Local adaptation and admixture

This is a very interesting and well-written paper, and I highly recommend it (it's open access).

The authors deal with the problem of admixture between locally-adapted populations and newly introduced populations. Local adaptation is the occurrence of alleles and allele combinations that are well-suited to the local environment.

Admixture is a double-edged sword: on the one hand, it dilutes locally adapted gene pools by introducing foreign (non-adapted) alleles. On the other, it reduces homozygosity and inbreeding depression. Local adaptation is fitness enhancing, while inbreeding depression is fitness damaging. Whether one or the other wins out is probably case specific, and would depend e.g., on the level of adaptation, as well as the level of inbreeding depression in the population.

The paper does not address humans in particular, but a human example might be instructive. Consider the movement of tropically-adapted people into an arctic village. If inbreeding depression is substantial, then a proportion of the native-native (arctic) offspring would be homozygous for deleterious alleles and would not be able to compete successfully with invaders or native-invader offspring. On the other hand, invader and invader-native hybrids would lack locally adapted genomes (e.g. related to heat production) and would thus be at a disadvantage against functional natives.

The consequences of admixture in introduced populations are also interesting. Such populations lack local adaptation, and they are also often less genetically diverse (as they represent an often small subset of founders drawn from a larger non-native population). Thus, they benefit from admixture doubly: by receiving locally adapted variants, and by increasing their genetic diversity and thus becoming more capable of adaptation (*)

Thus, in considering the process of admixture between native and introduced populations, we must take into account a few factors:

• Inbreeding depression, generally reduced by admixture
• Genetic incompatibilities between divergent gene pools, generally exposed by admixture
• The potential for selection, generally increased by admixture
• The loss of local adaptation, generally reduced by admixture in the native population

The paper's utility is quite broad, and, while the authors do well not to limit themselves to a particular species, the implications for human societies are worth considering.

Proceedings of the Royal Society B doi:10.1098/rspb.2010.1272

Population admixture, biological invasions and the balance between local adaptation and inbreeding depression

Koen J. F. Verhoeven et al.

When previously isolated populations meet and mix, the resulting admixed population can benefit from several genetic advantages, including increased genetic variation, the creation of novel genotypes and the masking of deleterious mutations. These admixture benefits are thought to play an important role in biological invasions. In contrast, populations in their native range often remain differentiated and frequently suffer from inbreeding depression owing to isolation. While the advantages of admixture are evident for introduced populations that experienced recent bottlenecks or that face novel selection pressures, it is less obvious why native range populations do not similarly benefit from admixture. Here we argue that a temporary loss of local adaptation in recent invaders fundamentally alters the fitness consequences of admixture. In native populations, selection against dilution of the locally adapted gene pool inhibits unconstrained admixture and reinforces population isolation, with some level of inbreeding depression as an expected consequence. We show that admixture is selected against despite significant inbreeding depression because the benefits of local adaptation are greater than the cost of inbreeding. In contrast, introduced populations that have not yet established a pattern of local adaptation can freely reap the benefits of admixture. There can be strong selection for admixture because it instantly lifts the inbreeding depression that had built up in isolated parental populations. Recent work in Silene suggests that reduced inbreeding depression associated with post-introduction admixture may contribute to enhanced fitness of invasive populations. We hypothesize that in locally adapted populations, the benefits of local adaptation are balanced against an inbreeding cost that could develop in part owing to the isolating effect of local adaptation itself. The inbreeding cost can be revealed in admixing populations during recent invasions.

Link

(*) Selection can proceed faster at more diverse populations, as selection depends heterozygosity, and is, in fact, the differential survival of one allele over another at a locus.