Biologists are very comfortable with the idea that new species can be formed when geographic barriers put members of the same species in places with different selective pressures. This is called allopatry.
The December 18, 2009 issue of Science explores a different route to forming a new species that may be more common than originally believed. In sympatric speciation, members of the same species become adapted to different ecological niches of a geographically overlapping environment and mate assortively.
This is likely to happen where there is more than one survival strategy that works in the environment, but both are "extreme" approaches, while middle ground is selected against. This can be particularly powerful when fitness at one of the extremes is distinguished by "ornaments" that a member of a species can use to distinguish well adapted members of the opposite sex.
The analysis is highly relevant in marine environments, where there are many species but few truly isolated locations.
The even greater teaser is that the analysis could be relevant to continuing human evolution of socially, but not geographically isolated populations in an era where almost no one is geographically isolated in our new small world. Many studies, for example, have looked at the impact of millenia of Jewish community isolation and differential treatment relative to the general populations into which the Jewish diaspora spread on Jewish community population genetics.
No one is suggesting that there is more than one hominin species in existence at the moment on Earth. But, this kind of analysis is another context where anthropology is developing multiple different kinds of links to genetics. In particular, there is interest in whether neurodiversity may arise from, or be sustained by, this kind of sympatric population separation. In other words, perhaps human societies have different "ecological niches" for different neurological traits that are selected for with assortive marriage in some way, which may be interdependent with each other.