Host-pathogen coevolution, secondary sympatry and species diversification

摘要

The build-up of species locally within a region by allopatric speciation depends on geographicallyseparated (allopatric) sister populations becoming reproductively incompatible followed by second-ary sympatry. Among birds, this has happened frequently in remote archipelagos, spectacular casesincluding the Darwin's finches (Geospizinae) and Hawaiian honeycreepers (Drepanidinae), butsimilar examples are lacking in archipelagos nearer to continental landmasses. Of the requiredsteps in the speciation cycle, achievement of secondary sympatry appears to be limiting in neararchipelagos and, by extension, in continental regions. Here, I suggest that secondary sympatrymight be prevented by apparent competition mediated through pathogens that are locally coevolvedwith one population of host and are pathogenic in sister populations. The absence of numerouspathogens in remote archipelagos might, therefore, allow sister populations to achieve secondarysympatry more readily and thereby accelerate diversification. By similar reasoning, species shouldaccumulate relatively slowly within continental regions. In this essay, I explore the assumptionsand some implications of this model for species diversification.