The the Eurasian carrion crow/hooded crow complex, the North American Orioles are examples of allopatric speciation that has reached some sort of equilibrium where interbreeding is still possible, but there are indications that barriers to hybridization are arising within the two populations. As such, they make interesting examples of speciation in progress and an illustrate of one mechanism for speciation.
I have written before about how and why the carrion crow, Corvus corone, and the hooded crow, C. cornix, are regarded as different species even though they can and do interbreed to produce fertile hybrids - normally a sign that the interbreeding populations are at best subspecies. The decision to classify them as different species was based on the evidence that there is a degree of female sex-selection which favours the hooded crow plumage pattern, so the hybrids tend to be reproductively less successful so any zone of interbreeding remains narrow and needs to be replenished. In other words, hybridization is not leading to significant gene flow between the two species. This sex-selection behaviour is a partial barrier to hybridization.
Just to recap on how speciation is maintained, especially in birds. Basically there are two forms of barrier to hybridization:
- Post-zygotic. Where any result of mating fails to produce a viable offspring. This can be, for example, where there are different numbers of chromosomes or other cytological incompatibilities which means the zygote fails to develop to full maturity, or the offspring is sterile.
- Pre-zygotic. Where fertilization of the egg fails to take place. These can be many and varied but, amongst birds, the commonest are the courtship and mating rituals, including plumage and song, that normally ensure mates of the right species are chosen.
However, their ranges overlap in the Great Plains, where they are known to interbreed along the river valleys that cross the plains. Here, they have a whole range of intermediate plumages and produce fertile offspring. Normally, when this happens between two related species that have evolved particular niche adaptations, a hybrid will lack the specialisation of either, so will be at a disadvantage in either part of the range. This acts as a strong selection pressure to evolve effective barriers, so driving complete speciation. However, as in the case of the orioles and the crows, there has not been much divergence in terms of habitat or food so not much specialisation.
Now, scientists from Cornell Laboratory of Ornithology have shown that the two species are remaining relatively free from genetic ingression from the other and the zone of hybridization has not only failed to increase, but has actually shrunk since it was first recognised in the 1950s. Natural selection pressures are acting against hybridisation in some way. This is unlike two other freely-hybridizine species - the blue-winged and golden-winged warblers - which appear to be re-merging back to a single species. This partial barrier, as with the Eurasian crows, is enough to justify both being given full species status.
A similar process is believed to be responsible for the many European finches which can and do interbreed in captivity but which rarely do in the wild due to their elaborate plumage and song which act as pre-zygotic barriers to hybridization. Once these barriers have been breached, there is little in the way of post-zygotic barriers because there has been no selection pressure to evolve them given the success of the pre-zygotic barriers.
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