Bromeliad population genetics reveals species cohesion against the odds

A traditional view of the nature of species holds that populations within species maintain genetic cohesion through substantial intraspecific gene flow and that the whole genome of a species is protected from interspecific gene flow by strong reproductive isolation. Both aspects of this view have been challenged. Reproductive barriers between recently evolved species can be incomplete and permeable to gene flow, and in geographically structured environments, intraspecific gene flow may be limited. Whether species that combine these features can evolve as cohesive evolutionary units remains an open question. In this issue of Molecular Ecology, Palma-Silva et al. (2011) investigate this issue in an ideal system. They characterize gene flow within and between four sympatric species of Pitcairnia bromeliads on isolated rock outcrops, called inselbergs, in Brazil (Fig. 1). They show that despite very little intraspecific gene flow between inselbergs and substantial introgression, each species manages to maintain genetic integrity. Furthermore, certain regions of the genome appear to introgress more easily than others. This, taken together with previous studies of premating isolation in these species (Wendt et al. 2001, 2002), suggests that their reproductive barriers are strong but permeable. These data reinforce recent work suggesting that speciation must be thought of not as a whole-genome phenomenon but rather on a locus-by-locus basis, with neutral loci readily exchanged between species, but genes that contribute to reproductive isolation, so-called speciation genes, relatively unlikely to introgress (Wu 2001; Feder & Nosil 2010; Rieseberg & Blackman 2010; Nosil & Schluter 2011).