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Breeding unicorns

Are there any limits to the production of hybrid animals?

Published: 19 January 2014 (GMT+10)

T.S. wrote in asking us to help him spot the flaw in his friend’s reasoning about hybrids:

Hi, a friend of mine told me of an interesting discussion he had today with some people at university. He apparently won an argument saying because donkeys and horses have a different number of chromosomes, yet you can cross breed them to create mules, then you must be able to cross breed any animal family, he gave the example of a narwal and horse creating a unicorn.

I argued that because they are from a different “family” they cant possibly create offspring, but could not find sufficient evidence to convince him.

Any help with this would be much appreciated.

CMI’s Keaton Halley responds:

Hi T.S.,

Unfortunately, your friend’s argument is logically invalid. (To understand validity, please read Logic and Creation, especially the sections on validity and soundness. Or you can purchase Dr. Sarfati’s lecture, Leaving Your Brains at the Church Door?) Here’s an argument with the same form, and the conclusion obviously doesn’t follow from the premises. Compare:

  1. Donkeys and horses have different numbers of chromosomes but can produce offspring.
  2. Horses and narwhals have different numbers of chromosomes.
  3. Therefore, horses and narwhals can produce offspring.
  1. Sparrows and turkeys have different sizes but can lay eggs.
  2. Hamsters and kangaroos have different sizes.
  3. Therefore, hamsters and kangaroos can lay eggs.
We can’t tell whether two animal groups can hybridize just by counting their chromosomes.

See, one similarity doesn’t mean they share everything in common. If donkeys and horses can breed, that only shows that having the same number of chromosomes is not a necessary condition for breeding. But it does not prove that having a different number of chromosomes is sufficient. It shows that some animals with mismatched chromosomes can hybridize, not that all can. Successful reproduction depends on a whole lot of complicated biological factors, so we can’t tell whether two animal groups can hybridize just by counting their chromosomes.

One of these factors is called ‘synteny’ (i.e., gene order). When the chromosomes pair up in a donkey/horse hybrid during cell division, each daughter cell gets the same genes, even if the number of chromosomes is not exactly the same. This does not work for things like dogs and cats because the genes are dispersed differently among the respective chromosomes of the two species. Even if fertilization were possible among dogs and cats, the resulting hybrid would go into genetic meltdown before real embryogenesis even got under way.

If crossing horses and narwhals really could produce a unicorn, then why hasn’t anybody bred one?

Also, if crossing horses and narwhals really could produce a unicorn, then why hasn’t anybody bred one? I’d pay to see a real unicorn. Really, this example doesn’t serve your friend’s point; it just demonstrates the absurdity of his view.

One other thing: I wouldn’t say that the family level is always the boundary above which animals cannot hybridize. The Linnaean classification system doesn’t correspond perfectly with the biblical grouping of organisms into distinct ‘kinds’, so creationist scientists have tried to develop their own methods for determining what constitutes a biblical ‘kind’. In some cases, a kind may represent a larger group than a family, and in other cases it may represent a smaller group within a family. Our website contains lots of information about how creationists reach these conclusions. See our Speciation Q&A page, for example.

Hope that was helpful to you. God bless,

Keaton Halley

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