Ahem. Donkeys and horses have different numbers of chromosomes. They can interbreed, although the offspring are sterile. There are plenty of zebra-horse hybrids–it appears there is currently a fad for producing them. Just run a google search on the term “zorse” and you’ll find a dozen or two websites with zorses for sale.
Humans and Chimps have different numbers of chromosomes. I can’t remember whether humans or chimps have more, but basically some time in the last 5 million years since the last human-chimp common ancestor either a human chromosome fused/fissioned, or a chimp chromosome fused/fissioned. But the interesting thing is, the genes themselves are pretty much all the same…so the chromosomes can line up together, albeit with a break in one pair.
This is why matings can happen, but the offspring are typically sterile. However, there are other factors. For instance you can have translocations…chunks of a chromosome can break off and be glued to another chromosome, or put back on backwards. The more translocations and fissions and fusions you have the less likely it is that a viable zygote can be formed.
It isn’t easy to tell in advance if two related species can interbreed. Recently there was a successful hybridization of a dromedary camel and a llama. Other times externally identical creatures cannot mate. There are many examples of insects that appear identical belonging to separate biological species. So there really is no way of telling if human chimp breeding would be successful or not. You’d have to carry out the experiment…which most scientists would be unwilling to do–if only to avoid being tarred with the “Stop playing God!” brush.
And then you have to add in pre-zygotic isolating mechanisms. For instance, closely related species could have different breeding seasons, or inhabit different ranges. Or they could have different secondary sexual characteristics. If you look at South American monkeys you often find closely related species living near each other. These closely related species will often be very similar in size, shape and gross anatomy but have wildly different mustaches, fur crests, swellings, markings, etc. Perhaps Homo sapiens sapiens body hairlessness and long head hair is a remnant of a similar situation.
So…when we get to the classification of humans and apes, it is becoming more and more clear that separating humans into Hominidae and putting chimps, bonobos, gorillas, and orangutans into Pongidae is indefensible. Any natural group that includes both chimps and gorillas must also include humans, since chimps are more closely related to humans than they are to gorillas. And we can’t lump gorillas in with orangutans, since they are more like people than they are like orangs. So we could include chimps and bonobos and gorillas into Hominidae, and orangs into Pongidae. Or we could put gorillas into Gorillidae by themselves. Or we could toss orangs into Hominidae too.
We could reclassify chimps and bonobos from Pan troglodytes and Pan paniscus to Homo troglodytes and Homo paniscus. I wouldn’t neccesarily go that far…but thinking this way makes it hard to justify the various pre-human genera of Australopithecus, Ardipithecus, etc. Now, if we had extant examples we could do some breeding experiments, examine the secondary sexual display characteristics, and get some pretty good answers. But since all we have are scraps of bone it is very much a matter of opinion whether Lucy et al deserve their own genera.
If they’re two different species, then they’ll no longer be able to interbreed.
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