Evolution Of A Stick Insect: How Environments Influence Speciation
Groups of organisms belonging to the same species sometimes evolve separate characteristics to better exploit the resources and environment available to them. Eventually, they become a new species. Now, scientists have discovered through observations and genomic studies that these new species may follow the same evolutionary trajectory, even if they are evolving at geographically separate locations.
The research was carried out by evolutionary biologists at Rice University, the University of Sheffield, and eight other universities, and looked at the genome of a Southern California stick insect, which is in the process of evolving into two distinct species.
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"Speciation is the evolutionary process that gives rise to new species, and it occurs when barriers prevent two groups of populations from exchanging genes," said co-author of the study Scott Egan in a press release. "One way to study how speciation occurs is to look for examples where partial reproductive barriers exist but where genes are still exchanged." The Timema cristinae, a close relative of the plant-eating stick insect that looks like twigs, is a good example of this. These insects have highly camouflaged appearances, which keep them safe from birds and other predators.
There are more than 12 unique species of Timema that feed on specific plants in California and northern Mexico. One of these species is found in two distinct varieties. One variety feeds on the thin, needle-like leaves of a shrub called Adenastoma and features a distinct white stripe on its back that serves as camouflage. The other variety has no stripe and feeds on Ceanothus, a plant with wide green leaves where the stripe would stand out.
"Populations of T. cristinae on the two host plants have evolved many differences in their physical form while still exchanging genes," said Egan. "These same populations have also evolved barriers to gene flow. We call this process 'speciation with gene flow,' and evolutionary biologists have long wondered if the genetic basis for this process is highly repeatable, and if the genes involved are spread out across the whole genome or in a few discrete regions."
To check this theory, biologist Patrik Nosil and his team conducted detailed genomic and ecological tests for over four years. They first sequenced the genome of T. cristinae to identify which portions of the genome corresponded to particular biological functions. They then collected about 160 T. cristinae from different geographic locations in the wild and split them between ecotypes on the two host plants.
"We resequenced the genome of each individual that we collected and looked at which genes were differentiated between populations adapted to different host plants," Nosil said. "Because we also conducted an experiment in the field measuring evolution in real time, we gained information on how natural selection is pulling these populations apart."
For example, the team found that many of the genetic differences were related to the biochemical function of metal ion binding, and metals are known to influence differences in pigmentation and mandible shape between the two T. cristinae ecotypes.
It is a known fact that Timema do not migrate to long distances, so the team expected to find evidence of common genes in insects collected from specific geographic locations. The genomic tests confirmed this, but they also revealed a pattern in the way that natural selection was playing out at each of the localities.
"In particular, we found that there were regions of the genome that exhibited significant differences between populations from host plant one and host plant two, regardless of where the individuals were collected," Nosil said.
That suggested that evolution might be occurring in the same repeatable fashion at each location. To further test this, the team devised an experiment to gather genomic data from individuals that were actively under selection.
"We took individuals from a mixed population of the striped versus the no-striped ecotype, and we transplanted them back into nature onto the two host plants in five different sets," Egan said. "We allowed them to go an entire generation, and then we resampled those populations, resequenced the genome of the survivors and compared those to the ancestors that we started with a year before. We tried to match up the allele frequency shifts in this experiment with the genome-level differentiation that we observed in our genome-resequencing populations. And what we found was that many of the regions that were highly differentiated in nature were the exact same regions that were responding to our selection experiment."
Showing natural selection at work in this study is an important milestone for evolutionary biologists who are trying to understand how the environment influences speciation.
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