Biologists observe recurring evolutionary changes, over time, in stick insects

A long-standing debate among evolutionary scientists goes something like this: Does evolution happen in a predictable pattern or does it depend on chance events and contingency? 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," would life on Earth evolve, once again, as something similar to what we know now, or would it look very, very different?

Now researchers  report evidence of repeatable evolution in populations of stick insects in the paper "Evolution repeats itself in replicate long-term studies in the wild," in Science Advances. 

The team examined three decades of data on the frequency of cryptic colour-pattern morphs in the stick insect species Timema cristinae in ten naturally replicate populations in California. T. cristinae is polymorphic in regard to its body colour and pattern. Some insects are green, which allows the wingless, plant-feeding insect to blend in with California lilac (Ceanothus spinosus) shrubs. In contrast, green striped morphs disappear against chamise (Adenostoma fasciculatum) shrubs.

Hiding among the plants is one of T. christinae's key defenses as hungry birds, such as scrub jays, are insatiable predators of the stick insects.

Bird predation is a constant driver shaping the insects' organismal traits, including coloration and striped vs. non-striped.

Scientists observed predictable 'up-and-down' fluctuations in stripe frequency in all populations, representing repeatable evolutionary dynamics based on standing genetic variation.

A field experiment demonstrates these fluctuations involved negative frequency-dependent natural selection (NFDS), where cryptic colour patterns are more beneficial when rare rather than common. This is likely because birds develop a 'search image' for very abundant prey.

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

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

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.

On long time scales, a new mutation in the species could introduce a rare trait. That's about as close to truly random as you can get.

Rare things are easily lost by chance, so there's a strong probability a new mutation could disappear before it gains a stronghold. 

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.

Replicated, long-term studies from natural populations, including research on the famous Darwin's finches, are rare.

Because most of this work is restricted to one or few populations, it is difficult to draw inferences on repeatability among multiple evolutionary independent populations.

Such studies are challenging to implement not only because they take concerted effort, but also because you can't rush time.

Patrik Nosil et al, Evolution repeats itself in replicate long-term studies in the wild, Science Advances (2024). DOI: 10.1126/sciadv.adl3149. www.science.org/doi/10.1126/sciadv.adl3149