Why Paleontologists Missed Speciation

Source: Ernst Mayr, “Speciation and Macroevolution,” Evolution 36(6), 1982, pp. 1119-1132.

Mayr argues that paleontologists often studied macroevolution without adequately addressing the origin of the taxa whose transformations they traced. They could describe trends through time, but the origin of new species, and therefore the origin of higher diversity, remained under-theorized.

He takes George Gaylord Simpson’s Tempo and Mode in Evolution as an example. Simpson’s book was foundational for the evolutionary synthesis, yet Mayr notes that Simpson made “no reference to species or speciation” in that work. For Mayr, this omission is not a minor oversight. It is symptomatic of a larger problem: paleontology, by its nature, often sees vertical sequences better than geographic population structure.

The fossil record tends to preserve widespread, abundant forms. Small peripheral populations, precisely the kinds of populations that Mayr thinks matter most for speciation, are unlikely to fossilize. This creates an observational trap. Paleontologists see long intervals of relative stability and sudden appearances of new forms. Saltationists interpret this as evidence for large jumps. Gradualists blame the incompleteness of the fossil record. Mayr’s solution is subtler: the missing action often happened in small, isolated populations outside the main fossil spotlight.

Mayr also explains why paleontologists were not simply careless. Their data were often not fine-grained enough to track speciation. He quotes the idea that paleontological “data just aren’t sensitive enough to analyze evolutionary kinetics.” In other words, fossils are powerful, but not omniscient. They show patterns, not always the population processes that produced them.

This gap allowed saltationist arguments to reappear again and again. Opponents of gradualism pointed out that nature does not display smooth transitions between genera, families, and higher taxa. The fossil record often shows discontinuity. If population genetics and geographic speciation are supposed to explain macroevolution, where is the evidence?

Mayr accepts the challenge, but redirects it. If speciation is key to macroevolution, then the correct question is not simply whether fossil sequences show every intermediate. The question is how species originate, where they originate, and whether their origins are likely to be visible in fossils.

This prepares the ground for the rest of the article. Mayr will argue that most important speciation, especially speciation relevant to macroevolutionary novelty, often occurs in small, peripheral isolates. Such populations are exactly the ones least likely to be preserved. The fossil record’s apparent gaps may therefore reflect the geography and demography of speciation, not the failure of gradual evolutionary mechanisms.

Key quote: “If, as I have always claimed, speciation is the key to the solution of the problem of macroevolution, it is necessary to review recent developments in the theory of speciation.”

Takeaway: Mayr does not dismiss paleontology. He argues that fossil patterns must be interpreted through population systematics, because the fossil record often misses the small, local populations where new species arise.