Evolution is both predictable and random

Have you ever wondered if evolution follows a predictable pattern? If we could rewind the evolutionary clock, would life on Earth unfold in a similar way, or would we end up with something entirely different? This age-old question has puzzled evolutionary scientists for centuries.

Zachariah Gompert, an evolutionary biologist at Utah State University, believes it’s not a simple either/or question. “The answer isn’t ‘completely random’ or ‘completely deterministic and predictable,'” he says.

Gompert and his colleagues recently published a remarkable study that sheds light on this fascinating topic.

Predictable evolution of stick insects

The researchers examined three decades of data on the frequency of cryptic color-pattern morphs in the stick insect species Timema cristinae. These insects are masters of disguise, blending seamlessly with their surroundings to avoid becoming a bird’s next meal.

Some sport a vibrant green hue, camouflaging them against California lilac shrubs, while others have green stripes that disappear against chamise shrubs.

Gompert and his team discovered a fascinating pattern: the frequency of striped stick insects fluctuated predictably over time in all ten populations they studied. 

“We observed predictable ‘up-and-down’ fluctuations in stripe frequency in all populations, representing repeatable evolutionary dynamics based on standing genetic variation,” Gompert explains.

Predators drive evolution

So, what’s behind this predictable pattern? The answer lies in the relationship between the stick insects and their predators, such as scrub jays. These hungry birds are constantly on the lookout for a tasty insect snack.

“Bird predation is a constant driver shaping the insects’ organismal traits, including coloration and striped vs. non-striped,” Gompert notes.

The researchers conducted a field experiment that revealed a fascinating phenomenon called negative frequency-dependent natural selection (NFDS).

When a particular color pattern becomes too common, birds become better at spotting it, making those insects more vulnerable to predation. This gives the rarer color pattern an advantage, leading to its increase in frequency.

“At short time scales, evolution involving existing variations can be quite predictable,” Gompert explains. “You can count on certain drivers always being there, such as birds feeding on the insects.”

Unpredictable side of evolution

While short-term evolutionary patterns can be predictable, things get a bit more chaotic over longer periods. “The populations might experience a chance event, such as a severe drought or a flooding event, that disrupts the status quo and thus, the predictable outcomes,” Gompert explains.

On even longer time scales, new mutations can introduce rare traits into the mix. “That’s about as close to truly random as you can get,” says Gompert. These rare mutations can easily disappear by chance before they have a chance to become established.

“Indeed, another species of Timema stick insect that also feeds on chamise either never had or quickly lost the mutations making the cryptic stripe trait. Thus, the evolution of stripe is not a repeatable outcome of evolution at this long scale.”

Study significance

Gompert’s research is unique because it involves replicated, long-term studies from natural populations.

Such studies are rare due to the time and effort required to track evolutionary changes over many years. However, they are crucial for understanding the complex interplay between predictability and randomness in evolution.

Gompert is passionate about sharing his research with others. He has developed interactive educational programs for both undergraduate students and the general public, making the fascinating world of evolution accessible to all.

Predictable evolution and its randomness

The recent study on stick insects reveals the link between predictability and randomness in evolution. Over short periods, evolutionary changes in these insects can be predictable. Clearly, birds’ predation drives these predictable patterns.

However, over long periods, evolution becomes less predictable. Chance events, like severe weather, and new mutations influence these long-term changes. This study shows that while some evolutionary patterns can be anticipated, others depend on random occurrences.

Gompert’s work highlights the balance between deterministic and random factors in evolutionary processes, offering new insights into how species adapt and change over time.

So, the next time you spot a stick insect blending seamlessly into its environment, take a moment to appreciate the fascinating evolutionary forces that shaped its appearance. It’s a reminder that life on Earth is a tapestry woven from both predictable patterns and the unpredictable twists and turns of fate.

The study is published in the journal Science Advances.

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