Hawaiian exotics show genetic drift

An invasive bird population in Hawaii provides a window into genetic drift – evolutionary changes typically seen over millennia.

Understanding genetic drift, or the random mutation of genes over time, is vital for the conservation of critically endangered species with small populations, such as the Hawaiian honeycreeper, the I’iwi (Vestiaria coccinea).

The native birds of Hawaii are experiencing one of the worst rates of extinction in the world, due to a combination of habitat loss, introduced predators, climate change and diseases such as avian malaria.  

When an entire avian fauna is threatened, researchers tend to focus upon imperilled native species. 

But one avian ecologist, Jeffrey Foster, from the Northern Arizona University in the US, realised that a small songbird’s recent introduction to Hawaii provided a unique opportunity to study genetic changes in a relatively short time scale as the birds colonised Hawaii from island to island.

“Japanese Bush-Warblers first arrived on the Big Island when I was living there in the 1990s,” says Foster. “The idea that one could study this invasion in progress totally blew my mind.”

The bush- warbler was introduced to the large island of Oahu in 1929, then subsequently colonised smaller, outer islands by the mid 1990s. Foster and his colleagues at measured changes in genetic diversity in the bush-warblers by comparing blood and muscle samples from 147 birds living on five islands between 2003 and 2005.

Population genetic theory predicts that genetic drift in small populations eventually produces loss of diversity. Foster’s study, published in the journal Auk: Ornithological Advances, found that the genetic diversity of the introduced bush-warblers followed this prediction. The genetic richness, measured by the presence of gene variants known as alleles, was highest at the point of introduction, where there were more founding birds, and lowest on the islands farthest from Oahu, which had smaller populations.

There were, however, some variations to this pattern. The birds on one island, Kauai, had nearly equal genetic diversity to their presumed source, which may indicate that multiple individuals colonised it.   

The finding also illustrated one of the principle outcomes of genetic drift.

“The birds on Kauai, the island just west of Oahu, appear to be more distinct than those birds on islands east of Oahu,” says Foster, “suggesting that over time birds on the respective islands may continue to diverge genetically.”

Silvereyes (from the Zosterops genus ) are an oft-used species for studying genetic diversity changes in a population when islands are colonised, and testing the predictions of genetic theory. Silvereyes move in flocks, and so the founding populations on remote islands are made up of numerous individuals with their component genetic diversity.

In contrast, the Japanese bush-warbler is a solitary bird, thus the colonisation process on islands consists of just a few individuals at a time. The study of bush-warblers enables researchers to gain an insight into the process of genetic drift that occurs in smaller populations. 

Foster says the species is just one example of inter-island colonisations by introduced birds in the Hawaiian Islands, and suggests that this study system can serve as a model for contemporary evolution.