Signatures of local adaptation along environmental gradients in a range-expanding damselfly (Ischnura elegans) (2018) Dudaniec et al., Molecular Ecology http://doi: 10.1111/mec.14709

The Crux

Terrestrial organisms aren’t always stationary entities, they often move around the landscape searching for food, potential mates, or more ideal environments. Over time, these movements may introduce the species into new environments, as some change allows the species to expand their historical range.

An interesting aspect of this shifting of the species range is how the organisms at the edge of the distribution are maladapted to the novel environments, as most of the species will be adapted to conditions at the core of the species range. To overcome this, they must adapt to the new conditions. Successful adaptation is dependent on changes in gene frequencies away from the historical genotypes, with an increase in genes that promote survival in the new habitats. The authors in this study used molecular techniques to identify genes that new environments might select for.

Did you know: Range expansions Organisms typically inhabit a home range, in that a given species is usually found in a given area. When the environment (or the species) changes, this range can then shift or expand. For example, the Inca Dove was adapted to live in human-made pueblo houses in Mexico, but then arrived in Laredo, Texas in 1866. It then continued to expand its range as human settlements created suitable habitat. Currently, Inca Doves have been seen as far north as Kansas and Arkansas, and as far west as southern California.

What They Did

In order to detect local adaptation at the edge of the species range, the authors used the blue-tailed damselfly (Ischnura elegans) as their model organism. This insect’s range extends from northern Spain to the middle of Sweden, and many aspects of its life history and genetics are well studied, making this an ideal species for the question at hand.

The authors used a three-step approach to identify genes under selection from the environment across a latitudinal gradient. First they wanted to identify candidate genes, then they analyzed any relationships between those genes and environmental gradients, and finally they characterized environmental adaptations using prior knowledge of gene function, experimental data, and the associations between allelic turnover (how much the genes changed) and environmental factors.

What Did They Find Out?

The strongest driver of genetic change was maximum summer temperature, meaning that the warmer the location the more the genes of the species in that location were changing. After maximum summer temperature, the strongest drivers of genetic change were annual rainfall and wind speed.

Problems?

The methods used to identify the candidate genes under selection at the range margins of the species are vulnerable to detection of false positives. This means that the genes detected as an outlier are not actually outliers. The authors addressed this issue in their methods by using models that account for false positives, and by also removing any outlier genes identified by the models.

So What?

The authors of this study used a clever experimental design to figure out how species adapt to different environmental conditions as they expand their range. They not only identified genes under selection, but also determined the gradient of genes along a latitudinal gradient. By doing so, they were able to analyze this genetic gradient and compare it to changes in the environment.

Our planet is warming at a scary rate, and life on this planet is going to change. Some species will move from where they used to be into areas where they were once couldn’t survive. Using methods like those in this study, scientists will be able to identify if and where species are adapting to these new environments.