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[–] jxfaith ago  (edited ago)

Breed is more accurate than species here. Best definition I can come up with for breed is a set of geographically concentrated traits, often distinguishable by visual qualities. Think dogs: coloration, face shape, hair color, stature.

The major scientific requirement for speciation is sexual isolation: distinct species cannot reproduce between their populations. Highly similar populations can often reproduce but yield sterile hybrids. Considering the viability of mixed-race offspring among humans, it's inappropriate to consider other races as different species.

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[–] ditch-digger 0 points 2 points (+2|-0) ago  (edited ago)

Incorrect.

http://rafonda.com/interbreeding_between_species.html

The biological species concept was developed by Ernst Mayr, in 1942. Here it is, as first formulated, and quoted in Douglas J. Futuyma's EVOLUTIONARY BIOLOGY (1998): "Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups". The "reproductive isolation" can be genetic (non-fertility), geographic, or behavioral; there is NO criteria that says (as is commonly believed) that if two populations can interbreed they are the SAME species. There is NO criteria that says that two distinct species CAN'T interbreed. Consider the example of wolves, coyotes and dogs: three distinct species that can interbreed. In fact, all species of the genus Canis can mate and produce fertile offspring (Wayne et al., 1997, re: A. P. Gray, Mammalian Hybrids). This is so common, that biologists actually use a different formulation of Mayr's definition: they say, "If two populations can NOT interbreed, they are NOT the same species." That is a very different statement. Note that this is an empirical definition, and gives no guidance in regard to extinct taxons, but the bottom line is: nothing in the biological species concept contradicts the idea that erectus and sapiens could and DID interbreed

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[–] jxfaith ago  (edited ago)

How is

"If two populations can NOT interbreed, they are NOT the same species."

Contradicted by

distinct species cannot reproduce between their populations

I am aware of wolves, coyotes, etc, crossbreeding. I would consider that broadly the biological exception to the rule. My statement, while perhaps not as logically formal as the textbook definition you provided, works sufficiently in almost every case. When last this topic came up (as it seems to be a rather popular pet argument for some people on this site), I wasn't able to dig up any other examples of extant species interbreeding. At least none as well-documented as coyotes/wolves/dogs. And even so, such hybridization is an exception rather than a norm. A modest percentage of individuals per generation, to my understanding. Certainly on a different level than interracial marriage among humans.

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[–] kittysaysmeownow ago 

explain the Liger then.