Untrue. Reticulated, Masai, and Rothschild's giraffes have coexisted in Kenya for hundreds of thousands to millions of years and have not interbred.
From
Giraffes: Equals stick together
"At any rate, distance is not the limiting factor that prevents interbreeding among the four giraffe species. "Masai, Reticulated, and Northern Giraffes all occur in East Africa in adjacent regions; yet they almost never interbreed. This is additional evidence of the fact that they are independent species. It is possible that the giraffes' reproduction depends on the rainy season, whose timing differs from one habitat to another," explains Winter. To gain deeper insight into the resulting extremely rare occurrence of hybrids, the team plans to conduct a further study in conjunction with the Kenya Wildlife Service in the near future."
It still suggests that ultimately the giraffes interbreed so rarely here because though in close proximity spatially, they are still separated into non-connecting habitat pockets with different seasonal triggers that make timing their reproduction together uncommon - basically, saying they are not
actually living together in the exact same place all the time. I would be interested to see what would happen if you mixed some of the herds up more directly - but giraffes also seem to imprint, to an extent, on the spot pattern of their mothers (and peers in infancy) and this may influence adult mate choice too. I suppose this is enough to separate some related bird species into different species to some people (hooded/carrion crow for example) where the recognition of self is mostly learned rather than innate. I don't find it completely convincing by itself as a measure of species validity, but it is interesting to me because I did not know about these forms living close together. Thank you for tagging me to this thread again!
Re: ducks (also referenced in the giraffe thread), a majority of the numerous mallard hybrids are either less fertile than the parent species or behaviorally maladjusted in ways that reduce reproductive success - since few crosses after the F1 are found in many of these examples - and many more distant ones (like eider crosses) are almost surely sterile. These cannot all be classified together as a species unit even to me. I think because the ability to interbreed is the ancestry condition in any lineage, an
inability to do so is a more viable marker for whether two species are distinct anyway. But mallards do have viable multigenerational hybrid populations with some other species like both pacific and american black ducks. This is less well defined in my view.
Even if they may not truly be the same "species", fertile hybrids that can breed with parent lineages or with one another like the mallard or the ruddy x white-headed duck are not really rare, and must surely have occurred with some frequency over evolutionary history. They can have a real, viable position in the tree of life, but one which is difficult for us to categorize. I would still lean toward these species not being
truly distinct as different units, because they seem to cross very freely and the hybrids are highly fecund, but only if there is no loss of fertility in the hybrid descendants over time, and not if they can only serve to move genes back into one parent species or the other. The ruddy and white-headed duck are not each other's closest genetic relatives, suggesting (but not confirming) the same level of intermixing is possible between them and other
Oxyura (but maybe other species have lost the ancestry condition of being cross-fertile. Any like that would more concretely be distinct!
)
For another example, long-term loss in fertility without an outcross to one parent or another in grey wolf x coyote crosses are the main reason I agree they are not simply extreme clinal variations of one species (though they are still quite close, and hybrids viably introduce genes from one species into the other often in
Canis.) Without backcrossing, wolf x coyote crosses seem to lose some fertility in subsequent generations, or this was the conclusion of an old study I was aware of (but using inbred stock and domestic dogs in place of wild wolves, may not accurately represent the situation to modern scrutiny.)
