Yes! This explains a lot of the reasoning and makes perfect sense to me. I can see this now with the gulls I study as well.
Basically it looks as though the Green and Striated Herons will remain split for the foreseeable future.
Ok - I will try to explain what issues I think have not been (and should be) addressed, and
could affect the reasoning. Sorry if it becomes really long... :eek!:
- The variability of the phenotype in pure populations of
striata and
virescens covers most if not all of the variation in the complex, thus almost any bird could arguably be called a "pure phenotype"; this makes arguments based on the frequency of these phenotypes in contact zones weak. If you look at the frequency distribution of the scores within
striata-type birds (1 to 4) on Tobago and Trinidad in Hayes (2006), it's easy to see that they are entirely different, with the score that dominates on Trinidad (1) almost absent on Tobago (statistically also, the difference is very highly significant - χ²=37.67, df=1, p=9E-10). Nothing is known about the inheritance of the phenotypes, hence we have no idea how a F1 hybrid looks. If Hayes' data are representative, it is far from unlikely that most of the "
striata" on Tobago are not at all "pure" in the same sense as those on Trinidad.
- There is no evidence in Hayes' (2006) data set that the frequency of
striata phenotypes has changed on Tobago. The frequencies of birds with scores 1-3 (pooled) in historical and modern samples do not differ significantly (χ²=0.29, df=1, p=0.59); neither do the frequencies of birds with scores 1-4 (pooled) (χ²=0.72, df=1, p=0.40). The main change visible in the data seems to be that, in the modern sample, birds with scores 6 and 7 appear at frequencies almost identical to those of the birds with scores 5 and 6, respectively, in the historical sample. This could imply that the brown-necked part of the population is shifting towards a browner-necked phenotype, but I see no clear implications for the gray birds. Hence, in any case, that "the frequency of pure phenotypes has increased on Tobago" sounds like an overstatement to me.
(More disturbingly, this could also simply result from phaeomelanin-based brown coloration fading out paler in old specimens... If this possibility is addressed somewhere, I missed it.)
- Assortative mating could be real, but remains a hypothesis, and is not the only possible explanation to the persistence of a local polymorphism; regular immigration of individuals differing from the dominant local phenotype could produce the same result. The local population on Tobago is dominated by brown-necked birds, and there is a potential source of gray-necked immigrants close by. If there is some level of assortative mating, it is certainly far from perfect, anyway, given the distribution of the scores on the island.
- Competitive exlusion is a hypothesis explaining the fact that the frequency of
striata phenotypes remains low on Tobago, that is built on the hypothesis of assortative mating; if the birds breed together, the frequency of
striata phenotypes on Tobago could remain low simply because they get systematically diluted in the local population, in which case competitive exclusion would not be necessary to explain what we see.
- "Essential reproductive isolation" is a rather vague term, which may be a problem in itself. But considering that imperfect assortative mating, as such, is evidence for essential reproductive isolation seems extreme to me. Assortative mating occurs at a lot of levels within species, including between morphs (in snow geese, for example, as a result of imprinting on the phenotype of the parents).
- Last, I actually have a different reading of the CBSC. My reading is that the CBSC allows to accept interbreeding taxa as full species
at the condition that they each appear to be "a system of populations representing an
essentially monophyletic,
genetically cohesive, and
genealogically concordant lineage of individuals that share a common fertilisation system through time and space, [and]
represent an independent evolutionary trajectory" (emphasis mine). IOW, essential (but not complete) reproductive isolation is a necessary
but not sufficient condition for species status. In the case of
Butorides spp., the other criteria are untested, and untestable with available data. Any evidence pointing to a significant disconnection between the phenotype and the phylogenetic/genealogical history of the populations would be quite problematic for species status under this concept, even in the presence of strong assortative mating.
Cheers,
L -
