Marlowe Starling writes:

When she was a graduate student in the 1970s, the evolutionary biologist Kerstin Johannesson regularly walked the shores of a Swedish archipelago, scanning the ground for pebbles that moved: marine snails. Her adviser, a taxonomist, had tasked her with describing the species present there by documenting their traits. She noticed that snails with thicker shells stayed on the shore, while those with thinner shells seemed to prefer wave-battered rocks, and in between the two habitats were snails with intermediate shell thickness. While they seemed like distinct species, Johannesson couldn’t help but wonder whether these snails might instead be different types of the same one.

Around 50 years earlier, the botanist Göte Turesson had had a similar revelation in a similar setting. As he walked Sweden’s shores, he noticed that saltbush plants from different stretches of coastline had distinct traits — earlier or later flowering times, or shorter or longer stalks — and between habitats, those traits fell somewhere in the middle. He bred the plants in his home garden and found that these distinct traits had a genetic basis even though they arose from the same species. In 1922, he published his results and coined the term “ecotype” to describe a subpopulation of a species adapted to a hyperlocal habitat.

At that time, the definition of a species was even less clear than it is today. Genes were still theoretical, and the structure of DNA wouldn’t be discovered for another 30 years. Turesson “struggled to be accepted,” said Johannesson, now the director of Tjärnö Marine Laboratory at the University of Gothenberg. How can a species contain multiple distinct phenotypes — or sets of traits — without separating into two species? “He had quite a job to try to convince his colleagues that there were inherited differences and local adaptation within species,” she said.

It wasn’t until the early 2000s, when whole-genome sequencing became accessible to evolutionary biologists, that Turesson’s ideas about ecotypes could be tested. By comparing the DNA sequences of ecotypes across the tree of life — from marine snails to stickleback fish to stick insects and more — scientists can study adaptation and speciation, the process by which new species form, at a molecular level. [Continue reading…]