On repeat: Biologists observe recurring evolutionary changes in stick insects over time

A long-standing debate among evolutionary scientists goes something like this: Does evolution occur in a predictable pattern or does it depend on chance events and contingencies? That is, if you could turn back the clock, as celebrated scientist Stephen Jay Gould (1941-2002) described in his famous metaphor “Replaying the Tape of Life,” life on Earth would once again evolve as something similar to what do we know now, or would 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.’ And yet, when examining short timescales, we can find predictable, repeatable evolutionary patterns.”

Gompert and colleagues report evidence of repeatable evolution in stick insect populations in the article “Evolution repeats itself in replicates long-term studies in the wild”, in Scientific progress. Authors on the paper include Gompert’s longtime collaborator Patrik Nosil and other researchers from France’s University of Montpelier, Brazil’s Federal University of São Paulo, the University of Nevada, Reno and Notre Dame University.

The team examined three decades of data on the frequency of cryptic color pattern changes in the stick insect 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-feeding insect to mingle with California lilac (Ceanothus spinosus) shrubs. In contrast, green-striped forms disappear against chamise (Adenostoma fasciculatum) shrubs.

Hiding among plants is one of T. christinae’s main defense mechanisms, as hungry birds, such as scrub jays, are insatiable predators of the stick insects.

“Bird predation is a constant factor that shapes the organismal properties of insects, including color and striped versus non-striped,” said Gompert, associate professor in the USU Department of Biology and the USU Ecology Center. “We observed predictable ‘up and down’ fluctuations in stripe frequency across all populations, representing repeatable evolutionary dynamics based on standing genetic variation.”

He says a field experiment shows that these fluctuations are associated with negative frequency-dependent natural selection (NFDS), with cryptic color patterns being more favorable when they are rarer than usual. This is probably because birds develop a ‘search image’ for very abundant prey.

“On short timescales, evolution can be quite predictable with existing variations,” says Gompert. “You can count on certain drivers to always be there, like birds that feed on the insects.”

But on longer timescales, evolutionary dynamics become less predictable.

“The population can experience a chance event, such as a severe drought or a flood, that disrupts the status quo and therefore the predictable outcomes,” says Gompert.

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

“Rare things are easily lost to chance, so a new mutation is likely to disappear before it becomes established,” he says. “Indeed, another species of Timema stick insect that also feeds on chamise never had or rapidly lost the mutations that cause the cryptic stripe trait. Thus, the evolution of the stripe is not a repeatable result of evolution on this long scale.”

Gompert notes that replicated long-term studies of natural populations, including research 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 conclusions about repeatability between multiple evolutionarily independent populations,” he says. “Such studies are challenging to implement, not only because they require concerted efforts, but also because you cannot rush the time.”