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almightySapling t1_jbkhsfx wrote

Reply to comment by Tyrosine_Lannister in Is there a fertile creature with an odd number of chromosomes? by TheBloxyBloxGuy

Was there a period where sapiens and neanderthals couldn't interbreed? I guess what I'm trying to understand is what formally makes them different species in the first place.

Seems to me that "hybrids," as a concept, have less to do with biology and more to do with our arbitrary classification of it.

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Rather_Dashing t1_jcym0rf wrote

Species is just an arbitrary classification. Interbreeding is only one factor used to determine what is a species. Its thought that only female neanderthal human hybrids were fertile and not males, so that one justification for considering us seperate species. Just how likely an offspring is to be fertile could also be taken into account. If two species have to breed a million times to produce 1 fertile offspring, it doesnt mean the two are the same species, there is never going to be considerable gene flow between those two groups.

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