Views from Understanding Evolution: Parsimonious Explanations for Punctuated Patterns

Abstract

Antievolution groups have frequently pointed to the debate prompted by Niles Eldredge and Stephen Gould’s (1972) proposal of punctuated equilibria with triumph. “Look,” they’ve claimed, “biologists can’t even agree among themselves how evolution works. The theory must be failing.” In the article included in this issue, “Editor’s Corner: The Early “Evolution” of “Punctuated Equilibria”,” Niles Eldredge (2008) takes on that mischaracterization. He describes the roots of punctuated equilibria and reveals how it builds on (not tears down!) established evolutionary theory. The idea of punctuated equilibria was inspired by a mysterious pattern in the fossil record. Many fossil organisms have retained the same anatomy for millions of years, appearing to evolve in body form hardly at all—a phenomenon that Eldredge and Gould termed stasis. Then, suddenly, the organisms seem to have experienced a jolt of evolution, quickly evolving into related, but new, species (Fig. 1a,b). Paleontologists see this pattern in many (although far from all) fossil organisms—from coral-like marine bryozoans to large terrestrial mammals, like horses (Gould and Eldredge 1993). Such observations are at odds with the pattern we might typically imagine evolution generating: slow and steady change (Fig. 1c)—in which, for example, tracing fossils through rock layers that correspond to a slow cooling in Earth’s climate reveals the gradual evolution of traits suited to colder temperatures. This picture of slow evolution, which Eldredge and Gould termed phyletic gradualism, fits well with one of the keystones of evolutionary theory: natural selection, the process responsible for adapting populations to changes in their environments. It is easy to imagine natural selection, for example, slowly transforming a delicate tropical species into a hardy, well-insulated, coldtolerant species over many millions of years. In some cases, we do observe such gradualistic patterns in the fossil record, but in many others, we observe evolution in apparent fits and starts. If the evolutionary mechanism of natural selection helps explain apparently gradual evolution, what mechanism can help explain punctuated patterns? Do we need to throw out everything we know about how evolution works or search for an as-yet-undiscovered evolutionary process? No. The beauty of punctuated equilibria is that the idea explains the apparent jolts of evolution seen in the fossil record with evolutionary processes that we already know to be at work in the world: speciation, migration, natural selection, and genetic drift. Eldredge and Gould simply showed how these mechanisms could work together to produce punctuated patterns in fossils. To see exactly how their idea works, it will help to review two modes of speciation: allopatric and peripatric speciation.