Island Life: Evolution in splendid isolation

From the time of Charles Darwin’s voyage of the Beagle in the 1830s, the study of island-dwelling forms has played a central role in the development of evolutionary and conservation biology principles. Islands harbour relatively few species; successful colonisation can be limited by dispersal capacity, the ability to exploit a new environment and chance. Those species that succeed live in a community with fewer competitors, predators and parasites, and as such, novel selective pressures can rapidly generate new evolutionary outcomes in ‘splendid isolation’. 

However with diversification potential comes the vagaries of chance events such as extreme weather or the introduction of a new disease. My group and I exploit island systems to study the underlying evolutionary processes of divergence and to understand how small and often isolated populations persist and diversify.

An interesting feature of island fauna is that we often see repeated patterns of change in a range of characteristics that form the so-called ‘insular syndrome’. Repeated changes are seen in behavior,such as increased tameness; ecology, such as having a wider ecological niche; morphology, such as changes in body size; life history,such as having fewer offspring but living longer; and population-level characteristics, such as having high population densities. 

We are interested in understanding why island living produces these repeated patterns. Silvereyes, of the white-eye bird family Zosteropidae, show several features of the insular syndrome. The species is a prolific natural coloniser of southwest Pacific islands, conveniently producing a suite of island populations of different ages from very recent (~100 years) to very ancient (>100 thousand years). 

We use these island colonists to investigate the rate and patterns of change and have found that colonisation of islands produces rapid evolutionary shifts early on in the colonisation history. Island birds grow more slowly and for longer than mainland birds, given their limited island resources. This strategy is suited to low predator environments that islands provide, where the selective pressure to fledge quickly and avoid predation in the vulnerable nestling phase is removed.    

We are currently characterising the genomic changes that accompany island colonisation in these birds using populations of different ages as proxies for stages of the speciation process.  From this work, we ultimately aim to map phenotypic changes to genomic changes and link evolutionary pattern and processes via genomic and environmental mechanisms.