Evolution Might be Repeatable: Clovers Reveal Predisposition to Traits

Can evolution be repeated? That's a good question, and it's one that scientists wanted to find out. After looking at 25 initial body plans exhibited by strange, soft-bodied fossils found in the Burgess Shale in the Canadian Rockies, scientists wanted to learn a bit more about whether evolution will select the same body plans every time.

The 25 body plans that the researchers examined eventually wound up becoming four. The other 21 were quickly eliminated during the course of evolution. In this case, the scientists wanted to know whether the same body plans would be selected again if they were to rewind the clock. Although this particular scenario was impossible to repeat, the researchers decided to look at related species that have independently evolved the same traits and ask if the same genes are responsible and, if so, whether the same mutations led to the trait.

The researchers examined 27 species of clovers in the genus Trifolium. Six of these species displayed what is called balanced polymorphism.  In some environments, natural selection favors plants that release hydrogen cyanide to discourage nibbling. In other environments, plants that do not release cyanide are favored. The polymorphism in the six species all evolved independently.

So what did they find? It turns out that chance played little part in the evolution of these species. Instead, the clover species seemed to be predisposed to develop the trait.

"We see exactly the same genetic mechanism-and it's kind of a weird mechanism-underlying the repeated evolution of the acyanogenic (cyanide-less) trait in different clover species," said Ken Olsen, one of the researchers, in a news release. "If you look at life on the planet, there's such an incredible diversity of life forms and traits that we tend to think anything goes. But when we look more carefully, we see there are constraints. There aren't any living species of limbed vertebrates with six toes, for example; it's five toes or fewer."

The findings reveal that evolution may actually be predisposed to certain paths. This means that, in fact, evolution may actually be repeatable.

The findings are published in the journal Philosophical Transactions of the Royal Society B.