Victoria Schlesinger in Aeon (Marbled Salamander. Photo by Michel Gunther/Biosphoto/Corbis):
Before there is a species, there’s a muddled period of innumerable changes as a group of individuals diverges, gene by gene, from their ancestors into a new species. The point at which those tiny changes add up to a separate species has been debated since the days of Aristotle. Further complicating matters, our basic litmus test for delineating species – viable offspring – is shaky at best. We know that when grizzly bears and polar bears mate, or coyote and wolf for that matter, the two species produce hybrid young – a combination individual that reflects some of the traits of each parent. It’s no wonder that roughly 26 concepts compete for the definition of species. Species are not so much a set of fixed traits but a temporary collection of them along a fluid continuum. The field guides belie variety within a species because it is so copious and ever-changing that you couldn’t get it on paper if you wanted to.
Scientists have long recognised the incredible diversity within a species. But they thought it reflected evolutionary changes that unfolded imperceptibly, over millions of years. That divergence between populations within a species was enforced, according to Ernst Mayr, the great evolutionary biologist of the 1940s, when a population was separated from the rest of the species by a mountain range or a desert, preventing breeding across the divide over geologic scales of time. Without the separation, gene flow was relentless. But as the separation persisted, the isolated population grew apart and speciation occurred.
In the mid-1960s, the biologist Paul Ehrlich – author of The Population Bomb (1968) – and his Stanford University colleague Peter Raven challenged Mayr’s ideas about speciation. They had studied checkerspot butterflies living in the Jasper Ridge Biological Preserve in California, and it soon became clear that they were not examining a single population. Through years of capturing, marking and then recapturing the butterflies, they were able to prove that within the population, spread over just 50 acres of suitable checkerspot habitat, there were three groups that rarely interacted despite their very close proximity.
Among other ideas, Ehrlich and Raven argued in a now classic paper from 1969 that gene flow was not as predictable and ubiquitous as Mayr and his cohort maintained, and thus evolutionary divergence between neighbouring groups in a population was probably common.