Genetic population structure is mainly shaped by distance effects.
European settlers introduced several animals to Australia for transportation or farming purposes, while others were brought in as pets or for hunting. From the 1850s onward, Common Starlings (Sturnus vulgaris) were released in several locations, such as Melbourne, Brisbane and Adelaide. Although they lost their ability to migrate, these birds managed to get a foothold and spread across the island. The population dynamics during this invasion shaped the current genetic make-up of Common Starlings in Australia.
It is possible that the introduced populations already differed genetically and diverged even more during the invasion. This divergence can follow a neutral path of isolation-by-distance where neighboring populations continue to exchange gene while distant ones accumulate genetic differences. Or perhaps different populations adapted to local conditions in distinct environments, resulting in isolation-by-environment. A recent study in the journal Molecular Ecology tested these patterns using 568 starling samples from 24 localities across Australia.
The genetic analyses uncovered two main clusters that correspond to southern Australia (from Western Australia to Tasmania) and northern Australia (New South Wales and southern Queensland). These two groups are separated by an extremely arid region between Victoria and New South Wales. Interestingly, the Great Dividing Range – a mountain range in the east – did not have a big effect on the population genetic structure, suggesting that Common Starlings can cross this mountainous area. If you are not familiar with the geography of Australia, you can check out the map below.
A match between introduction site and genetic ancestry indicated that the genetic make-up of the founder populations partly influenced the present-day patterns. However, the main process shaping the genetic population structure of the Common Starling appears to be isolation-by-distance. As these birds spread across Australia, gene flow between distant populations diminished, giving rise to the correlation between genetic and geographical distance we see today.
The authors reported that isolation-by-environment was not significantly correlated with genetic differentiation. This finding can be explained by phenotypic plasticity (i.e. one genotype produces more than one phenotype when exposed to different environments) or by local adaptation driven by a small number of genetic loci. The latter explanation is supported by further analyses in which the researchers tested for genetic variants under positive selection. This search uncovered “several hundred unique candidate loci (375) for local adaptation” and “a total of 25 proteins were identified”. These candidate loci were associated with differences in aridity, precipitation and temperature, suggesting that these environmental factors may be important in driving adaptive variation across the Common Starling’s invasive range.
The paper discusses a few of these candidate genes, such as CACNA1C which plays a role in behavioral plasticity (an important trait for an invading species). However, I always remain reluctant to tell “Just So Stories“. The uncovered genes are interesting starting points for further research, but not the final story. Moreover, the authors also write that “many of the loci were reported for only one of the exploratory methods, and all the annotated proteins identified were unique to their identification method.” More analyses are thus needed to understand the detailed genetic consequences of the Common Starling’s conquest across Australia.
Stuart, K. C., Cardilini, A. P., Cassey, P., Richardson, M. F., Sherwin, W. B., Rollins, L. A., & Sherman, C. D. (2021). Signatures of selection in a recent invasion reveal adaptive divergence in a highly vagile invasive species. Molecular Ecology, 30(6), 1419-1434.
Featured image: Common Starling (Sturnus vulgaris) © Marie-Lan Nguyen | Wikimedia Commons