When Does Speciation Occur?

A curious feature of evolution by natural selection is that it depends crucially on events that “almost never” happen. For instance, speciation, the process in which a new species is generated by wandering away from its parent species, is an exceedingly rare event, but each of the millions of species that have existed on this planet got its start with an event of speciation. Every birth in every lineage is a potential speciation event, but speciation almost never happens, not once in a million births. Mutation in DNA almost never happens—not once in a trillion copyings—but evolution depends on it. Moreover, the vast majority of mutations are either deleterious or neutral; a fortuitously “good” mutation almost never happens. But evolution depends on those almost rarest of rare events.

Consider an intuition pump about a remarkable possibility: At this time, so far as we know, there is just one species of hominid on the planet, Homo sapiens. But suppose that fifty years from now, all but a lucky handful of our descendants are wiped out by a virus that leaves only two groups of survivors: a thousand Inuit living on remote Cornwallis Island off Greenland, and a thousand Andamanese living in splendid isolation on islands in the middle of the Indian Ocean. These two populations have been isolated from each other for thousands of years and have developed distinct physiological differences in response to their very different environments, but we have no good reason to question the standard assumption that they are members of our species. Suppose these populations remain both geographically and reproductively isolated for another ten thousand years, eventually repopulating the globe with two species—as they learn when they finally encounter each other and discover they have no interest in mating with each other, and besides, their few inadvertent attempts at mating are fruitless, a standard mark of allopatric speciation, reproductive isolation produced over time by geographical isolation. They might wonder, When precisely did the speciation occur? It is probably the case that their last common ancestor lived more than thirty thousand years ago, but speciation didn’t occur then and there (and so far as we know, it hasn’t occurred yet), but it still might emerge in a few thousand years that speciation had occurred at some time before the two populations were reunited. Did speciation occur before the dawn of agriculture or after the creation of the Internet? There is no nonarbitrary answer we could comfortably defend. There must have been a last common ancestor, or concestor to use Dawkins’s (2004) term, living maybe thirty thousand years ago, and the offspring of this individual could eventually turn out in the fullness of time to have been the founders of two different species, but it still isn’t settled today, one way or the other, whether a speciation event started then.

Here we would have an event, a birth that could turn out to have played a pivotal role in human (and post-human) history, one that occurred in a precise time and place but didn’t acquire its special status until millennia of sequelae had fixed that role, which was never a foregone conclusion. All it would take to prevent that birth from ever having been a speciation event would be a single boatload (or planeload) of islanders making a journey that led to the “premature” reuniting of the branches. One can imagine that speciation actually occurred at some precise but unknowable moment in the interval between initial isolation of the lineages and the eventual demonstration of their status as two species, but how could such a tipping point be defined? Presumably as the earliest time at which, thanks to the accumulation of chromosomal divergence between the two lineages, if any cross-lineage matings had been attempted, they would have proved infertile. But surmises about such counterfactuals are of scant significance.

The building of the transcontinental railroad split the herds of American bison into reproductively isolated populations, but Buffalo Bill Cody and his colleagues nipped that potential speciation event in the bud in no time by pretty much wiping out all but one of the populations. Populations of conspecifics often get divided by environmental events into two (or more) isolated groups, which stay reproductively isolated from each other for a few generations, and almost always either these groups reunite again or one group goes extinct. So although such first steps to speciation must happen fairly often, they almost never lead to speciation, and when they do, it is an outcome that takes many hundreds of generations to establish. Absolutely nothing about the circumstances of the initial separation could tell you whether they were the beginnings of a speciation event, even if you had perfect physical knowledge of every molecule in the world at the time. The very concept of a species is a sorta concept. Domestic dogs, coyotes, and wolves are named as species, and yet coydogs and dog-wolf offspring are quite common, so “officially” maybe we should consider these as three mere varieties, not even subspecies, of Canidae. Generalizations about the possibility of hybridizing—creating offspring from parents of different species—are hard to establish, which is not surprising when you reflect that every individual of every species exhibits subtle differences from every other individual of that species. This does not particularly worry biologists; they have learned not to fret over definitions or essences, since the processes that create all the intermediate cases are well understood.