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Message: <blockquote data-quote="l_raty" data-source="post: 1194360" data-attributes="member: 24811">Hi Niels,I was mainly referring to the Panama contact zone. Historically, changes in the interpretation of what was going on in this contact zone played a major role in the treatment of the complex. What happened is about this:<ul> <li data-xf-list-type="ul">To quantify the variation, Payne defined an index, based on 9 voucher specimens (<a href="http://www.geocities.com/secaribbirds/butorid2.jpg" target="_blank">http://www.geocities.com/secaribbirds/butorid2.jpg</a>), that he thought showed a complete smooth gradation from the purest gray-necked (score 1) to the darkest purplish-brown-necked birds (score 9). Monroe & Browning claimed that the specimens representing scores 5 and 6 in this voucher series were young birds, the neck color of which bridged the two taxa artificially. Hayes found that all of the vouchers had attained adult neck coloration.</li> <li data-xf-list-type="ul">Using his index, Payne found that the population of central Panama showed an increased variability and a high proportion of intermediate birds; he concluded that the two taxa interbred there. Monroe & Browning claimed that this population was merely a bit higher on the index, but not more variable than South American populations, if North American migrants were excluded from Payne's sample; they suggested these birds were pure striata and no hybridization was occurring. Hayes re-analyzed the data, excluding specimens collected in winter to avoid contamination by northern migrants, and found that the increased variability was real; he concluded that the two taxa interbreed there.</li> </ul>So, as far as this contact zone goes, we are basically back to the findings of Payne, that resulted in the lump.Here is Hayes' (2002) discussion:(I think the role of species concepts is evident.)"Extensive hybridization between taxa often has been interpreted as evidence for lack of reproductive isolation, requiring that the taxa be considered conspecific according to the traditional biological species concept (BSC; Mayr 1970). However, the degree of hybridization must be considered, inasmuch as more than 10% of bird species retain the ability to interbreed and produce viable offspring with other closely related species, including non-sisters (Grant and Grant 1992). Johnson et al. (1999) proposed a new comprehensive biologic species concept (CBSC) applicable to birds: “An avian species is 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, represent an independent evolutionary trajectory, and demonstrate essential but not necessarily complete reproductive isolation from other such systems” (emphasis added). Assuming phenotype is correlated with genotype, the presence of “pure” phenotypes within a hybrid zone, even when hybridization is extensive, provides evidence of assortative mating; in this case, the two taxa demonstrate essential reproductive isolation and should be considered specifically distinct. In contrast, when all individuals within a hybrid zone are intermediate, free interbreeding is inferred and the two taxa should be regarded as conspecific. In New World Butorides, the seemingly continuous variation in neck color from purplish-brown in the north to gray in the south strongly implies polygenic control of the deposition of gray eumelanin and rufous phaeomelanin pigments in the distal barbules of neck feathers (Schodde et al. 1980). Each currently recognized subspecies appears to differ in the combination of alleles coding for neck coloration, with intermediate combinations occurring where the ranges of B. virescens and B. striatus meet. The presence of apparently pure B. virescens and B. striatus phenotypes within the contact zone suggests that although the two forms frequently hybridize, assortative mating does occur. However, the sample sizes of museum specimens are small, and it remains uncertain whether both forms actually breed within the hybrid zone. In Tobago, a large body of recently collected data clearly indicates the presence of mostly “pure” phenotypes in an apparent hybrid zone (neck scores range from 1 to 8; Hayes unpubl. data), providing further evidence that assortative mating occurs. Thus, the two forms appear to maintain essential reproductive isolation and should be regarded as specifically distinct. However, further colorimetric, morphological, behavioral, and genetic studies are needed to shed light on the extent of gene flow and the stability of the hybrid zone." Having re-checked the papers, there are indeed several points with which I do not really feel comfortable in Hayes' interpretations, but explaining in detail would be a bit long.Best,Laurent -</blockquote>

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