Research on stick insects shows predictable short-term evolution, but long-term changes introduce unpredictability due to random events and new mutations.
The researchers present evidence of repeatable evolutionary patterns in California stick insect populations, showing that while short-term evolution may be predictable due to constant environmental pressures such as predation, long-term evolutionary outcomes involve more chance due to events such as mutations and climate change.
There is a long-standing debate among evolutionary scientists that goes something like this: Does evolution follow a predictable pattern or does it depend on random events and contingencies? In other words, if we could turn back time, as the famous scientist Stephen Jay Gould (1941-2002) described in his famous metaphor “Replay the Tape of Life”, would life on Earth re-evolve like something similar to what happening now? do we know it now, or would it be very, very different?
A bush jay with a Timema stick insect in its beak. Credit: Henri Truchassout
The complexity of evolution
“If you frame the question as either/or, it’s too simple,” says Zachariah Gompert, an evolutionary biologist at Utah State University. “The answer is not “completely random” or “completely deterministic and predictable.” And yet, looking at short time frames, we can find predictable and repeatable evolutionary patterns.
Gompert and colleagues report evidence of repeatable evolution in stick insect populations in the May 24, 2024, online issue of the Journal of the American Association for the Advancement of Science. Scientists are making progress. Contributing authors to the paper include Gompert’s longtime collaborator Patrik Nosil and other researchers from the University of Montpellier in France, the Federal University of São Paulo in Brazil, the University of Nevada, Reno, and the University of Notre Dame. The research is supported by the National Science Foundation and the European Research Council.
Green Timema cristinae morph stick insect mixes with California lilac bush (Ceanothus spinosus). Credit: Aaron Comeault
Results of research on stick insects
The team examined three decades of data collected on the frequency of cryptic color forms in stick insects. species Timema cristinae in ten naturally breeding populations in California. T. cristinae is polymorphic with regard to the color and pattern of its body. Some insects are green, which allows wingless plant-eating insects to blend in with California lilacs (Ceanothus spinosus) bushes. On the other hand, the green striped shapes disappear on the dresser (Fasciculated adenostomy) bushes.
Hiding among plants is one of T. christinae Hungry birds, such as jays, are voracious predators of stick insects.
Striped Timema cristinae morph stick insect is confused with chamomile bush (Fasciculated adenostomy). Credit: Moritz Muschick
Evolutionary models and natural selection
“Avian predation is a constant factor that shapes insect body characteristics, including color and whether they are striped or not,” says Gompert, an associate professor in the University’s Department of Biology and Ecology Center. “We observed predictable fluctuations in the frequency of stripes in all populations, which represents a repeatable evolutionary dynamic based on constant genetic variation. »
He says the field experiment shows that these fluctuations involved 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 pattern” for the most common prey.
Zach Gompert, a biologist at Utah State University, and his colleagues observe repeated evolutionary changes over time in stick insects; publish the results in the issue of May 24, 2024 Scientists are making progress. Credit: M. Muffoletto
The predictability and randomness of evolution
“On short timescales, evolution involving existing variation can be quite predictable,” says Gompert, who received a 2019 CAREER grant from the National Science Foundation to support his research. “You can count on the constant presence of certain drivers, like birds that feed on insects. »
But over longer timescales, evolutionary dynamics become less predictable.
“People could be faced with a random event, such as a severe drought or flood, that would disrupt the status quo and thus the predictable outcomes,” explains Gompert.
Challenges of evolutionary studies
In the long run, a new mutation in a species could introduce a rare trait, he says. “It’s as random as it gets.” »
“Rare things are easily lost by accident, so there is a high probability that a new mutation will disappear before it takes root,” he says. “Indeed, another kind That’s it the stick insect, which also feeds on the shirt, never had or quickly lost the mutations that created the mysterious trait of stripes. Therefore, the evolution of the stripe is not a repeatable result of evolution on such a long scale.
Gompert notes that long-term, repeated studies of natural populations, including studies of Darwin’s famous finches, are rare.
“Because most of this work is limited to one or a few populations, it is difficult to draw conclusions about reproducibility across multiple independent evolutionary populations,” he says. “Such studies are difficult to conduct, not only because they require a joint effort, but also because it is impossible to squeeze time. »
Gompert, who was named a ScholarGPS Senior Scholar, developed, with USU colleagues, an interactive, research-intensive introductory biology laboratory course to introduce undergraduate students to research. He and his colleagues also developed an interactive presentation on evolution for all ages, called “Nabokov’s Butterflies,” which was presented at the USU College of Science’s Science Unwrapped 2022 public program.