Allopatric speciation occurs when a population is separated geographically. The gene flow between the subpopulations is severely restricted or does not occur at all. Over time genetic differences between the two subpopulations accumulate. If the two populations are separated long enough two genetically different populations arise. Simply put, allopatric speciation is the result of geographic isolation. When the barrier is removed, the two subpopulations may or may not be able to mate with each other depending on the length of separation.
The black headed grosbeak (Pheucticus melanocephalus), which I discussed recently, is an example of allopatric speciation. During the Pleistiocene glaciers separated avian populations. Following the retreat of the glaciers, the North American central plains continued to form a geographic barrier. In the case of the black headed grosbeak, a western resident, the rose-breasted grosbeak ( Pheucticus ludovicianus) evolved on the eastern side of the central plains, resulting in a closely related pair. The treeless plains originally maintained the geographic fragmentation because the habitat of both birds is large trees with a diverse understory.
Over time as cities began to build up throughout the Central Plains, the two grosbeak species were able to expand their territories because of the large ornamental trees planted along city streets and urban and suburban yards and gardens. Today where the ranges of the two grosbeak species overlap they are able to mate and form hybrids.
Over time it will be interesting to know if human activity (planting trees in the plains) altered or reversed the separation of the two grosbeak species – stopped the speciation “midstream” , so to say. Unfortunately this question will not be answered in my lifetime.
This female black headed grosbeak was photographed along Lower Hat Creek (Shasta County CA) last summer. It is not a winter resident in our area.