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A tank full of cichlids. Credit: OakleyOriginals. CC BY 2.0

The Innovation That Doomed The Very Fish It Helped

A few hundred thousand years ago, a group of fish in an African lake evolved something new—a set of strong chewing jaws in their throats. They became better at eating a wide variety of tough foods. They thrived. They diversified into endless forms most wonderful. And in doing so, some of them doomed themselves.

This is a counter-intuitive story because evolutionary innovations are typically seen as good things. The advent of sharp eyes allowed animals to sense each other over long distances, triggering an enscalating arms race between voracious predators and armoured prey. Wings allowed dinosaurs to conquer the skies. Silk allowed spiders to become master trappers. Nutrient-making bacteria allowed bugs to thrive on an impoverished diet of sap, and become the bane of plants. Each of these breakthrough adaptations allowed their owners to exploit some previously inaccessible opportunity, and allowed obscure species to diversify into vast families. They opened doors.

But the trouble with evolution is that you have no ideas where those doors will lead. You could end up in paradise, or in a one-way corridor to a crematorium.

The cichlid fish (pronounced “sick-lids”) exemplify this lesson. They are icons of evolution. In the limited confines of East Africa’s Great Lakes—Victoria, Malawi, and Tanganyika—the cichlids have diversified into a staggering number of species in a very short span of time. Lake Malawi alone has around 500 unique cichlid species. Some play dead to lure their prey in, others chase their victims down. Some eat only snails, others graze on algae, and yet others specialise on the scales of other fish.

These varied lifestyles depend on a crucial innovation—a second set of jaws in the cichlids’ throat. Many fish have similar “pharyngeal jaws” but the cichlids’ ones are exceptionally strong. This allows them to crush, chew, and process whatever food they grab with their mouths, including tough morsels like snails and scales.

But the pharyngeal jaws come with a cost. They’re strong because they consist of separate bones that have fused together, and this fusion limits the extent to which cichlids can open their mouths. Their restricted gape makes predatory cichlids less competitive than other hunting fish, which can swallow much bigger prey.

For cichlids in Lake Victoria and Malawi, this wasn’t a problem, since they diversified in the complete absence of any wider-mouthed fish. It didn’t matter that they were less competitive, because they had no competitors. That changed in the 1950s, when fisherman introduced the voracious, big-mouthed Nile perch into Lake Victoria. Its arrival was apocalyptic for the cichlids, and many species quickly disappeared.

But this mass extinction didn’t hit all the cichlids equally. Matthew McGee from the University of California, Davis found that the fish-eating species were most likely to have gone extinct, which suggests that the perch hadn’t eaten the cichlids into oblivion.

Instead, it had outcompeted them.

McGee found that the fish-eating cichlids had mouths of similar size to the Nile perch but, because of their fused pharyngeal jaws, they could only open theirs half as wide. As a result, they take many hours to deal with prey that the Nile perch can swallow in a few minutes. Their innovative throat jaws, which had stood them so well in the pre-perch era, had suddenly turned them into inferior competitors.

The cichlids aren’t alone in having chewing pharyngeal jaws. Other groups of fish, including wrasses, damselfish, and flying fish, independently hit on the same innovation. McGee studied the diets of these families to see what kinds of food they specialised in throughout their evolutionary history. He found that they were unusually quick to diversify into species that tackle hard-to-eat foods, like tough plants or crunchy animals. By contrast, they were unusually slow to produce species that tackle hard-to-catch foods like other fish. So these other groups were subject to the same trade-off—chewing throat jaws for tough foods, or wide mouths for swallowing other fish. You can’t have both.

McGee writes that for “nearly half a century”, scientists have seen the pharyngeal jaws of cichlids and wrasses as “a classic example of evolutionary innovation that opened up new niches”. That’s still true, but his study reveals that this innovation wasn’t wholly beneficial. In some cases, it was an timebomb of extinction, waiting to detonate.

“Adaptation by natural selection contributes to a good fit between an organism and its current environment,” writes Geerat Vermeij, also from University of California, Davis, in a related commentary. “What it cannot do is anticipate the future.”