Stick insects species
© Rosa Marin

Study: Natural selection affects new species

A study involving walking stick insects has shown how natural selection can also impede the formation of new species.

Scientists at the UK’s University of Sheffield and the University of Colorado Boulder in the United States, which received funding from the European Research Council, studied a plant-eating stick insect species from California called Timema cristinae; it is known for its cryptic camouflage that allows it to hide from hungry birds.

The species comes in several different forms – one is green and blends in with the broad green leaves of a particular shrub species, while a second green variant sports a white, vertical stripe that helps disguise it on a different species of shrub with narrow, needle-like leaves.

While Darwinian natural selection has begun pushing the two green forms of walking sticks down separate paths that could lead to the formation of two new species, the team found that a third melanistic, or brown variation of T. cristinae, appears to be thwarting the process. The brown version is known to successfully camouflage itself among the stems of both shrub species inhabited by its green brethren.

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 walking stick species also is favoured by natural selection because it has a slight advantage in mate selection and a stronger resistance to fungal infections than its green counterparts.

The paper’s second author Samuel Flaxman said: “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.”

Adding his thoughts, Aaron Comeault, a Colorado University graduate student and lead study author who conducted the research while at the University of Sheffield, said: “This movement of genes between environments slows down the genetic divergence of these stick insect populations, impeding the formation of new species.”

A paper on the subject appeared in a recent issue of the journal Current Biology.