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A long-standing debate among evolutionary scientists goes something like this: Does evolution occur in a predictable pattern or does it depend on random events and contingencies? That is, if you could turn back the clock, as the famous scientist Stephen Jay Gould (1941–2002) described in his famous metaphor “Replaying the Tape of Life,” would life on Earth have re-evolved as something like what we know now or will it look very very different?

“If you frame it as an either/or question, it’s too simplistic,” says evolutionary biologist Zachariah Gompert of Utah State University. “The answer is not ‘completely random’ or ‘completely deterministic and predictable.’ Yet, by examining short time scales, we can find predictable, repeatable evolutionary patterns.”

Gompert and colleagues report evidence for recurrent evolution in stick insect populations in the paper, “Evolution is recurrent in repeated long-term studies in the wild,” in Scientific progress. Authors of the paper include longtime Gompert collaborator Patrik Nosil and other researchers from France’s University of Montpellier, Brazil’s Federal University of Sao Paulo, the University of Nevada, Reno, and the University of Notre Dame.

The team examined three decades of data on the frequency of cryptic color pattern morphs in the stick insect species Timema cristinae in ten naturally replicated populations in California. T. cristinae is polymorphic in body color and pattern. Some insects are green, allowing the wingless, plant-eating insects to blend in with California lilac (Ceanothus spinosus) bushes. In contrast, the green striped morphs disappear against checkered bushes (Adenostoma fasciculatum).

Hiding among plants is one of T. christinae’s key defenses, as hungry birds such as jays are voracious stick predators.

“Bird predation is a constant driver shaping insect body traits, including coloration and banding versus non-banded,” said Gompert, associate professor in USU’s Department of Biology and the USU Ecology Center. “We observed predictable ‘up-and-down’ fluctuations in banding frequency in all populations, representing recurrent evolutionary dynamics based on persistent genetic variation.”

He says a field experiment shows that these fluctuations involve negative frequency-dependent natural selection (NFDS), where cryptic color patterns are more beneficial when they are rare than common. This is probably because birds develop a “search image” for very abundant prey.

“On short time scales, evolution involving existing variation can be quite predictable,” says Gompert. “You can count on certain drivers always being there, like birds eating insects.”

But on longer time scales, evolutionary dynamics become less predictable.

“A population may experience a random event, such as a severe drought or flood, that disrupts the status quo and thus predictable outcomes,” says Gompert.

In the long term, a new mutation in the species could introduce a rare trait, he says. “It’s as close to truly random as you can get.”

“Rare things are easily lost by accident, so there’s a high chance that a new mutation will die out before it gets stronger,” he says. “Indeed, another species of stick insect, Timema, which also feeds on shirt, either never had or quickly lost the mutations that create the mysterious stripe trait. Therefore, the evolution of the stripe is not a repeatable result of evolution on this long scale.”

Gompert notes that repeated long-term studies from natural populations, including studies on the famous Darwin’s finches, are rare.

“Because most of this work is limited to one or a few populations, it is difficult to draw inferences about repeatability among multiple evolutionarily independent populations,” he says. “Such studies are difficult to do, not only because they require a concerted effort, but also because you can’t rush the time.”