Abstract
I consider a variety of contact zones between divergent species or subspecies that have come into contact since the last period of ice age glaciations, with my goal being to reconstruct a typical speciation scenario for boreal forest birds. Morphologically distinct groups often hybridize in narrow zones, many of which appear to be stable tension zones, in which selection against hybrids is balanced by gene flow into the zone. To reach full reproductive isolation requires millions of years of divergence, an order of magnitude longer than each glacial cycle. Hence hybridization between diverging groups is likely common during divergence into full species, and need not prevent further divergence. I summarize several case studies of the dynamics that can occur during avian speciation, including ring speciation in greenish warblers, selection against intermediate seasonal migration in Swainson’s thrushes, and a possible case of hybrid speciation in yellow-rumped warblers.
Darren E. Irwin
Acting Director, Beaty Biodiversity Museum
Professor, Department of Zoology, and Biodiversity Research Centre University of British Columbia, Vancouver, BC, Canada