When a red-bellied dace and a finescale dace (freshwater fish in the carp and minnow family) mate with each other, they produce a hybrid with a very special ability: it can reproduce asexually. This asexual hybrid should have a tremendous evolutionary advantage over its sexually reproducing forefathers.
In sexual populations, two individuals need to get together to reproduce, but in asexual populations every individual can reproduce on its own, giving asexuals twice the reproductive potential. Theoretically, the asexual advantage should enable the hybrids to outcompete sexual dace living in the same pond. But in reality that doesn't happen. Sexual and asexual dace are known to live side-by-side.
So why doesn't the hybrid dace take over? According to a study by researchers from the University of British Columbia, it's because the hybrids aren't as healthy. Using swimming speed as a proxy for overall health, the researchers found that hybrids performed worse than at least one of the parent species in a series of speed tests.
The results suggest that at minimum, the hybrid has no physiological performance advantage over the sexual species, and is probably at something of a disadvantage. The lower physiological performance may counteract the hybrids' reproductive advantage, preventing them from taking over. The results offer one possible explanation for why sexual reproduction has stayed dominant in vertebrates
The research is published in the May/June 2011 issue of the journal Physiological and Biochemical Zoology.
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