“Can we predict what evolution is going to do? Does evolution repeat itself?”
It’s a good question, and one that Zacharia Gompert, evolutionary biologist and associate biology professor at USU, investigates using a species of small, flightless stick insect that mostly comes in two primary color morphs, a green one and a green striped one.
“And they blend in to different extents on two different plants they feed on, they feed on a lilac, a California lilac that has broad green leaves were the one that's all green blends in better. And then this other plant adenostoma that has more narrow needle-like leaves," says Gompert. "And that white stripe essentially splits the body of the stick insect and makes it look like two little needles.”
Generally, the all green ones are more common on the lilacs and the striped ones on the adenostoma.
“And that's driven largely by predation by birds so that, you know, the birds can more easily detect the green ones when they're on the wrong plant and vice versa,” he says.
So then is this the predictability that he's was referring to? Simply that a color morph better suited to blend in does better than one ill suited to its surroundings? Well, no. It turns out there’s more to it than that.
“One of the cool things we found more recently, though, is that that advantage can start to evaporate at the extremes," says Gompert. "So when a given color pattern becomes really, really common, say the green one, on the California lilac, where it blends in better, it starts to suffer actually a greater proportion of the predation”
The idea here is that the birds essentially begin to hunt for the more common morph. And the more common the morph is, the more they hunt for it.
“And when they hit about a frequency of 80, 90, 95% it actually flips where they start actually not surviving quite as well.”
Meaning that the very insects that blend in better become disproportionately hunted.
“And so what that does is it prevents either of those forms from being the only form found on either of those plants in nature.”
Because then after one morph is over-hunted the other morph gains in population until it again becomes the primary target, thus allowing the first morph to repopulate.
“They actually fluctuate up and down some from year to year, such that years of increase are usually followed by years of decrease.”
So this is the predictability then. The ‘up-and-down’ fluctuations in morph frequency from year to year. But Gompert, of course, didn’t just study a few cases. All told his study lasted over 30 years, involved 10 locations, and 48,349 individual insects.
In the end, he was able to conclude that in the short term, at least one aspect of evolution does indeed repeat itself.
You can read Gombert’s findings for yourself since they are published in the American Association for the Advancement of Science journal ‘Science Advances.’
