Room of One's Own

Natural History, Nov, 1997 by Stephen Jay Gould

The conventional theory for speciation--called allopatric, and meaning "living in another place"--holds that a population can only gain the potential to form a new species by becoming geographically isolated from the ancestral group, for only strict spatial separation can guarantee the necessary cutoff from contact with members of the parental population. Much research into the process of speciation has focused on the modes of attaining such geographic isolation--new islands rising in the sea, continents splitting, rivers changing their courses, and so on.

A contrasting idea--called sympatric, or "living in the same place"--holds that new groups may speciate while continuing to inhabit the same geographic domain as the parental population. The defense of sympatric speciation faces a classic conundrum, and most research on the subject has been dedicated to finding solutions: if isolation from members of the parental population is so crucial to the formation of a new species, how can a new species arise within the geographic range of the parents?

This old issue in evolutionary theory remains far from resolution, but we should note, in the context of this essay, that proposed mechanisms usually follow the Holy Sepulchre principle of graining the new group a room of its own within the spatial boundaries of the parental realm--and that such "internal isolation" may be achieved by either the spatial or the temporal route. The best-documented cases of the spatial strategy invoke a process that goes by the technical name of "host specificity," or the restriction of a population to a highly specific site within a general area. For example, to cite an actual (although still controversial) case, flies of the genus Rhagoletis tend to inhabit only one species of tree as an exclusive site for breeding and feeding. Suppose that some individuals within a species that lives on apple trees experience a mutation that leads them to favor hawthorns. A new population, tied exclusively to hawthorns, may soon arise and may evolve into a separate species. The hawthorn flies live within the same geographic region as the apple flies, but members of the two groups never interbreed because each recognizes only a portion of the total area as a permissible home--just as the six sects of the Holy Sepulchre never transgress into one another's territory.

The same principle may work temporally as well. Suppose that two closely related species of frogs live and reproduce in and around the same pond, but one species uses the day-lengthening cues of spring to initiate breeding, while the other waits for the day-shortening signals of fall. The two populations share the same space and may even (metaphorically) wave and wink at each other throughout the year, but they can never interbreed and can therefore remain separate as species.

In the second, and philosophically far more interesting, strategy for securing a requisite room of one's own, species may share the same region, but avoid the need for a natural equivalent of the Status Quo, because they do not perceive each other at all, and therefore cannot interfere or compete--blessedly benign ignorance rather than artfully negotiated separation. This fascinating form of imperception, which can also be achieved by either spatial or temporal routes, raises one of the most illuminating issues of intellectual life and nature's construction: the theme of scaling, or strikingly different ways of viewing the world--with no single way either universally "normal" or "better" than any other--from disparate vantage points of an observer's size or life span.