4. Application of neontology to paleontology Now we are ready to apply these concepts from the biology of extant organisms to that of fossil organisms. This proceeds on the simple inference that past life went about its business in much the same way as present life does.
Some of the predictions of Punctuated Equilibria are as follows: (A) Punctuated Equilibria postulates that speciation events comprise most of the evolutionary change seen in adaptation. This is a consequence of the inhibitory effects of gene flow, genetic homeostasis, and large population sizes (6 above). The adaptations of newly speciated daughter populations are forever excluded from the ancestral population because of reproductive isolation (2 above). (B) PE explains the abrupt appearance of new species in the fossil record. The splitting of lineages (1 above) in the mode of allopatric speciation (2 above) followed by ecological dispersal and succession (5 above) would result in geologically abrupt appearance of the daughter species everywhere except the limited geographic area where the speciation took place. Most speciation takes place as peripatric speciation, which is confined to a limited geographic region, and after which ecological principles argue for relatively rapid reintroduction and spread into new habitats for the daughter species. Since the critical change occurs in such a small region and in such a limited population, the probability of finding specimens which document the transition from ancestral to daughter species is very low. A population which can exploit resources untapped by current populations will grow and spread at somewhere near its theoretical intrinsic rate of increase. The cases of introduced species in modern times (the starling in North America, for example) demonstrate the extreme rapidity in which a species may spread across large geographic extents. (C) PE explains the relative stasis of most species. A species may produce a few daughter species during its duration (3 above). Large interbreeding populations are unlikely to change much due to genetic homeostasis and gene flow from far-flung parts of the range (6 above) [Eldredge & Gould emphasize homeostatic mechanisms over gene flow.]. (D) PE asserts "species selection" as the way in which major adaptive trends proceed. Closely related species are often likely to overlap in niche space (5 above). Ecological processes may cause the displacement and possible extinction of certain species due to competition with other species. If adaptive change in large populations is largely inhibited (6 above), then each species represents a "hypothesis" that is "tested" in competition. This is one of the more controversial points in PE. (E) PE also makes a statement concerning the pattern of fossils found. This pattern has both geographic and stratigraphic components. If peripatric speciation is the mode of speciation, then the place where transitional fossils between a parent and daughter species will be found will be limited in geographic region (2 above). Because the time needed for transition from parent to daughter species is short compared to the total residence time of either, the stratigraphic extent of transitional fossil sequences will be very brief (4 above). 5. PE vs. Phyletic Gradualism Punctuated Equilibria could have been advanced simply upon the basis of features of geology, taphonomy, geography, and taxonomy. However, that is not how Eldredge and Gould chose to do it. Instead, they codified what they saw as an inaccurate and incorrect "picture" of the fossil record, labelled it as "phyletic gradualism", and demonstrated that their PE was to be preferred on several points. The essential features of "phyletic gradualism" are described by Eldredge and Gould. In this Darwinian perspective, paleontology formulated its picture for the origin of new taxa. This picture, though rarely articulated, is familiar to all of us. We refer lo it here as "phyletic gradualism" and identify the following as its tenets: - New species arise by the transformation of an ancestral population into its modified descendants. - The transformation is even and slow. - The transformation involves large numbers, usually the entire ancestral population. - The transformation occurs over all or a large part of the ancestral species' geographic range. These statements imply several consequences, two of which seem especially important to paleontologists: - Ideally, the fossil record for the origin of a new species should consist of a long sequence of continuous, insensibly graded intermediate forms linking ancestor and descendant. - Morphological breaks in a postulated phyletic sequence are due to imperfections in the geological record. [E&G 1972] While it is nice to have terminology by which obtuse opponents of a theory may be conveniently labelled, there is no actual point to even bringing up "phyletic gradualism" in establishing PE. Eldredge and Gould quoted from Darwin in their 1972 paper to establish their concept of phyletic gradualism. They claim that Darwin set the task of later workers to search out confirmation of phyletic gradualism. That view is, unfortunately, just so much hokum. Nothing can be effected, unless favourable variations occur, and variation itself is apparently always a very slow process. The process will often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection. I do not believe so. On the other hand, I do believe that natural selection will always act very slowly, often only at long intervals of time, and generally on only a very few of the inhabitants of the same region at the same time. I further believe, that this very slow, intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed. [Charles Darwin, Origin of Species 1st Edition 1859, p.153] The above quote from Darwin also shows that Darwin did not embrace three of the four antecedent conditions that Eldredge and Gould specified for phyletic gradualism, and the single one that Darwin did embrace is also a tenet of any theory of speciation. Some people may embrace phyletic gradualism, but it is incorrect to attribute the concept to Charles Darwin. The quote above is noteable on several points. The "free intercrossing" bit is easily recognizable as a forerunner of the concept of gene flow, though Darwin was probably concerned there with blending inheritance. Darwin makes explicit that there is no constancy of rate implied with the comment on intermittent action. Darwin also recognized that change would be more likely to occur in a sub-population. Whether Darwin meant by "of the same region" much the same thing as the modern concept of allopatric speciation is disputable. Darwin did think that a daughter species arose from a population of the parent species. So do punctuated equilibrists. Darwin did think that the transformation would be slow, but he did not think that it would be "even". Darwin did not think that the transformation would involve large numbers, and certainly not the entire parent population. Darwin did not think that the transformation would occur across the entire ancestral range. But on the view of all the species of a genus having descended from a single parent, though now distributed to the most remote points of the world, we ought to find, and I believe as a general rule we do find, that some at least of the species range very widely; for it is necessary that the unmodified parent should range widely, undergoing modification during its diffusion, and should place itself under diverse conditions favourable for the conversion of its offspring, firstly into new varieties and ultimately into new species. [Charles Darwin, Origin of Species 1st Edition 1859, p.391] It is difficult to extract meaning from the above without recognition that Darwin was well aware of the importance of geographical distribution in the production of new species. Only a small portion of the world has been geologically explored. Only organic beings of certain classes can be preserved in a fossil condition, at least in any great number. Widely ranging species vary most, and varieties are often at first local, -- both causes rendering the discovery of intermediate links less likely. Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they do spread, if discovered in a geological formation, they will appear as if suddenly created there, and will be simply classed as new species. [Charles Darwin, Origin of Species 1st Edition 1859, p.439] The above quote comes from the famous section on the imperfection of the goelogical record. However, Darwin makes it clear that geographic location makes a difference in the finding of intermediate forms. "Both causes" in the above could not make discovery of intermediate links less likely if Darwin expected the transformation of the entire parent population. Gould has said that history cannot be done on the basis of selective quotes and qualifying footnotes, but rather must be a matter of general tenor and understanding. However, I think the onus is upon Eldredge and Gould to demonstrate that phyletic gradualism actually exists outside of their description. There have been persons that have advanced positions that could more or less be termed "phyletic gradualism". A feature of Gould and Eldredge 1977 is the discussion of various purported examples of change in the phyletic gradualist mode. Of many examples, G&E only found one to meet their criteria for establishing phyletic gradualism. Most examples were disputed by G&E because the original work ignored the geographical dimension. Richard Dawkins has a chapter in "The Blind Watchmaker" which goes into some detail on how "phyletic gradualism" is in many ways a strawman of Eldredge and Gould's creation. It is a recommended read. --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] Visit IAGI Website: http://iagi.or.id IAGI-net Archive 1: http://www.mail-archive.com/iagi-net%40iagi.or.id/ IAGI-net Archive 2: http://groups.yahoo.com/group/iagi Komisi Sedimentologi (FOSI) : F. Hasan Sidi([EMAIL PROTECTED])-http://fosi.iagi.or.id Komisi SDM/Pendidikan : Edy Sunardi([EMAIL PROTECTED]) Komisi Karst : Hanang Samodra([EMAIL PROTECTED]) Komisi Sertifikasi : M. Suryowibowo([EMAIL PROTECTED]) Komisi OTODA : Ridwan Djamaluddin([EMAIL PROTECTED] atau [EMAIL PROTECTED]), Arif Zardi Dahlius([EMAIL PROTECTED]) Komisi Database Geologi : Aria A. Mulhadiono([EMAIL PROTECTED]) ---------------------------------------------------------------------
