Same lake, different fish Eawag/David Marques

Some thought it was impossible. But a population of stickleback fish that breed in the same streams is splitting into two separate species before our eyes, and at rapid speeds.

Three-spine sticklebacks were introduced to Lake Constance in Switzerland around 150 years ago – a blink of an eye in evolutionary terms. But since then, the fish have begun splitting into two separate types: one that lives in the main lake (pictured above left, female top, male in breeding colours below), and another that lives in the streams that flow into it (above right).

The main lake dwellers are bigger, with longer spines and tougher armour. In theory, these differences could be due to lifestyle rather than evolution – perhaps lake fish survive longer and grow larger.


But David Marques of the University of Bern and colleagues have found that there are already clear genetic differences between the two types. “We could be glimpsing the beginnings of two species,” he says.

What makes this finding extraordinary is that both types of fish breed in the same streams at the same time of year. They have been interbreeding all along, and still do, yet they are splitting into two genetically and physically different types.

Splitting apart

This kind of speciation, known as sympatry, was once thought to be extremely unlikely, says Chris Bird of Texas A&M University Corpus Christi, who studies how organisms are evolving by analysing their genomes. The conventional view is that speciation almost always requires two populations to be physically separated to prevent interbreeding, for example, living on different sides of a mountain, or on different islands in an archipelago.

This is because when animals mate, a process called recombination mixes up gene variants, meaning the genes of a mother and a father will be shuffled together in future generations. As long as interbreeding continues, it’s unlikely that two groups with distinctly different genetic traits will arise.

But Marques’ team found that the genetic differences between the two fish types are concentrated on the parts of chromosomes that are less likely to undergo recombination. As a result, the sets of gene variants that give the two types their distinct characteristics are less likely to get split up.

Rapid change

We cannot know for sure that the Lake Constance sticklebacks will continue evolving until they become two non-interbreeding species, says Marques. But evidence for sympatric speciation is growing, from mole rats in Israel to palms on Lord Howe Island, Australia, leading some evolutionary biologists, including Bird, to think it could be surprisingly common.

There is another case where sympatric speciation seems to be occurring nearly as fast as in the sticklebacks, Bird points out: apple maggots evolved from hawthorn maggots within two centuries of apples being introduced to North America.

As for the speed of the sticklebacks’ separation, there are now innumerable other examples of recent evolution that show how fast it can happen, from cancers becoming resistant to drugs and bedbugs becoming resistant to pesticides, to fish getting smaller to avoid becoming our dinner. It’s possible that such rapid evolution may even be the norm, rather than the exception.

Journal reference: PLOS Genetics, DOI: 10.1371/journal.pgen.1005887