Host-pathogen coevolution, secondary sympatry and species diversification.

The build-up of species locally within a region by allopatric speciation depends on geographically separated (allopatric) sister populations becoming reproductively incompatible followed by secondary sympatry. Among birds, this has happened frequently in remote archipelagos, spectacular cases including the Darwin's finches (Geospizinae) and Hawaiian honeycreepers (Drepanidinae), but similar examples are lacking in archipelagos nearer to continental landmasses. Of the required steps in the speciation cycle, achievement of secondary sympatry appears to be limiting in near archipelagos and, by extension, in continental regions. Here, I suggest that secondary sympatry might be prevented by apparent competition mediated through pathogens that are locally coevolved with one population of host and are pathogenic in sister populations. The absence of numerous pathogens in remote archipelagos might, therefore, allow sister populations to achieve secondary sympatry more readily and thereby accelerate diversification. By similar reasoning, species should accumulate relatively slowly within continental regions. In this essay, I explore the assumptions and some implications of this model for species diversification.