How Species are Made

While the Grants don’t know exactly which long‑term pressures have acted on Darwin’s finches since they arrived on the Galápagos, they do know that evolution has not always gone in circles. It has turned a single common ancestor into 14 species, each with its own peculiar adaptations. The evidence of this evolution is inscribed in the genes of the birds.

As populations of finches experience natural selection and become isolated from one another, their DNA becomes more and more distinct. The Grants enlisted the help of German geneticists to look for the genetic differences between all 14 species of Darwin’s finches. They also compared their DNA to an Ecuadorian bird called the grassquit, which ornithologists have suggested might be the closest living relative of the Darwin finches on the mainland. The researchers then compared the sequences and drew a family tree. Whenever they found two species with genes that were more similar to each other than to any other species, they joined their branches together, the node representing their common ancestor. They then joined them to more distant relatives, until all of the birds were united on a single tree.

Their results, which they published in 1999, show that all of the finches do indeed descend from a common ancestor. All 14 species are more closely related to one another than any of them is to the grassquit. An ancestral population of grassquit‑like birds arrived at the Galápagos Islands a few million years ago and gave rise to four different lineages of finches. The first to branch off on its own were the warbler finches, a group of species that use a slender beak to catch insects. The next branch to split off were the vegetarian finches, which have a stubby beak that they use to eat flower blossoms, buds, and fruit. Finally, two more lineages evolved: the tree finches adapted to catching insects in trees (the woodpecker finch, for example, ferrets out insects from cracks with cactus spines, which it holds in a chisel‑shaped beak), while the ground finches emerged, including the seed‑eating G. fortis.

Ornithologists have classified the ground finches into six different species, but according to the tree built by the Grants and their German colleagues, they are barely formed. Their genes are clearly distinct from the other finches on the Galápagos, but it is next to impossible to tell them apart from one another. Although the ground finches look and behave differently, they are still mating with one another and successfully producing hybrids. They are, in other words, six species in the process of being born.

While the finches of the Galápagos may be diverging quickly, the biggest explosion of speciation occurring on Earth has been taking place in Lake Victoria and the other Great Lakes of East Africa. Lake Victoria spreads over 27,000 square miles of East Africa, its floor practically as flat as a pool table. It is home to a group of fishes known as cichlids. There are 500 species of these small, brightly colored animals in Lake Victoria, none of which live anywhere else on Earth. Each species has some feature that makes it unique from the other residents of the lake. Some of them scrape algae off rocks with their teeth. Some of them crush shellfish, some pluck the eyes of other species of cichlids. Some species perform courtship rituals in which males build underwater sand castles for females to inspect. Some of them carry their young in their mouths.

A group of geologists came to Lake Victoria in 1995 hoping to look back at the past few hundred thousand years in the lake’s mud. The rivers that flow into it carry with them pollen, dust, and dirt, which gets buried year after year on the lake bottom. The geologists thought that if they drilled into the lake bed, they would draw up a core of mud recording hundreds of thousands of years of river flow, which would tell the story of the surrounding woodlands and savannas during that time. But they had drilled down only about 9 meters–in other words, into mud that had formed about 14,500 years ago–when all traces of a lake disappeared.

Their drills showed that 14,500 years ago, the deepest parts of Lake Victoria were covered in grass. It seems that during the Ice Age a cool, arid climate dried up the rivers that fed the lake, and its vast supply of water simply evaporated. Over the past few million years ice ages have come and gone, and Lake Victoria has emptied out and filled up again along the way. The last time the glaciers melted, the lake swelled to its current size in a few centuries.

A dry lake is no home for a fish. The ancestors of Victoria’s cichlids must have lurked in nearby streams, and when the waters came back to the lake, a single species of cichlid slipped in as well. The cichlids that live in Lake Victoria today are all close relatives of one another, and only distantly related to cichlids in other lakes and rivers. Just as brothers and sisters have similar genes, so do these fish. Their genes show that a single lineage of mouth‑brooders came to the lake after it refilled, and then, in the time that it took for humans to build civilization, 500 species were born. Look into the waters of Lake Victoria with an understanding of evolution, and you see a biological explosion.

This evolutionary boom seems to be a case of the right animal showing up at the right place at the right time. Cichlids are the perfect fish for quick specializations. For one thing, they have an extra set of jaws at the back of their mouth that can be used to break up food, leaving their front jaws free to evolve into new kinds of grabbing tools. Meanwhile, their teeth have shown a surprising evolutionary flexibility, having turned into pegs, spikes, and spatulas. As a result, their bodies can be reshaped by evolution into a staggering number of forms.

Cichlids may also have been primed for an evolutionary explosion by their elaborate sex lives. Male cichlids go to great lengths to attract females, dancing complicated jigs or building bowers out of sand and gravel. If a female likes what she sees, she releases her eggs, which the male fertilizes. The choices that females make for their mates are influenced by their genes; as a result, some females may have a preference for a particular shade of red, or a particularly steep angle of a bower wall, or a particular jig in the mating dance. These preferences may spread through the females until they become indifferent to any other male. With enough time, these tastes may isolate a population of fish and turn it into a new species.

When cichlids entered Lake Victoria 14,000 years ago, they were released from evolutionary constraints that their ancestors had faced in their rivers. Rivers are rapidly changing places, subject to sudden floods, droughts, and shifting courses. Under such conditions, evolution doesn’t favor fish that are too specialized for one particular part of a river; ones that are adapted to surviving all sorts of unexpected conditions will thrive. But the cichlids that colonized Lake Victoria entered a much more stable place, where they could adapt to special habitats such as rocky shores or deeper waters with sandy bottoms. They could rapidly evolve specialized ways of living and not get punished for their change.

Biologists are now studying the genetic differences of cichlids to figure out exactly how different species formed in Lake Victoria, but they are running out of time. In the 1950s and 1960s, a new fish was introduced to Lake Victoria. The Nile perch, which lives in some other lakes of East Africa, can grow up to 2 meters long on a diet of fish such as cichlids. It was brought to Victoria as a new source of food for the people who live around the lake. The Nile perch thrived and the catch of fishermen on the lake multiplied tenfold. But it thrived by devouring cichlids.

Meanwhile, farming and logging have caused massive soil erosion, sending topsoil into Lake Victoria, turning what were once clear waters dark. The cichlids, so attuned to the appearance of their potential mates, can’t distinguish their markings anymore and end up mating with other closely related species. The reproductive isolation that was pushing these fishes into hundreds of new forms is breaking down.

The silty murk and the Nile perch together have eliminated over half of the cichlid species in Lake Victoria in only 30 years. Humans may have ended this burst of speciation just as they’ve gotten acquainted with it.