Speciation Is Not One Thing

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

Mayr next turns to a terminological problem: “speciation” has not always meant the same thing to everyone. For those working in the vertical tradition, speciation often meant phyletic transformation, one species gradually becoming another through time. For naturalists in the horizontal tradition, speciation meant the multiplication of species, the splitting of populations into separate evolutionary lineages.

This ambiguity, Mayr argues, has confused debates about punctuated equilibria and phyletic gradualism. Some defenders of gradualism were still imagining species transformation along a single line, while others were talking about the origin of daughter species through population splitting.

Mayr’s preferred definition is clear: speciation is the production of new daughter species. Once this is established, he surveys different proposed modes of speciation.

The most important, for Mayr, is allopatric speciation. He quotes his own earlier definition: “A new species develops if a population which has become geographically isolated from its parental species acquires during this period of isolation characters which promote or guarantee reproductive isolation when the external barriers break down.”

This definition is classic Mayr: geography first, reproductive isolation later. Species do not usually arise by magical internal splitting within a single continuous population. They arise because populations become spatially separated, diverge, and eventually become reproductively isolated.

Mayr then reviews nonallopatric models. Sympatric speciation, the origin of reproductive isolation within the dispersal area of a single deme, is treated skeptically. He acknowledges that it is conceivable but doubts its major importance. He is even more dismissive of stasipatric speciation, saying there is “no evidence whatsoever” for it. Parapatric speciation also fails, in his view, to produce convincing evidence, with many proposed cases better explained as secondary contact between previously isolated populations.

Still, Mayr makes an important point: for the debate about punctuated versus phyletic evolution, the exact mechanism may matter less than expected. Whether new species arise peripatrically, sympatrically, or by disruption, the fossil result could still look punctuational. New species can appear suddenly in the fossil record if they originated in small or localized populations.

This is a crucial conceptual move. Mayr is not merely defending one speciation mechanism. He is arguing that species multiplication changes the expected pattern of macroevolution. Once speciation is understood as branching rather than simple transformation, apparent discontinuity becomes less mysterious.

Key quote: “For those in the horizontal tradition it meant the multiplication of species, that is the establishment of separate populations that are incipient species.”

Takeaway: Mayr’s argument depends on defining speciation as branching, not just transformation. Once species originate as daughter lineages, macroevolution becomes a history of branching populations, not a single ladder climbing through time.