While surfing the Internet I came across a new scientific article describing progress in characterizing the donkey genome. This reminded me of a brief conversation I had last year at church where someone asserted that the fact that mules (the product of mating horses and donkeys) are sterile is evidence that, in contrast to evolution, God created various kinds of animals that reproduce after their own kind [1].
I don't know where the notion that mules represent evidence against evolution came from, but it has been around for a long time. For example, Joseph Fielding Smith wrote in Man, His Origin and Destiny (1954):
There are various breeds of dogs, but they do not breed with cats. The cat family, composed of the domestic animal and the wild varieties, may mix. The horse and the ass are not of the same family and while man has been able to obtain from them the mule, the mule is rudely and humorously spoken of as being "without pride of ancestry and hope of posterity." The Lord decreed that they should not mix. This determining factor is a sufficient answer to organic evolution [2].
What's strange about this is that reproductive isolation is the expected and required
outcome of evolution, not a barrier to it. It's a confusion of the outcome with the input [3]. Reproductive isolation occurs through
a variety of mechanisms and sends the genetics of two proto-species on their own trajectory. Once separated, the two gene pools may eventually go on to further sub-divide through reproductive isolation. One species becomes two, then two become four, and so on. At some point, descendants of a lineage can no longer interbreed with their cousins in the other lineage and the genetic isolation is complete. Along the way, the different pools accumulate different variants, leading to differences in shape, color, physiology, and lifestyle. At a simplified level, this how evolution works and how the variety of life on earth has come to pass.
Just limiting ourselves to currently living animals we can see reproductive isolation in various stages of progress, and it's the reason that defining what constitutes a species can be difficult. For example, giraffe and okapai are separate species with a common ancestor of about 11.5 million years ago. But
based on genetics there is now good reason to consider giraffe as four different species (even though they can interbreed in captivity) with a few sub-species, rather than one species with nine to eleven sub-species. (This is the kind of thing taxonomists fight over.)
Back to my conversation, it was a group setting and the topic of evolution was a distraction from the business at hand, so I didn't get much chance to respond. Next time I hope to be quick enough to turn the tables and say something like, "the fact that mules are sterile is evidence
for evolution, not against it." That will probably cause some surprise, and will hopefully lead to a deeper discussion.
Notes:
1. I have addressed this issue before (
link). Also, did you know that the phrase "after their own kind" is
not found in the scriptures? Also, there have been a few
documented cases of fertile mules.
2. In fairness to President Smith, the field of genetics came into existence around the time that he became an apostle, and the molecular structure of DNA had only recently been determined when this was published. Biology has come a long way since then.
3. This is really a generalization about animals. New species of plants have been known to form through hybridization. And all kinds of things happen with microbes.
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