[I tried sending a version of this note earlier today, but it didn't go through; I apologize if the earlier version re-emerges from the ether.]
The question is not whether any hybridization at all is occurring, but whether hybridization is a plausible explanation for a major change in the average appearance of an entire taxon. >To me it makes no more sense to dismiss hybridization amongst those two >species as a mechanism for primary darkening than it does to invoke it. The hypothesis should not be dismissed, but nor should it be invoked uncritically. Instead, let’s do our best to evaluate it on its merits. Despite all the deficits in our knowledge, I believe that the hybridization hypothesis can be challenged on two fronts: (a) the mechanism’s theoretical ability to produce the effect in question (i.e., a major increase in primary pigmentation across an entire taxon); and (b) the extent to which we are observing the corollary consequences that would be expected if this mechanism were operating at a scale sufficient for its efficacy in (a). Regarding (a), although gene flow is one of four mechanisms that can alter gene frequencies in a population (the other three are mutation, selection, and random drift), it is not by itself expected to cause huge changes--for instance to cause a trait that was previously common to become rare. Much more typically, it might introduce a new allele into a population (i.e., raise this allele’s frequency from near zero to slightly above zero), thereby potentially allowing natural selection to gain traction if the allele proved favorable in the new population. Gene flow could easily add some dark-winged birds to a white-winged population, thereby causing the white-winged trait to drop from, say, 100% to 99%; but in the absence of selection, astonishingly high rates of hybridization would be required to transform a whole taxon from >90% white-winged to >90% gray-winged—rates that are not only improbable in themselves, but which would also produce many other observable consequences. > A Western birder might find the idea that two similar gull species *not* > hybridizing to be alien to their experience, for example. Consider any example of hybridization that you like. One thing that you will notice is that wherever the hybrids are at all common, at least one (and usually both) parental types will also be common. Wherever one finds lots of Mallard x Black Duck hybrids, one finds hordes of Mallards and Black Ducks. Even at the epicenter of Glaucous-winged x Western Gull hybridization, Glaucous-winged Gulls are abundant in migration. This brings us to point (b). If hybridization with Thayer’s Gulls were occurring on the massive scale necessary for flipping a trait value in North American Iceland Gulls, one would expect to see a lot of Thayer’s Gulls wherever the hybrids (i.e., Kumlien’s Gulls, under this view) were common. This is manifestly not the case along the New England and Long Island coast, where the frequency of convincingly Thayer’s-like birds is so close to zero that it is actually shocking—one would think we’d see more than we do strictly via vagrancy, even if there were zero hybridization going on. Heavily pigmented Iceland Gulls are not in any way anomalous. They are the norm in coastal northeastern North America, and their increasing occurrence in Europe may be due to vagrancy, or it may be due in part to gene flow and natural selection acting on Greenland-breeding populations. What reason is there to doubt that, by and large, these pigmented birds return north and breed with each other, that their offspring are also pigmented, and that they all continue to live and act (and sound—thanks, Pete) like Iceland Gulls? The alternative is to posit that, upon returning north, the palest ones are systematically knocked up by randy, vagrating Thayer’s Gulls that, despite their depraved penchant for miscegenation, still assiduously conform to traditional Thayer’s Gull migratory routes, eschewing the wintering areas of their bewildered mates and progeny! Shai Mitra Bay Shore ________________________________ The Campaign for CSI: For College and Community<http://www.csi.cuny.edu/foundation/> -- NYSbirds-L List Info: http://www.NortheastBirding.com/NYSbirdsWELCOME http://www.NortheastBirding.com/NYSbirdsRULES http://www.NortheastBirding.com/NYSbirdsSubscribeConfigurationLeave.htm ARCHIVES: 1) http://www.mail-archive.com/nysbirds-l@cornell.edu/maillist.html 2) http://www.surfbirds.com/birdingmail/Group/NYSBirds-L 3) http://birdingonthe.net/mailinglists/NYSB.html Please submit your observations to eBird: http://ebird.org/content/ebird/ --
