EPIGENETICS , PALEONTOLOGY, AND ... EPIGENETICS , PALEONTOLOGY, AND EVOLUTION Stan P . Rachoo t i n

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Text of EPIGENETICS , PALEONTOLOGY, AND ... EPIGENETICS , PALEONTOLOGY, AND EVOLUTION Stan P . Rachoo t i n

  • EPIGENETICS , PALEONTOLOGY, AND EVOLUTI ON

    Stan P . Ra cho o t i n and Kei t h S t e war t Thoms on Pe abo dy Museum of Na tu ra l Hi s t ory , Yale University ,

    New Haven , Co nne c t i c u t 0 6511 , U. S .A .

    ABSTRACT

    'Three requirements of evolution are persistence with chang e, internally cohe s ive individua l s of limi t ed dura t ion, and interaction with environment. We recas t th es e proper t i es in megaevolutionary terms. Harologous structures persist over geological t ime as they evo lve in f orm and func t ion . Spec ies, which arise through a greater or lesser genetic r evolution, behave as individuals. The interac t ion of genes and envi- ronments- -development- -under lies both the origin of species and the continuity of haro- logies. The properties of epigenetics- -self assemb ly , feedback, a lternative pathways to the sane devel opmental end, canpensati on for the inevitable irregularities of devel- opment--allow a redefinition of gene t ic revolution in terms of the establishment of new, stable developmental patterns : a typological r esult produced in ac ceptably popu- l ati ona l ways . Harologues, s imi larly , are the products of evo lving epigenetic sub-sys- tems. Al though developmental systems retain ancestral potentials that renain unex - pressed for tens of millions of years, such potentials are so burdened with other devel- opmental pathways built upon them that they hold little promise for evolutionary change. But where a recent developmental change ha s occurred, no such "buf fering" exists . As errors occur , sane will be ep igenetically acconm:>dated and , if adaptive, gene t i cally assimilated . These are the quanta of evolution . If a r ecent adaptive breakthrough has occurred, i t wi ll be " suppor t ed" at first by interacting ep igenetic sub-systems. The poorly canalized epigenetic system roost; closely associated with the new adaptation will be under intense selection to build up an epigenetic environment that assures the pre- dictable expression of the new trait . Until this happens, fur ther " exper iments" along the l ines of t he initial change can occur and be assimilated . Depend ing on eco logical mi lieu, t his i s the stuff of adaptive r adiation , evo l u t i onary trends, or allanetri c change. Speciation, which can occur without any adaptive change, is th e result of anal- ogous changes in the epigenetic systems responsible for mate r ecognition . We accept the neo-Darwinian reliance on natural selection acting in populations. But we choose to concentrate on i ts action on developmental processes, rather than on traits , the obvious and experimentally tractable end -products of such processes. The normal features of epi- genet ics fortui tousl y l ay down the lines of l ea st r esistance to evo l utionary change. We find ourselves l ooking a t an intrinsic and emergent s i de to evolution, a view that in the past was held, on similar gener a l grounds, by Bateson, Goldschnidt, and Waddington . The story of evolution i s perhaps less the warfare of selfish genes than the YX>rking out of the potentials of selfless epigenes.

    I NTRODUCTION

    The title of our paper, "Epi genetics, Paleontology, and Evolution", i s de liber- ately chosen to r eca ll the title of the conference that was held at Princeton Univ - ersi ty in 1947 on "Genetics , Paleontology, and Evol ut i on" (Jepsen e t a l . 1949). I t included not only th e subjects of gene tics , speciation, and paleontology, but canpara- tive anatany, ecology, and systematics as well . The synthe tic theory that came of age with that symposium ha s been the nota- ble success of organ isrnal biology of thi.s century . In a day when theories in o ther parts of biology change wi th the seasons, it i s astonishing to f ind that 33 years later the study of evolution is no l es s vigorous for having r emained in the sane node . We =uld be pleased if our contri -

    bution i s seen as a na tural offshoot from the syn thet ic stock. But though the syn- thetic theory has provided many answers , i t has not adequately framed a l l the ques- tions. llich renains to be explained , es- pecially when we take the long pa leontolo- gical per sp ective . The answers to these questions are, we think , t o be found in a considera t i on of the evolutionary implica- t ions of development and the developmental impl i cations of evo l ution, topics that were notably l acking in the 1947 conference .

    Our goa l in t his paper i s to se t in a developmental con text the problems of mega- evo lution : the nature of adaptation , the tempos and nodes of evol ution , the framing of us eful general izati ons about the origin and diversifica tion of major groups . As we go about this , we find ourselves r ein-

    G.G . E. Sc udder & J . L. Rev e a l (ed s .) I EVOLUTION TODAY, Proce edings of t he Second Inter - na ti o na l Congress o f Sys tematic a nd Evolut i onar y Bi olo g y, pp . 18 1- 19 3 . 19 81 .

  • 182 EVOLUTION TODAY Rachootin & Thomson

    terpreting sane of the ftmdarnenta l con- cepts of evolut ion, including variation, spec ies, and what i t i s that selection acts upon . These reinterpretations cane out of a series of generally accepted premises about development , which, how- ever, have unexpected impl ications when viewed over the time scale familiar to the paleontologist. Our paper is an outline of a way of ret:hinking what everyone knows about evolution. We ask you to rearrange your mental furniture and consider sane changes that we think are suitable for sane circurns tances . Al though the rear- rangement is new, the pieces are not . Those that look unfamiliar we have only brought down from the attic .

    In the broadest terms. we ask what me- ga -evolution ought to require of theories at the level of genetics and development, and also at the level of speciation. We expect that nothing at the l evel of mega- evolution contradicts what happens at these l ower levels, bu t that we will not fully understand these l ower l evel s un til we actively search them for phenCID2I1a that may seem peripheral to the population bio- logist, but which are required to explain the data of the paleontologist and the roorphologist. We trace the emergent mega- evolutionary phenanena down to their hid- den roots, which we find to be developmen- tal, and then ask what this mega-evolution- ary view of development suggests about the nature of species and speciation .

    We also ask you to allow us to break one other convention of evolutionary theory, concerning intrinsic and extrinsic factors in evolution. Extrinsic factors are the relations of an organism to the external environnent- -adaptations to local condi- tions, the sorting-out of congeners in sympatry by character displacement, and such chance effects as the genetic sample borne by the traditional gravid female com- pared to the population from which she de- rives, or the effect of a river changing its course and cutting off a population. We accept such extrinsic factors as the carrron property of all m:xIem evolution- ists. But although we all believe in such factors, they are remarkably difficult to demonstrate in anyone case . We define intrinsic factors as the adaptation of the genane to itself . Developmental mechan- isms, especially epigenetics, the self- organizing and correcting properties of developmental pathways, are the means by which the intrinsic side of evolution i s manifested . Intrinsic factors will be just as difficult to demonstrate as ex- trinsic factors, all the roore so because biologists are not accustaned to t:hinking much about them. But there is no a priori reason why they rrust be mystical.

    "Intrinsi c" and "extrins i c" bear close relation to the distinction that Mayr has drawn be tween typological and population- al thinking . Throughou t the paper, we will be contrasting a populational, or , rrore generally, an extrinsic approach with a complementary intrinsic approach that is developmental and typological. Note that both approaches are ways of t:hinking, not hypotheses about nature . The mind has a hard time holding = com- pletely different views of the worId si- mul taneously, but it is our guess that having two ways and switching when i t seems appropriate i s better than having one way that we a lways be lieve to be right, and another, which we seem to fall into , but which we have decided is always wrong.

    Development is studied in a typological m:xIe. Development shows directedness; in an experimental manipulation, the embryo " tri es" , if you will, to develop normally, according to type, in spite of the insult. Results in a devel opment al experiment turn on the behaviour of particular, crucial embryos. This is a far cry from popula- tion thinking. The dichotomy is apparent in the 1947 symposium, which neglected de- velopmental biology, but not because there were no evolutionists interested in it . Indeed, embryology dominated evolutionary studies in the las t century , and was the guiding light to such twentieth century vorkers as William Bateson, Richard Gold - schmidt, and C. H. Waddington. The prob- lem was that the typological and the pop- t l a t i ona l could not be fused into a sin- gle approach. How, for instance , could Goldschmidt 's macro-evolution, which took developmental saltations as the means of getting from one major group to another, be reconciled with the gradual, popula- tional approach that was then being fash- ioned, an approach that accounted for me- ga -evolution by an aCCl.IIllllation of micro~ evo lutionary events? The r esolution on the part of those who formulated and pop- ularized the synthetic view--Mayr, Dob- zhansky, Simpson, Stebbins--was to take population t:hinking as right, true , and rrodern , and typological thinking as wrong, false and old-fashioned . A sympathe t