Stick Insects and the Predictability of Natural Selection
An international team of scientists endeavored recently to determine whether or not evolution’s effects on a particular species can be predicted. They chose stick insects as their target of observation for the requisite analysis, aided by 25 years of preexisting data. They compared the first half of that dataset to the second half to find out if there were any predictive indicators they could extrapolate from natural selection, and they ultimately found that evolution is extremely difficult — but not impossible — to predict.
In this study, scientists succeeded in presaging a select few evolutionary shifts, but the majority of adaptations were virtually unpredictable. They concluded that these were mostly adaptations for which they lacked sufficient data to account, and that says a great deal considering that a quarter-century’s worth of data on a genus thoroughly studied by a broad variety of research teams was at their disposal. One of the researchers on the team, in fact, had been routinely hiking the mountains of California with nets and capturing stick insects since the early 1990s, capturing over 34,000 individuals in that timeframe.
Utah State University biologist Zach Gompert says, “Evolution often appears random, even when driven by the deterministic process of natural selection because we just aren’t aware of all the environmental fluctuations and other factors taking place that drive change.” In essence, that’s what studies like these reaffirm, the fact that we still don’t truly understand natural selection — the definition of which has never been fixed or absolute but, rather, has changed more than once since Darwin first coined the term in his unfurling of the theory of evolution and its rudimentary principles. “If we had a better understanding of the mechanisms at play, we might have a better picture of evolutionary change and its predictability.”
The team found that they could classify the stick insects into three categories just based on appearance, though: brown, green with a vertical stripe on the back and green with no stripe. Previous studies have already linked these variations to the kinds of plants on which stick insects choose to hide. One species, in particular, has an almost completely white population whereas another species is predominately green. They camouflage themselves on this basis, and the brown species likewise camouflage themselves on the stems of plants; however, the dominant coloration of a certain species can change over time.
The team focused on trying to make predictions about the changes of coloration, attempting to determine what patterns would be most dominant from one year to the next. They did this for several years, publishing their findings in Science, and their work was funded by a grant from the Canadian Natural Sciences and Engineering Research Council as well as another from the European Research Council. The study was also supported by computational resources the team received from the University of Utah’s Center for High-Performance Computing.
“We used a rare and unique dataset of 25 years of field data documenting the evolution of cryptic body coloration in terms of frequencies of three ‘morphs’ — flavors, if you will—of stick insects,” Grompert explains. He’s an assistant professor in the Biology Department and at the Ecology Center. “Using the first 10-15 years of the data, we tried predicting, or forecasting, the changes that would occur in the subsequent years of the data.” The brown stick insects they called melanistic morphs.
“These insects are cryptic, meaning they visually blend into their surroundings to hide from predators,” according to Gompert. He says that they got rather accurate predictions for green morphs over green-striped morphs and vice versa, but they never proved accurate at all when it came to the melanistic morph dominating seemingly at random. They relied on genomic analysis, so they were able to demonstrate in each case how natural selection appeared to effect evolutionary adaptation.
“With the green versus green-striped morphs, the cause of selection was a simple and well-understood facilitation of predictability,” Gompert explains. “In contrast, with the melanistic morph,” and by this he means green versus melanistic as well as green-striped versus melanistic, “natural selection was more complex and tied to variation in weather and climate, making it harder to predict from past patterns of change.” The research team compared results to other well-established studies, even Darwin’s own classic finch study as well as that of the scarlet tiger moth. Neither of those had proven to be very predictable either.
“Our findings support previous discoveries and suggest evolution of morph frequencies in these stick insects is indeed a result of selection,” Gompert elucidates. “They also suggest poor predictability of environmental variation and how it affects selection, rather than random evolutionary processes, might be the main limits on predicting evolution.” Basically, the team was able to show that evolution is not random per se but that it’s still far from predictable.
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