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Message: <blockquote data-quote="MJB" data-source="post: 1941555" data-attributes="member: 88928">What is a species?Russlac,Your starting point for trying to establish what a species (and as a corollary, subspecies) may be, has to be that there is no simple answer. I've done a fair amount of digging on this subject and have a draft article that will need much more expert advice and knowledge than I can summon, to get it into a final state! However, one of Darwin's principal conclusions was that species, not being fixed, evolve in response to evolutionary pressures. However, there is another aspect to ‘not being fixed’ whose implications may be less obvious, and that is that the rates of evolution of contemporaneous populations of related organisms may differ, even over a short period of time. One example will serve: one of the after-effects of post-glaciation periods is that returning populations whose ‘new’ breeding areas have become geographically separated could well experience differing sets of evolutionary pressures; these might also differ from any pre-glaciation sets. Therefore, classically-driven speciation would proceed at different rates, affecting the ability of individuals from different populations to interbreed in varying ways.Amongst birds, there are many examples of biology's tendency to be messy - in the Yellow Wagtail/Citrine Wagtail complex, the genetic isolation of some populations is variable across the breeding distribution. I think I'm right in saying that 'Black-headed Wagtail' hybridises infrequently with other 'subspecies' in the north and west of its distribution, but often, even to the extent of forming a cline, to the east and south. Here you see the difficulty in attempting any definition of subspecies that would fit, certainly covering many pages to cover every eventuality. The term 'species' has reasonably been called a 'convenient fiction'; it suits perhaps 95% of birds without any major difficulty.If you Google the subject 'species concepts', you will probably find reasonably researched arguments that will explain why present humans genetically can be regarded as a single species. Yet in relatively recent time, Neanderthal Man, another human species died out, the evidence suggesting that for quite a period communities existed in much of Europe of both Cro-Magnon (us, essentially) and Neanderthal peoples, probably mostly amicably, despite the ferocious portrayals in 1920s and 1930s pulp fiction. Hope that helps, but it's an enormous subject that has been explored deeply in philosophical as well as scientific terms. Beware of the world of 'woo' that may attempt to portray speciation as revealed by 'crystal pyramids' or the sudden apperance of the word 'quantum' in a Web advert!MJB</blockquote>

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