Hybrid zones are often used as a window with which to gaze upon the evolutionary process (Barton and Hewitt 1989). With the advent of genomic tools, it is possible to detect the genomic signatures and the architecture underlying reproductive isolation. In the case of the black-capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadees, a narrow, but long zone of contact stretches from New Jersey to Kansas in which hybrids experience strong intrinsic selection.
Most studies investigating hybrid zones are snapshots of a particular time and place. Yet, as Taylor and colleagues (2014) highlight in their new Evolution article:
Stochastic processes (biological or from sampling) can, however, generate variation in genetic patterns among loci that can mimic forms of selection, including reproductive isolation and adaptive divergence.
This is especially relevant as stochastic processes can confound outlier approaches with which to detect non-neutral loci. However, utilizing different geographic and temporal samples, comprising different sets of admixed individuals, is one method whereby it is possible to determine whether patterns of differentiation and introgression are the result of selection or of stochastic variation.
In the case of these two chickadee species, the hybrid zone is moving northward at a rate closely matching natal dispersal distance. Taylor and colleagues found:
consistently low introgression for highly divergent loci [with some localized on the Z chromosome] between P. atricappillus and P. carolinensis in this moving hybrid zone. This is strong evidence that these loci may be linked to genomic regions involved in reproductive isolation between chickadees.
Reduced geographic introgression and higher differentiation of sex-linked loci may be driven by lower recombination rates or a higher proportion of infertility alleles on sex chromosomes. Undoubtedly, the genomic architecture of reproductive isolation between these two species is complex, but the sex chromosomes may be further along the speciation continuum than autosomes (see the comprehensive examination of genomic architecture in flycatchers by Ellegren et al. 2012).
Though fine-scale distribution within chromosomes was beyond the scope of this current study, it appears, nonetheless, that a small proportion of loci contribute to reproductive isolation. Do other spatiotemporal studies also find consistency of genomic architecture? Do relatively few loci contribute to reproductive isolation?
Barton, NH and GM Hewitt (1989) Adaptation, speciation and hybrid zones. Nature 341: 497-503. doi:10.1038/341497a0
Ellegren, HL, et al. (2012) The genomic landscape of species divergence in Ficedula flycatchers. Nature 491: 756-760. doi:10.1038/nature11584
Taylor, SA, RL Curry, TA White, V Ferrretti, I Lovette (2014) Spatiotemporally consistent genomic signatures of reproductive isolation in a moving hybrid zone. Evolution 68: 3066-3081. doi: 10.1111/evo.12510