Experimental biology … may reveal what happens to a hundred rats in the course of ten years under fixed and simple conditions, but not what happened to a billion rats in the course of ten million years under the fluctuating conditions of earth history. Obviously the latter problem is more important.
—George Gaylord Simpson, 1944, Tempo and Mode in Evolution
Last Sunday, Feb. 12, we celebrated the 203rd birthday of two of the most important figures in world history, Abraham Lincoln—and Charles Darwin. To mark the occasion properly, I spent part of my weekend visiting the Creation Museum in Santee, California, with Carrie Poppy and Ross Blocher of the podcast “Oh no, Ross and Carrie!” (more on that trip in my March 7 post). But I thought I’d mark this anniversary with a discussion of another important anniversary in the history of evolutionary science.
It was 40 years ago this year that the most frequently cited paper in the history of paleontology was published. That was none other than the legendary 1972 article by Niles Eldredge and Stephen Jay Gould which proposed the “punctuated equilibrium” hypothesis. (Full disclosure: I took seminars from Niles while I was a student at the American Museum of Natural History, and Steve Gould was very interested in and supportive of my research even though I was not his student in a formal sense). At the time the paper came out, the dominant concept about speciation was the allopatric speciation model. In a nutshell, good biological evidence showed that new species arise not in the large mainland populations (with their extensive gene mixing) but in small isolated populations with unusual gene frequencies (peripheral isolates), usually living separate (allopatric) from the mainland population. Once these allopatric populations were no longer isolated but remixed with the mainland population, they would be genetically and behaviorally distinct from their parent species. Thus, they would be no longer capable of interbreeding, which is part of the definition of a biological species.
Even though the allopatric speciation model was accepted by biologists as early as 1942, it took paleontologists 30 years to recognize its implications. In their historic 1972 paper, Niles and Steve pointed out that if you took Ernst Mayr’s allopatric speciation model seriously, it would predict that species should arise in a normal biological time frame: a few years to a few hundred years at most. That’s a geologic instant, the difference between one bedding plane and the next in strata that span millions of years. The allopatric speciation model also predicted that species should arise in small, peripherally isolated areas, so they were unlikely to be fossilized in the few places for which we have a good fossil record. Rather than slow gradual change through millions of years of strata (the “phyletic gradualism” model), the allopatric speciation model accepted by biologists should give a fossil record where species seem to appear suddenly without any gradual transition preserved (“punctuation”), and then persist for long periods of time without change (“equilibrium”). Continue reading…comments (11)