Frontiers of Taxonomy: Why Are There Two Types of Streamertails?<\/strong><\/em><\/p>

Red-billed and Black-billed Streamertails share Jamaica, with Red-billed exclusively occupying most of the island except the easternmost 5% or so, where Black-billed predominates.\u00a0 They overlap to a limited extent, as small numbers of Red-billed occur within Black-billed\u2019s tiny range.\u00a0\u00a0They are obviously very similar to one another, but also readily distinguishable by sight\u2014at least in the bill color and general coloration of the males.\u00a0 Their vocalizations and mating displays also differ.\u00a0 More subtly, their bills differ in shape, as Red-billed\u2019s is somewhat broader than Black-billed\u2019s.\u00a0 Their overall measurements are close, but Red-billed averages slightly larger than Black-billed.<\/p>

In light of the clear differences between them and their persistence as distinct forms, it is peculiar that they are widely regarded as conspecific: the Streamertail (Trochilus polytmus<\/em>).\u00a0 As of August 2019, this remains the official classification of the pertinent taxonomic authority, the American Ornithological Society.\u00a0 It seems certain that the AOS will eventually recognize the two as separate species\u2014this does not appear to be a close case by contemporary standards.\u00a0 The AOS\u2019s anachronistic position is a vestige of \u201cLumping Fever,\u201d which overwhelmed taxonomists in the 1900s but has gone out of fashion and is unlikely to return as long as biologists continue to value biological diversity.<\/p>

Disagreement over the species status of Red-billed and Black-billed Streamertails raises a central issue in the delineation of species: the significance of interbreeding.\u00a0 The streamertails present an analytically clean example, almost as if they were a laboratory experiment designed to decompose this issue.\u00a0 They are extremely similar (similar enough to plausibly be one species), but also easy to tell apart (different enough to plausibly be two species).\u00a0 They are confined to the same island, where they occupy the same general habitats, but mostly in different locations, although no physical barrier separates them.<\/p>

On the surface, the streamertails appear to be sibling species, western and eastern counterparts, but this conclusion is complicated by the fact that they have been reported to interbreed at least occasionally.\u00a0 Based on these reports, ornithologists governed by the \u201cbiological species concept\u201d decided that the streamertails must be races of a single species.\u00a0 The \u201cbiological species concept\u201d reduces the definition of a \u201cspecies\u201d to a set of organisms capable of producing fertile offspring.\u00a0 This is among the classic criteria for determining what constitutes a species, but it is inadequate to describe the dynamic diversity of the living world.\u00a0 Taxonomists who reflexively apply the concept seek evidence of any interbreeding and prioritize it above all other factors\u2014though in cases such as the streamertails it becomes clear that this approach fails to comprehend the core issue of taxonomy: speciation.<\/p>

Rigid application of the \u201cbiological species concept\u201d to the streamertails shows how an exception can sometimes swallow the rule.\u00a0 Through reliance on a single factor, advocates of this approach ignore the main question: why are there two types of streamertails on Jamaica?\u00a0 Having coexisted in the same limited real estate for tens or hundreds of thousands of years while capable of interbreeding, why do they remain distinct?\u00a0 If they are the same species, then why have they not blended into a single gene pool?\u00a0 The answer must be either that they recognize one another as different so interbreeding is rare (i.e., assortative mating) or that their hybrids are disadvantaged and fail to propagate over many generations (i.e., natural selection), or both.\u00a0 Either response is tantamount to a description of speciation.<\/p>

Contrary to the \u201cbiological species\u201d analysis, occasional instances of interbreeding streamertails actually confirm their separateness by showing that the birds themselves experiment with their species limits.\u00a0 In other words, exceptional cases of hybridization prove that no merely circumstantial barrier prevents the two populations from merging, yet these populations have remained distinct through the millenia.\u00a0 So evidently the distinction between them is intrinsic and the inescapable conclusion is that they are separate species: Red-billed and Black-billed Streamertails.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t