Dispersal ability as a driver of genetic structuring and diversification
The dispersal ability of a species has significant effects on population dynamics, as it allows individuals to find suitable patches of habitat to exploit resources. It also has important consequences for the genetic structuring of populations. As described in a recent paper published in the journal Proceedings of the Royal Society B, researchers from the Ecology and Evolution on Islands group have studied these processes together, using data from spider and beetle communities on oceanic islands.
The dispersal ability of a species has significant effects on population dynamics, as it allows individuals to find suitable patches of habitat to exploit resources. Species that disperse well will have greater gene flow between populations, thus favouring genetic homogeneity. However, low dispersal ability reduces this flow, resulting in geographically isolated populations with marked genetic structuring. These populations can potentially evolve independently, giving rise to new species. Although the effects of dispersal ability on genetic structuring or species diversification have been investigated, there are few studies that integrate all three processes.
As described in a recent paper published in the journal Proceedings of the Royal Society B, researchers from the Ecology and Evolution on Islands group have studied these processes together, using data from spider and beetle communities on oceanic islands. First, a standardised sampling of 31 plots in laurel forests in the western Canary Islands was carried out. Genetic structuring was studied using a fragment of these arthropods’ mitochondrial genome as a marker. This revealed that beetles and spiders with lower dispersal ability have greater genetic structuring, explainable by geography.
Furthermore, phylogenetic trees were generated at the genus level to calculate three measures: average divergence, maximum divergence and diversification rate (number of species per genus divided by average divergence). Regarding beetles, higher diversification rates were found for wingless species (low dispersal ability) compared to winged species (high dispersal ability). This suggests a higher rate of species formation in wingless beetles. In addition, these non-winged beetles were found to have a lower mean divergence. That is, they are more recent than winged species. This pattern suggests that the rate of species extinction is also higher in wingless beetles.
In spiders, although the trends were similar to those in beetles, no significant differences between the two types of dispersal ability were found for any phylogenetic measure. These results indicate that dispersal limitation in spiders has less weight in species formation than in beetles. While the juvenile stages of beetles are not very mobile and live hidden in the soil or on plant stems and trunks etc., spider juveniles are much more mobile and can be dispersed passively, even in those species with lower dispersal ability.
In conclusion, a positive linear correlation was found between genetic structuring values and diversification rate. This suggests that population differentiation and diversification are associated, mediated by low dispersal ability. Furthermore, a younger divergence in wingless beetles indicates s a higher rate of extinction in this group. Thus, the overall model suggests that diversification rates appear to be influenced by both species formation and their extinction.
Spider species of the genus Dysdera are considered to be not very dispersive. This is why they have given rise to 47 different species in the Canary Islands. The image (courtesy of Pedro Oromí) shows a specimen of Dysdera verneaui, endemic to the island of Tenerife.