade heat. Molecular conservatism is aconsequence of this evolutionary constraint. The mode of converting so-lar energy into ordered living systems has been responsible for freezing themechanisms by which energy and materials are utilized by living organ-isms. This then implies that once the balanced system has been perfected,the amount of biomass in the biosphere could not have altered very muchsince biomass production ultimately depend on the total solar radiation in-cident on Earth and on the photosynthetic efficiency. The latter again couldnot alter very much because of molecular conservatism. I would suspectthat this situation has lasted since the Cambrian period. The approximateconstancy in the total biomass has been the reason for the constant speciesdiversity over the past billion or so years. In this context I would like tostress that the constancy I refer to does not imply that there have not beenfluctuations. What I assert is that on the average both biomass and speciesdiversity have been reasonably constant. Thus the long term ’economicgrowth’ in the biosystem has to be measured through species turnover ratherthan through an ever increasing biomass production and species diversity.
Some evolutionary biologists claim that the fundamental law of evolutionis that evolution proceeds in the direction such that all available ecologicalniches are occupied. I prefer to recast this in terms of the constancy in thebiomass and in species diversity.
The living world can be described in terms of various hierarchical lev- els such as the molecular level, the genotype level, the phenotype level,the population level, the species level, the ecological community level etc.
There exists a large amount of overlaps between the levels, but still such cat-egorization is useful since there exists at present no Grand Unified Modelfor the description of biological evolution at all levels. I have shown in thisreview that at each of these levels evolution, in the main, proceeds throughthe Darwinian mode in terms of the macroscopic variables pertaining to thelevel. But as we have seen, natural selection is often modified by stochasticeffects and by constraints in the system. Many of the controversies amongbiologists arise in trying to understand the nature of evolution at any hier-archical level in terms of models specifically formulated for the level aboveor below.
What can economists learn from a study of biological evolution ? Un- like biological evolution, economic evolution is Lamarckian in character.
The rate of economic evolution is also dependent on the variability that ispresent and it is possible to formulate an equivalent to Fisher’s FundamentalTheorem. The variability in economic systems arises through competitionand occasionally through new discoveries, the ’hopeful monsters’. Boththese can be incorporated into a model for economic evolution which par-allels much of the ecological theories in biology. For example, one hasthe concept of competitive exclusion for two products (species) occupyingthe same economic niche. Thus the variability in economic systems couldbe thought of as arising through the Red Queen effect and ’character dis-placement’. Free market economy, encouraging more competition, wouldthus generate more variability and hence higher rates of evolution than acontrolled economy. The great difference between biological evolution andeconomic evolution is in resource utilization. Economic systems, particu-larly capitalist economy, are often tuned to the concept of growth which, inthe ultimate analysis, implies increasing resource utilization. Economic sys-tems, like biological systems, are far from equilibrium. Thus if economic systems are to enjoy long term stability, it needs to adopt the biological con-straints, namely resource recycling. In this case the concept of economicgrowth has to be considerably modified. Pure free markets cannot be sta-ble for long periods and the constraints we need to impose are not the onesthat present socio-political systems apply. Whether we can achieve such abalanced economic evolution depends on our collective political will. Sincethe economic system is strongly coupled to the biological world, unless wecould bring about such a balance, we shall be also strongly perturbing thestability of the biological system. It would be an irony if the biological sys-tem that has enjoyed more than a billion years of stability faces instabilitythrough the actions of the species that has been endowed with intelligenceand means to utilize it more than any other species in the long journey ofbiological evolution.
I am very grateful to Professors B.C.Clarke FRS, A.J.Cain FRS, and J.C.Wilmottand to Drs. L.M.Cook, A.Wallace, and A.Hillel and to my wife Jean Manifor reading the manuscript and for many helpful suggestions. Many of theerrors that still exist in the text are due to my own pig-headedness in ignor-ing some of their comments.
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