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Veteran Member
Join Date: Mar 2001
Location: Louisville, KY, USA
Posts: 1,840
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So as not to derail the Evidence for ID thread started by Bubba, I will respond to DavidH's comments on PE on this thread.
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Davidh: I
So if you maintain that the transition fossils are present then punctuated equilibrium cannot be correct. If gaps are present then "aparently" punctuated equilibrium is proved.
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Incorrect. A speciation event is punctuated or gradual depending entirely upon the tempo of the transition. It has nothing necessarily to do with the presence or abscence of 'transitional fossils,' since that has just as much to do with preservability of the organisms, their geographic range, and sedimentation rates in their environments, as it does with evolutionary tempo. The key is simply that the transition occurs quickly compared to the time-ranges of the ancestor and descendent forms. All other factors being equal, 'transitional fossils' are much less likely to be preserved or discovered if the transitions tend to happen quickly. But even a very fast transition can leave fossil evidence if the geologic conditions are right.
As a counterfactual to the claim that punctuated means absence of transitional fossils, there are examples of evolutionary transitions in marine microfossil lineages that are 'punctuated,' yet documented by plenty of transitional forms. Check out the following two papers for examples of transitions that occur in a punctuated manner temporally, yet are documented by 'transitional forms':
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Wei, K-Y., and Kennett, J.P., 1988. Phyletic gradualism and punctuated equilibrium in the late Neogene planktonic foraminiferal clade Globoconella. Paleobiology 14, pp. 345-363.
"Substantial geographic coverage in paleontological study is essential in testing evolutionary models of phyletic gradualism and punctuated equilibrium. We present a multivariate morphometric study of the late Neogene planktonic foraminiferal clade Globoconella using specimens from four Deep Sea Drilling Project sites (DSDP 284, 207A, 208, and 588) along a latitudinal traverse in the southwest Pacific" (p. 345).
"The gradual transformation of G. (G.) conomiozea terminalis (a form retaining a keel) into G. (G.) sphericomiozea (a form lacking a keel) occurred during an interval of about 0.2m.y., with all measured morphological variables showing continuous and steady changes. The evolution of the central populations follows the model of phyletic gradualism. In peripheral populations, the origin of the descendent species G. (G.) pliozea from the ancestor G. (G.) conomiozea terminalis occurred very rapidly within an interval of less than 0.01m.y. . . The evolution os the Globoconella clade shows both phyletic gradualism and puncuated equilibrium. These two 'alternative' evolutionary models complement each other rather than being mutually exclusive. Both models are indespensible towards providing a complete picture of the evolution of Globoconella" (p. 345).
"During the interval of geographic isolation, central populations gradually evolved into a new chronospecies, G. sphericomiozea. It took about 0.17m.y. (from 5.14 to 4.97Ma) to transform the whole ancestral population of the highly conical, keeled morphocline (G. conomiozea terminalis) into the descendent subglobular, non-keeled species, G. sphericomiozea. The evolution in the central populations follows the model of phyletic gradualism. In contrast, the peripheral populations rapidly gave rise to a new species, G. pliozea, a form resembling flattened members of the ancestral populations. The origination of G. pliozea is inferred to be an allopatric speciation occurring within an interval of less than 0.01m.y. Following speciation, the new species remained in morphological stasis for about 0.6m.y. . . The evolution of G.pliozea follows the model of punctuated equilibrium" (p. 361).
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Lazarus, D.B., 1986. Tempo and mode of morphologic evolution near the origin of the radiolarian lineage Pterocanium prismatium. Paleobiology 12, pp. 175-189.
Pterocanium prismatium is a fossil radiolarian lineage commonly found in equatorial sediment cores throughout the world ocean. It first appeared in the fossil record ~4ma, and became extinct 1.8ma. Previous examination of phylogenetic pattern in late Neogene Pterocanium (Lazarus et al. 1985) observed the origin of P. prismatium from P. charybdeum in severak sediment cores from the equatorial Pacific and Indian Oceans. The P. charybdeum lineage is extant today in tropical and subtropical waters, and extends back in time well into the Miocene. The origin of P. prismantium from P. charybdeum thus represents biological speciation, as it involved the splitting of one ancestral lineages into two descendent ones (p. 175).
Morphometric examination of cladogenesis and phyletic evolution in two late Neogene sister lineages of marine microfossils (Pterocanium prismatium and Pterocanium charybdeum, Radiolaria) from two equatorial Pacific sediment cores was undertaken to better understand the rate of cladogenesis and its relation to subsequent phyletic change. The origin of P. prismatium from P. charybdeum ~4ma ago has been estimated to take place over an interval of ~500,000 yr. Results show that the speciation event consists of two distinct phases. The firsst phase, cladogenesis, occurred relatively rapidly (on the order of 50,000 yr). A second phase of relatively rapid divergent phyletic evolution away from the common ancestral state followed in both descendent branches and continued for at least 500,000 yr after completion of the cladogenetic event. Net evolutionary rates over the next 2ma appear to be much slower. Individual characters change by as much as 2 population standard deviations during cladogenesis, and by a total of approximately 3 standard deviations over 2.5ma of phyletic evolution. Up to 5 population standard deviations of change during -50,000 yr of cladogenesis, and 7 additional standard deviations of phyletic change over 500,000 are observed in multivariate (discriminant function) indices of morphologic difference. The measured pattern does not appear to be either strictly 'punctuated' or strictly 'gradual,' but instead shows features of both hypothesis (p. 175).
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12-01-2002, 07:09 AM
