Creatures That Can Almost-But-Not-Quite Breed

If two creatures try to mate, there are lot of ways for things to go wrong. For example, the egg may not fertilize at all. An embryo may spontaneously miscarry. And away down at the end of the spectrum, there is the possibility that the offspring will live and grow, but be infertile.

That is the case for horses and donkeys. The result of such matings is a mule, and mules are almost always sterile.

If a human-chimp hybrid were to exist, it would be sterile like a mule, and for the same reason. The genetic differences between humans and chimps are minor, but they include nine inversion mutations and a translocation mutation. Each mule cell has a set of horse chromosomes, and a corresponding set of donkey chromosomes. Egg and sperm cells are made by a process called meiosis. In meiosis, each chromosome must pair - lie side by side - with its counterpart so that corresponding genes can match up, one to one. But if a piece of donkey chromosome is inverted relative to its counterpart in horses, then gene-by-gene pairing cannot occur without elaborate looping and twisting. The chance of a successful cell division is low. So, the mule cannot make egg or sperm cells.

When a creature grows, the cells split by a different process, mitosis. Mitosis doesn't have to match things up: it only has to make copies. So, inversions and translocations don't prevent the mule from growing up to be an adult.

I'm not sure how Creationists or Intelligent-Design'ers explain this situation. (I don't know a purpose for an inversion mutation.) The Theory of Evolution explains it by saying that a horses and donkeys were both the same species, not that long ago.

Last modified: 2 April 1998

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