Evolutionary Stasis and the Uneven Tempo of Life

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

One of the great puzzles Mayr addresses is evolutionary stasis: why do some species appear to change very little for long periods? Living fossils, long-lived morphologies, and stable fossil forms all suggest that evolution is not a constant-motion machine.

Mayr argues that evolutionary rates vary dramatically and that this variation is linked to population structure. In particular, he repeatedly emphasizes that, other things being equal, the rate of evolution is inversely correlated with population size. Small isolated populations can evolve rapidly. Large widespread populations may be evolutionarily inert.

He writes: “rate of speciation is inversely correlated with population size.” This helps explain why widespread, populous species are often the ones paleontologists encounter in the fossil record. They are abundant, fossilizable, and visible. They are also expected to show the least evolutionary change.

This creates another observational bias. The fossil record overrepresents the very species least likely to show rapid evolutionary change and underrepresents small peripheral populations where new species may originate.

Mayr resists an overly rigid version of punctuated equilibria in which all established species become static after their origin. Instead, he proposes a full spectrum of rates: extremely rapid change in some peripatric speciation events, slow continuing change in some species, and near-total stasis in widespread populous forms.

This spectrum is important. Mayr is not replacing one dogma with another. He is arguing for pluralism. Evolutionary tempo depends on population size, isolation, ecology, genetic cohesion, and history.

His explanation of stasis is also holistic. The atomistic view might say that stabilizing selection simply removes mutations, keeping the genotype essentially unchanged. Mayr’s preferred view is different. The phenotype may remain stable even while the genotype turns over, because the cohesion of the genotype compensates internally. In this view, stasis is not genetic paralysis. It is dynamic stability.

This is a striking idea. A species can look still from the outside while molecular and genetic change continues within. The organismal form persists because internal developmental and genetic systems buffer change.

Key quote: “If the Limulus or Triops of today is morphologically almost indistinguishable from their ancestors of 100 or 200 million years ago, this does not mean that they still have the same genotype.”

Takeaway: Evolution does not tick at one speed. Some lineages sprint, some drift, some hold form while changing internally. For Mayr, population structure helps explain this uneven tempo.