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Message: <blockquote data-quote="mckaybailey" data-source="post: 1951471" data-attributes="member: 85190">Yes, the Song Sparrow is a great example. The problem with defining subspecies within a species like the Song Sparrow is that, from a population genetics standpoint, there is massive amounts of gene flow across its range. The forces of gene flow, which works to homogenize populations, and selection, which works to differentiate populations, act against one another and this results in large stretches of the range where one character state gradually changes to another state. For example, from a dark color to a lighter color. We call these areas of gradual change "clines."This gradual change makes it impossible to draw an objective boundary between subspecies with clinal differences. To make matters worse, different characters can have different cline distributions. For example, following Gloger's rule, plumage color might change from areas of high precipitation to areas of low precipitation. This cline would follow precipitation patterns. However, body size, following Allen's rule, might change from north to south. This cline would follow latitude patterns. So even if we could agree on optimal subspecies boundaries using plumage color, there would be different optimal boundaries if we looked at body size or other characters.The arbitrary nature of these kinds of subspecies wouldn't be such a big deal except for the fact that non-systematists seem to take it for granted that they are real. For example, the US endangered species act protects subspecies. What makes the matter all the worse is that there are really distinct subspecies mixed in among all the arbitrary ones. For example, Myrtle and Audubon's Warbler have a sharp boundary that is concordant in many characters (e.g. plumage, genetics, song, etc). To a non-specialist looking at a list, on paper the subspecies of Yellow-rumped Warbler look just like the subspecies of Song Sparrow. In fact, they are very different entities; some can be talked about objectively and planned for in a conservation context, and some can't.So to answer your question. I don't think there is any real reason that we would want to call desert forms of the Song Sparrow subspecies and not do the same for House Sparrows. Some might argue that House Sparrows in America should be divided into multiple subspecies. I would argue, based on my reasoning above, that it would be better to not recognize the desert forms of either species as subspecies. We can still acknowledge that there is geographic variation within these species (much like we do House Sparrows now), but this kind of geographic variation simply isn't amenable to having a name stamped on it.Bailey</blockquote>

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