Amphibious Assault Against Gradualism
A State of the Salamander Address was printed in PNAS recently.1 An international group of scientists looked for evolutionary ancestry and “Global patterns of diversification in the history of modern amphibians.” It would seem Mr. Darwin has a bit of frog in his throat:
The fossil record of modern amphibians (frogs, salamanders, and caecilians) provides no evidence for major extinction or radiation episodes throughout most of the Mesozoic and early Tertiary. However, long-term gradual diversification is difficult to reconcile with the sensitivity of present-day amphibian faunas to rapid ecological changes and the incidence of similar environmental perturbations in the past that have been associated with high turnover rates in other land vertebrates. To provide a comprehensive overview of the history of amphibian diversification, we constructed a phylogenetic timetree based on a multigene data set of 3.75 kb for 171 species. Our analyses reveal several episodes of accelerated amphibian diversification, which do not fit models of gradual lineage accumulation. Global turning points in the phylogenetic and ecological diversification occurred after the end-Permian mass extinction and in the late Cretaceous…. Approximately 86% of modern frog species and >81% of salamander species descended from only five ancestral lineages that produced major radiations in the late Cretaceous and early Tertiary. This proportionally late accumulation of extant lineage diversity contrasts with the long evolutionary history of amphibians but is in line with the Tertiary increase in fossil abundance toward the present.
Note: “accelerated diversification” can be considered a synonym for “abrupt appearance” for all practical purposes. “Because of its incompleteness, the fossil record of amphibians sheds little light on the time and rate at which modern taxa attained their current diversity,” they said; “…the timing and intensity of important macroevolutionary trends are obscured by fossil scarcity.”
Molecular evidence, however, failed to rescue Darwinian gradualism. Their charts show no clear upward trend in diversity over time, but peaks and valleys and a sudden burst of diversification in the most recent epoch. “Our results, inferred from extant taxa,” they said in conclusion, “provide evidence for substantial fluctuations in the history of amphibian net diversification and reject the hypothesis of gradual lineage accumulation.” See also the report on New Scientist and its discussion of “exploding frogs.”
1Roelants et al, “Global patterns of diversification in the history of modern amphibians,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0608378104c, published online before print January 9, 2007.
OK, another research project has falsified Darwin’s prediction. Keep up the good work.