The Missing Bridge Between Species and Big Evolutionary Change

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

Ernst Mayr begins with a deceptively simple problem: species and higher taxa seem to occupy different levels of biological organization. A species is one kind of entity; a genus, family, order, or major evolutionary novelty seems to belong to a larger architectural scale. The central question is: how does evolution move from one level to the other?

Mayr frames this as an old problem, almost as old as evolutionary thinking itself. Darwin’s answer was gradualist. Given enough small changes, qualitative difference could emerge from quantitative accumulation. Mayr summarizes Darwin’s position as follows: “If one would simply pile enough small differences on top of each other, one would eventually get something that is qualitatively different.”

But Mayr immediately complicates the story. Darwin, he notes, was “virtually alone” among his contemporaries in insisting on gradualism. Many nineteenth-century evolutionists were impressed by the apparent gaps between higher taxa and therefore leaned toward saltation, the idea that large evolutionary jumps were needed to explain novelty. This tradition persisted through thinkers such as Bateson, de Vries, Goldschmidt, Willis, and Schindewolf.

Mayr’s key complaint is that even after gradualism triumphed in the evolutionary synthesis, it did so in a distorted form. The synthesis emphasized gradual change along lineages, what he calls the “vertical” tradition. This meant thinking about evolution as a line moving through time, accumulating adaptation or specialization. But Darwin’s other great contribution, the “horizontal” origin of diversity through species multiplication, was neglected.

Mayr writes that the Darwinian “‘horizontal’ tradition of an origin of diversity, that is of a multiplication of species, and the role of this diversification in macroevolution was totally ignored.” This is the engine room of the article. Mayr is not merely defending gradualism. He is arguing that macroevolution cannot be understood unless speciation is placed at the center.

For Mayr, geneticists and paleontologists often jumped directly from mutation or genetic variation to macroevolutionary outcomes. Naturalists, especially zoologists, were different. They inserted the species level into the explanation: genes affect populations, populations give rise to species, and species are the raw material from which higher taxa emerge.

This gives us Mayr’s basic structure:

Gene → Population → Species → Higher Taxon

The missing bridge is speciation. Without it, macroevolution looks either mysterious or saltational. With it, macroevolution becomes a consequence of population-level processes unfolding in space.

Key quote: “If we define evolution as changes in adaptation and diversity, then the students of adaptation deal with what we might call the vertical dimension of evolution, while the students of diversity deal with the horizontal dimension.”

Takeaway: Mayr’s central move is to shift the conversation from “How do lineages slowly change?” to “How does the multiplication of species create the conditions for major evolutionary change?”