A common consequence of habitat loss and fragmentation is decreased genetic connectivity, or gene flow, among populations. When populations that were previously connected become restricted to small and isolated habitat fragments, they may be vulnerable to the negative effects of inbreeding depression. This contributes to an extinction vortex, ultimately leading to population demise.
A potentially powerful management strategy to mitigate these effects is human-assisted migration for the purpose of genetic rescue, i.e. an increase in population growth due to the introduction of new genetic material. There have been a handful of iconic examples of successful genetic rescue in conservation, leading to population rebounds in imperiled species like Florida panthers, Bighorn sheep, and greater prairie chickens. However, many uncertainties remain in predicting the consequences of gene flow, and assisted migration is still considered a risky strategy, typically only used as a last resort.
Unfortunately, it’s not possible to carry out the gene flow experiments needed to improve understanding of genetic rescue in endangered species like Florida panthers. Instead, we turned to a model system in ecology and evolution – Trinidadian guppies – to do a high resolution study of the effects of gene flow on small and isolated populations in the wild, in real time and over multiple generations.
Through individual tattooing and gaining genetic information from thousands of guppies, we were able to track survival and reproduction for six generations following the onset of gene flow from a different source population. We observed large increases in population size and found that, on average, hybrid guppies lived longer and had more surviving offspring.
Importantly, we also showed that traits and genes thought to be important in the local environment were maintained in the face of high gene flow. In sum, the small guppy populations we studied experienced genetic rescue, with no detectable costs from the added variation.
These results, and several other new studies on genetic rescue, support a growing call for a paradigm shift in how to manage small and fragmented populations. Rather than sticking to a default policy of inaction, the argument is that the default should be to evaluate restoration of gene flow to small and isolated populations.
This is timely for two reasons: (1) Thousands of small populations are at the brink of extinction, in part due to genetic factors associated with inbreeding depression and a lack of adaptive variation; and (2) Only very recently have we reached the point where genomic approaches are truly amenable for large-scale studies of wild populations in terms of cost and feasibility. Studying the genomes of organisms alive today provides a window into historical processes that have previously shaped them. This then informs key questions about which populations stand to benefit the most from assisted migration, and which populations and individuals to target as migrants.
Protecting and restoring habitat to facilitate natural connectivity still remains the highest priority. But genetic rescue has emerged as an attainable way to provide an often dramatic demographic boost, and to buy time for small and isolated populations that may otherwise be headed for extinction.
Fitzpatrick, S.W., Bradburd, G.S., Kremer, C.T., Salerno, P.E., Angeloni, L.M. and Funk, W.C. 2020. Genomic and fitness consequences of genetic rescue in wild populations. Current Biology 30:517–522.e5.
Ralls, K., Ballou, J.D., Dudash, M.R., Eldridge, M.D., Fenster, C.B., Lacy, R.C., Sunnucks, P. and Frankham, R. 2018. Call for a paradigm shift in the genetic management of fragmented populations. Conservation Letters 11:e12412.