USU study shows evolution can be repeatable, predictable

A close-up image of the Timema cristinae unstriped morph.

LOGAN – A recent Utah State University study reveals that evolution can be predictable. The study showed repeated patterns of adaptation and evolution of two different types of stick insects in the Santa Ynez Mountains in Southern California.

“There have been sort of long standing questions in biology about the extent as to which evolution is repeatable and predictable,” said Zachariah Gompert, one of the USU scientists involved in the study. “In other words if you imagine sort of rewinding things and play them again how often would you end up where we’re at now.”

According to Gompert, the stick bugs repeatedly adapt to two different populations of small plants they feed on. One of the plants is more broad-leafed, while the other is needle-shaped. The insects that feed on the needle-shaped plants have a white stripe down their back that shape their bodies into two needle shapes rather than one body. Both populations of insects have adapted to be the specific shade of green of their host plants.

Gompert said he wanted to see if the same patterns of evolution are seen across the two different populations. After studying the stick bugs, their adaptations and applying statistical models he discovered that 83 percent of the genome doesn’t have a predictable, repeatable pattern of evolution, but there is a big part of it that does.

“For the other 17 percent of the genome you do see these repeated patterns of divergence and 17 percent of the genome is much more than you’d expect by chance,” he said. “So you’re seeing some degree of repeated patterns of divergence. It is certainly not the whole genome. There’s a big kind of a chunk of the genome in a sense and more of a chunk than you’d expect.”

Gompert said that the part of the genome that has parallel divergence is an important part.

“That chunk contains more things that are genes than the rest of the genome does,” he said “There are reasons to think that more of the differences that matter are going to be in genes than outside of genes.”

Gompert said he did not expect to see such a large part of the genome showing this pattern.

“It suggests that, evolution is, at least in this case with stick insects, sort of more predictable, more repeatable than I think many people would have thought.”

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