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Message: <blockquote data-quote="Rasmus Boegh" data-source="post: 1468879" data-attributes="member: 1146">Of note that there are no fixed rules for genera, and as such the claim of how much a genus should/shouldn't include is to some extend arbitrary.It is a bit more complex. First, the vast majority of subspecies that are recognized are based on observable differences between populations. Indeed, while there are cases where genetic differences have been confirmed between different populations, but there are no other known differences (morphology, voice, etc.) between them, the general standard has been to maintain them in a single taxon. When using the BSC, a clade doesn't necessarily equal a taxon and vice versa (though some have argued for modified BSC where these are more equal). Strictly speaking this is only the case if using e.g. PSC. Secondly, lack of gene flow: In species, yes. In subspecies, no. Otherwise, for example the Australian population of the Varied Sitella would fall into a single subspecies, as there are large zones of intergradation where the currently recognized subspecies come into contact, i.e. there is geneflow. This is why the idea of considering them as separate BSC species was abandoned in the first place. Numerous subspecies come into contact, and if there was little or no intergradation, they would be species rather than subspecies (per BSC), though one can discuss how much gene flow is needed for two taxa to be separate spp. rather than ssp's (as in e.g. Corvus cornix/corone). So, if limiting it to allopatric speciation (which of course also applies for ssp's), then the isolation only has to have been there at some historical point, but when including parapatric and - especially - sympatric speciation, it becomes far more complex (as briefly noted in earlier posts). Finally, specifically regarding the case discussed here, the current absolute free movements between human populations is very recent. Depending on how you view it, it goes back perhaps a few hundred years, and that figure is so small that it is of little - if any - importance from an evolutionary point of view in a species with a generation length as humans. What genetics have confirmed is that 1) The spread of humans is relatively recent (this brings in possible issues of punctuated equilibrium, but that's for another thread), and the figure is generally too low for subspeciation to have had the chance of taking place. 2) While it would have been entirely sensible to believe that certain human populations have been very isolated (e.g. Americas versus remaining populations), geneflow appears to largely have persisted. Anyhow, the above issues aside, the result is the same; no human subspecies.</blockquote>

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