Natural Selection Can Advance Evolution, But Can Also Impede It
A study involving walkingstick insects led by the University of Sheffield in England and the University of Colorado Boulder shows how natural selection, the engine of evolution, can also impede the formation of new species.
The team studied a plant-eating stick insect species from California called Timema cristinae that is known for its cryptic camouflage, which allows it to hide from hungry birds. Some of the insects are green and blend in with the leaves of a particular shrub species; others are also green but they blend in with a different species of shrub.
Darwinian natural selection has begun pushing the two down separate paths that could lead to the formation of two new species, but the team found that a third variation — a brown one — appears to be thwarting the process because it is able to successfully camouflage itself among both shrub species.
Using field investigations, laboratory genetics, modern genome sequencing, and computer simulations, the team concluded the brown version of T. cristinae is shuttling enough genes between the green stick insects living on different shrubs to prevent strong divergent adaptation and speciation. The brown variant of the walkingstick species also is favored by natural selection because it has a slight advantage in mate selection and a stronger resistance to fungal infections than its green counterparts.
“This is one of the best demonstrations we know of regarding the counteractive effects of natural selection on speciation,” said Samuel Flaxman, second author on the new study. “We show how the brown population essentially carries genes back and forth between the green populations, acting as a genetic bridge that causes a slowdown in divergence.”
“This movement of genes between environments slows down the genetic divergence of these stick insect populations, impeding the formation of new species,” said Aaron Comeault, lead author of the study.
The new results underscore how combining natural history and cutting-edge genetics can help researchers gain a better understanding of how evolution operates in nature. It also shows how natural selection can sometimes promote but other times hinder the formation of new species.
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