Gene flow and Population Fitness

Fitness effects of gene flow (both advantageous and deleterious) have garnered plenty of recent press and scientific exploration. At the population level, the concepts and consequences are notoriously familiar. In the context of immigration, they reduce to existing genetic variation, and new variation introduced into the recipient “sink” population, or conversely, homogenizing effects, and loss of overall biodiversity at the species level. Emigration on the other hand (gene flow to a new environment) could result in local adaptation, and eventual speciation, or loss of genomic diversity due to pervasive inbreeding, and eventual extinction. Three recent manuscripts attempt to summarize/study these concepts, and provide neat springboards for future explorations

• Genetic rescue to the rescue – Whiteley et al. (2015) TREE

Genetic Rescue (GR) can be advantageous to small inbred populations in many ways, due to increased genetic diversity introduced into the “sink” population, often increasing fitness of the population, and its ability to evolve adaptively. But the central debate in the efficacy of genetic rescue lies in whether it delivers on its promise above, or results in what’s known as “outbreeding depression” – the overall reduction of genetic diversity (at the species level), leading to more homogenous populations with reduced diversity. Whiteley et al. (2015) in this excellent TREE review assess numerous studies that have measured population fitness in the context of genetic rescue in several taxa. They also provide an excellent review of the state of the science in utilizing genetic rescue for conservation efforts.

GR may not save imperiled populations over the long term (ultimately, sufficient habitat is required for that), but recent results show that GR can buy time by improving their fitness and increasing population sizes in the short term.
 

• Genetic rescue of small inbred populations: meta-analysis reveals large and consistent benefits of gene flow  – Frankham (2015), Molecular Ecology

“Continuing” the study above, many species are often isolated, and subject to acute inbreeding (depression), and hence loss of genetic diversity with heightened extinction risks. Genetic Rescue is thus just one generation of outcrossing, to metaphorically rescue the genetic diversity of an otherwise inbred, mostly homozygous population. However, genetic rescue can be implemented in many forms – (a) multiple augmentative gene flow events into the inbreeding population, (b) one large outcrossing event, (c) dispersal of the inbreeding population into an outbreeding population, etc. Genetic rescue is also logistically difficult. Frankham conducted a meta-analysis using data-sets collected from the literature of 156 species that were noted to be inbred, with low genetic diversity, with measured fitness data, and had comparable data from outbreeding populations of the same species. He computed ratios of mean fitness in the outbred versus the inbred population (genetic rescue ratio), and used these in multivariate regression models to identify predictor variables for genetic rescue. 145 out of the 156 species showed that the fitness effects due to gene flow into the inbreeding population were beneficial (2 equal, and 9 deleterious), with an average of 57.8% increase in composite fitness of the inbred population due to outcrossing. Frankham also has numerous recommendations for situations in which genetic rescue should be attempted, including assessing the risk of outbreeding depression, magnitude of maternal and zygotic inbreeding coefficients, environmental regimes of the inbreeding population, and monitoring for the need of augmented gene flow.

The limited use of augmented gene flow in conservation settings is not justified scientifically, given the result of this study.

• Effects of dispersal plasticity on population divergence and speciation –  Arendt (2015) Heredity

On the flip side, in the context of emigration to a new habitat, how does dispersal itself behave as a plastic phenotype? In other words, the decision on whether to emigrate or not may depend on numerous factors, including productivity, parasite load, and social context. Arendt (2015) describes numerous examples of plastic dispersal (to put this into context, Genetic rescue from the above 2 studies is “unconditional”, and hence not plastic). Arendt describes plasticity in (a) dispersal morphs of gametes – eg. different sized seeds, stem elongation that allows for differential dispersal, (b) settlement regime – phenotype dependent, semblance to known environments, quality of environment. However, emigration can just as well possess contrasting effects – dispersal, local inbreeding, lowers genetic diversity within, but increases between populations. Alternately, dispersal and subsequent gene flow due to hybridization (as above) could result in increase in genetic diversity, and promote adaptation to local environment. Arendt (2015) also discusses the population genetic consequences of dispersal (plastic and non-plastic) due to habitat choice with several examples from a variety of taxa.

Different theoretical models seem to reach different conclusions about whether plasticity in the decision to disperse increases or decreases population divergence relative to random dispersal.

As all three manuscripts put it, it’s an exciting time to be studying the genomic context of local adaptation and differentiation in the face of immigratory, or emigratory gene flow.
References
Whiteley, Andrew R., et al. “Genetic rescue to the rescue.” Trends in ecology & evolution 30.1 (2015): 42-49. DOI: http://dx.doi.org/10.1016/j.tree.2014.10.009
Frankham, Richard. “Genetic rescue of small inbred populations: meta‐analysis reveals large and consistent benefits of gene flow.” Molecular ecology (2015). DOI: http://dx.doi.org/10.1111/mec.13139
Arendt, J. D. “Effects of dispersal plasticity on population divergence and speciation.” Heredity (2015). DOI: http://dx.doi.org/10.1038/hdy.2015.21