A speciation rate of 10 years

From The Fishy Mystery of Lake Malawi by Ben Crair.

Cichlids are found all over the world, mainly in Africa and Latin America, but they’re especially abundant in Lake Malawi, where they’ve diverged into at least 850 species. That’s more species of fish than can be found in all of the freshwater bodies of Europe combined.

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Lake Malawi is what’s known as a meromictic lake: Its distinct water layers—generally three—don’t mix. This provides more environments for plants and animals to live in, and it also accounts for the lake’s stunning color; sediments stay on the bottom and the top layer is crystal clear.

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The Scottish explorer and missionary David Livingstone first told Europeans about Lake Malawi in 1859—the same year Charles Darwin published his revolutionary On the Origin of Species. Darwin famously formulated his theory of natural selection after observing the 14 different species of finches in the Galápagos Islands, among other phenomena. He theorized that the birds had evolved into different species because they’d been isolated in different habitats and adapted to different types of food. On one island, finches with thick beaks had outperformed thin-beaked neighbors at crunching seeds. On another island, finches with thin, pointy beaks had won out in the competition for insects. In each case, Darwin suggested, a bird with the physical advantage was able to survive longer and produce more offspring than run-of-the-mill birds, and the trait was passed down through the generations and amplified over millions of years. He called this process natural selection to contrast it with the artificial selection performed by an animal or plant breeder working to strengthen a pedigree or create a new hybrid.

If that’s the usual understanding of Darwinian evolution, the myriad cichlids of Lake Malawi pose a real challenge to it. Some 850 species have descended from the original cichlids that swam into the lake one or two million years ago. This extraordinary diversity has long puzzled evolutionary biologists, especially because, unlike the Galápagos finches, the cichlid species aren’t necessarily separated by geographical barriers. Many of them live together in the same populations, where nothing in the environment prevents them from mixing with one another. Different species of mbunas will all feed on the algae carpeting the rocks and the tiny creatures within it—and yet a fish will patiently seek a mate of its own species rather than breed with another.

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McKaye was confounded by the sheer number of cichlid species in Lake Malawi. How were the fish branching off into new species at such a fast rate while living together in the same environment? The answer, McKaye explains, is the cichlids’ fondness for beauty contests—run by the females.

For instance, within a mixed population of mbuna, the females—even the ones that are drab, with only a few brown or black markings—seek out males with extremely specific color patterns. Females of Labeotropheus trewavasae seek out blue males with red dorsal fins, rarely mixing with males of Labeotropheus fuelleborni, which look similar except their dorsal fins are also blue.

It’s not uncommon for an animal to have some degree of choosiness when it selects a mate. Darwin called this phenomenon sexual selection, and it’s familiar to anyone who has watched a nature documentary where birds perform elaborate courtship dances. But the reasons behind sexual selection are not always clear. Survival of the fittest should guide species toward practical traits like strength or ability to find food. How could an ornate train help a peacock exploit its niche?

Darwin believed some animals simply had a “taste for the beautiful,” an attraction to purely aesthetic traits that confer no fitness or advantage. The idea that female birds simply enjoy colorful feathers and elaborate dances did not catch on—the Yale ornithologist Richard Prum has said his colleagues treat it like a “crazy aunt in the evolutionary attic.” Still, there’s no question that female peacocks like fans of colorful feathers and female birds of paradise like elaborate courtship dances.

Some scientists believe these traits often signal a degree of overall good health that could support strong offspring and long-term vitality. But over the generations, sexual selection can exaggerate traits to the point where they would seem to actually impede survival—for instance, producing long, cumbersome ornaments or colors so bright that they draw extra attention from predators.

In the case of cichlids, the tastes of the females are so fixed and specific that it’s hard to see how they’d point to an evolutionary advantage for the male. “It can be a totally arbitrary trait,” says Alex Jordan, a cichlid researcher affiliated with the Max Planck Institute. Among sand-dwelling cichlids of Lake Malawi, for instance, some females are drawn to the males that move sand with their mouths to build the biggest bowers—crater-like structures or mounds on the lakebed. Other females favor the males that perform the most elaborate figure-eight dances. The differences keep getting more pronounced with every generation: The male offspring of the figure-eight swimmers may become even better at swimming figure eights, and the females may become even more attached to that particular trait. This creates a positive feedback loop that can create a new species of cichlid in as few as 20 generations. (Most cichlids reach sexual maturity at around 6 months.)

“In my lifetime, they could spin out another species or two,” McKaye told me. This is much faster than new species evolve through natural selection alone, which would require waiting for an advantageous mutation to randomly arise.