Intuition Pumps and Other Tools for Thinking by Daniel C. Dennett

Author:Daniel C. Dennett [Dennett, Daniel C.]
Language: eng
Format: epub
ISBN: 9780141970127
Publisher: Penguin Books Ltd
Published: 2013-04-30T00:00:00+00:00

44. 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

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