Stick Bug Study Suggests Evolution is Predictable

953 Stick Bug Study Suggests Evolution is Predictable
A stick insect resting upon one of its host plant species / Moritz Muschick
Certain aspects of evolution -- such as the creation of new species -- can be predicted and repeatable, a new study finds. An analysis of stick insect genomes reveals the role of natural selection in speciation: Natural selection can drive two different populations of the same species to develop similar traits by inducing similar genetic changes on their genomes. The findings also suggest that two very similar species might evolve separately if they were placed under the same environmental pressures. 
If you could rewind "the videotape of life," would animals evolve exactly the same way they did? “Our results suggest, at least partially, yes,” Zach Gompert from Utah State says in a news release. “Despite some idiosyncratic outcomes, there may be a repeatable component driven by selection.”
Parallel speciation is when populations of the same species adapt to different environmental conditions, and as they become more and more different over time, they diverge genetically -- splitting into two species. We know that natural selection can drive the repeated evolution of reproductive isolation (when different forms stop exchanging genes), but the genomic basis of parallel speciation is still unclear.
Luckily, here's an example of speciation caught in the act. The wingless walking stick Timema cristinae has two ecotypes -- variations of the same species who’ve adapted to different host plants. One feeds on the thin, needle-like leaves of a shrub called Adenostoma fasciculatum; this bug has a white stripe on its back for camouflage (pictured above). The other bug (pictured below) is a different shade of green, wider, has no stripe, and feeds on Ceanothus spinosus -- a plant with wide green leaves where the stripe would stand out. “Natural selection is pulling these populations apart,” Patrik Nosil from the University of Sheffield says. This pair of stick insects are native Southern Californians. 
To analyze the divergence between the ecotypes, Nosil and colleagues collected 160 T. cristinae from the wild and sequenced their genomes. When they compared genomes to see how different environments had affected them, the team found thousands of genomic regions with “accentuated divergence” between the two. Many of the diverging genomic regions involve genes that code for proteins. 
But to their surprise, only 17 percent of their DNA had changed in the same way. That suggests that, even though some evolution leading to host specialization is predictable, a lot of the changes are random, Science reports.
Then, to test natural selection’s role in speciation, they transplanted stick bugs from their preferred plant hosts to alternative plants, and watched what happened to their genomes. They checked the DNA from the bugs’ offspring a year later to see how the frequency of different versions of their genes (or alleles) shifted compared with the parents. These shifts mean one version must provide a better survival advantage over another. 
They found dozens of those shifts coinciding with the DNA differences between the ecotypes -- indicating those differences were due to selection, Nostril tells Science, not chance. The ecotypes’ genomes exhibited both converging and diverging regions after just one generation on a different host plant.
The findings suggest that natural selection can drive parallel evolution among ecotypes of a species -- by which different ecotypes develop similar traits -- by inducing parallel genetic changes in them. The results also suggest that replicate species appear to evolve separately under similar environmental pressures.
The work was published in Science this week. The collaboration also included researchers from University of WyomingUtah StateRice University, and Simon Fraser University
[AAAS via Science]
Images: Moritz Muschick (top, bottom), Aaron Comeault (middle)


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