T86 Punctuated Equilibrium I: 50 Years Later

Monday, 10 October 2022
8:00 AM - 12:00 PM
Mile High Ballroom 4B

SIGNIFICANT PHYLETIC EVOLUTION IS VERY RARE: EVOLUTION HAS BEEN PREDOMINANTLY PUNCTUATIONAL

Paper number: T86-5
Program: Technical Sessions
Time: 9:05 AM – 9:20 AM

Authors

Abstract

Eldredge and Gould stated that the fossil record couldn’t reveal whether the punctuational model of evolution is valid. Disagreeing with this, in 1975 I published three tests pitting the gradualistic and punctuational models against each other. They all favored the punctuational model. The test of adaptive radiation showed that many families of mammals arose within ~12 my of the start of the Cenozoic, at a time when an average mammal species survived for about 2 million years. Stacking ~ 6 of these more-or-less unchanging species end-to-end could not possibly create a new family. Punctuational steps were necessary. And the average duration of these species has recently been extended to 3 my! The second test examined what has happened to “skinny” clades – ones that underwent few speciation events. It turns out that these all underwent little evolution. Their extant representatives are all living fossil taxa. These clades support the punctuational model, according to which little evolution occurs without speciation.

Beedle showed that quantum speciation produced a new sand dollar family with a unique morphology and mode of life in California about 6 my ago. It’s been shown that a single genetic change can alter development to produce a distinctive new morphology in a small population, and biologists have demonstrated the punctuational origins of unusual jellyfish species in the saltwater lakes of Palau by way of very few genetic changes.

Examples of phyletic evolution are extremely rare. Darwin was frustrated to find that a species always looked the same at the top of a formation as at the bottom. Many paleontologists have also observed this without comment. G. G. Simpson concluded that an average animal species has survived for 5 my, but he never noted the anti-gradualistic implication of this observation. Cheetham showed a clear punctuational pattern for a clade of Bryozoa, and Yang and I showed that 19 bivalve lineages exhibited almost no evolution over 4-17 my. Hallam documented phyletic evolution for Gryphaea but then showed that 329 other Jurassic bivalve species experienced evolutionary stasis except for size increase. Both Hunt and Hopkins and Lidgard found that for large samples of fossil lineages phyletic evolution was much rarer than stasis. If you look at all the facts, you’ll see that the punctuational model has won out.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379421

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