I was watching a video of a Richard Dawkins lecture given on a cruise to the Galapagos Islands, and he described the first irreversible step to speciation, where two separated populations of the same species of animal or plant become two distinct species. I found his explanation of this quite enlightening since it also explained the reason why mules cannot reproduce.
Apparently, if two populations begin to diverge, there comes a point where they are no longer fully genetically compatible, but if individuals from the two groups are brought together, they are similar enough that they will try to mate–the individuals are not so different as to be sexually unappealing, yet. There will be offspring. But those offspring themselves won’t be fertile. This is the basic criterion for deciding that you now have two species instead of one.
Okay, here’s the greatly simplified explanation.
“Ordinary” Cell Division
You probably remember from high school biology that (almost) every cell of your body has 46 chromosomes, grouped in 23 pairs. It turns out that one chromosome of each pair came from your mother, and one from your father. When these cells divide, each chromosome is duplicated, and one copy goes into each half of the cell before division. This is called mitosis. Almost every cell in your body is the result of this process. For it to work, the two chromosomes of (say) pair #7 don’t even really have to match each other; the cell simply indiscriminately duplicates all the chromosomes it has. This is the process by which a fertilized egg develops into an adult.
It’s possible for two not terribly dissimilar creatures to create a fertilized egg, in fact, if one is a donkey and the other a horse, you get a mule. (And no Ezekiel 23:20 jokes here, please! Well, OK if you must.) There is no problem creating a viable embryo with one of each pair of chromosomes coming from a donkey and the other from a horse; the fertilized egg will divide repeatedly as they are wont to do, and develop into a mule.
Sex Cell Creation
But something quite different, more involved, and more finicky happens during the creation of sex cells, the sperm and ovum, called meiosis. Greatly oversimplified, what happens is that the matching pairs of chromosomes, one from the father and one from the mother, come together, and they actually swap pieces with each other. So now the two chromosomes in (say) pair #7 could swap just their central parts, so one chromosome ends up with the father’s DNA on the ends and the mother’s DNA in the middle, while the other is the opposite, with the mother’s DNA on the ends and the father’s DNA in the middle. Now from this cell with the scrambled DNA, two sperm (or ovum) cells are created, and each gets one chromosome from this newly-scrambled pair, since the sperm and ovum each will contribute half the genetic material of the eventual offspring. The net result of this is that the father’s contribution to the the offspring is a thorough mixture of his parents’ DNA; likewise for the mother.
Sex Cell Creation Fails in Mules
But as I said earlier, this is a very finicky process. If the two chromosomes of pair 7 are different enough from each other, the process doesn’t complete, and no sperm (or egg) cells are created. In the case of a horse, or a donkey, this is not a problem. In the case of a mule, one chromosome came from a horse, and one came from a donkey, and although they are similar enough that a living, breathing mule could develop from them, they are not similar enough for the chromosomes in a pair to find each other and exchange the material–so the male mule develops no sperm cells, and the female mule develops no egg cells. They are sterile.
Implications for Speciation
Once two populations diverge enough that their offspring are sterile, they won’t be rejoining each other, even if they interbreed a lot and have a lot of offspring. In fact, at that point, it’s evolutionarily bad for them to do so, and I imagine selection pressure will positively drive the two new species further apart in appearance, to the point where they show less and less interest in each other as mates. The individuals that look distinctly unattractive to the other species, and themselves find the other species unattractive, are more likely to have offspring that will pass their genes on.
So an admittedly very science-for-senators level of understanding of cell reproduction explains the curious phenomenon of being able to mate with another species, resulting in a mule and no grandchildren. I found the explanation very illuminating and hence interesting; hopefully you did too. And if you didn’t find it interesting, well, I can quote Richard Dawkins (who in turn was quoting an unknown editor of New Scientist): “Science is interesting, and if you don’t agree, you can fuck off.“