When a pair of freshwater fish species reproduce together, they create hybrids who are able to reproduce asexually. This ability should offer the hybrids an unstoppable evolutionary advantage...so why aren't we bowing down before our asexual fish hybrid masters?
The red-bellied dace and finescale dace are relatives of the minnow and carp, and their reproductive pairing is what creates these asexual hybrids. Since the hybrids don't need to find a mate in order to reproduce, they should have twice the reproductive potential of other fish. That means they should be able to create up to twice as many offspring without any extra energy exertion, and even reducing the population to a single fish wouldn't necessarily be enough to wipe it out.
When you look at it that way, asexual reproduction seems to have massive benefits over its sexual counterpart. With the ability to overwhelm other species in terms of sheer numbers, you'd think the hybrid fish would rule all ponds in which they appeared (and, eventually, the world). So why hasn't that happened? Researchers at the University of British Columbia have the answer:
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.
In a larger sense, the researchers speculate that this is one good reason why asexual reproduction hasn't made an evolutionary comeback in vertebrates. While asexual reproduction may be better when it comes to quantity of offspring, sexual reproduction is apparently still the far better option when it comes to quality.