ASSORTATIVE MATING AND THE ADAPTIVE LANDSCAPE

Assortative mating at loci under recent natural selection in humans

Genetic correlation between mates at specific loci can greatly alter the evolutionary trajectory of a species. Genetic assortative mating has been documented in humans, but its existence beyond population stratification (shared ancestry) has been a matter of controversy. Here, we develop a method to measure assortative mating across the genome at 1,044,854 single-nucleotide polymorphisms (SNPs), controlling for population stratification and cohort-specific cryptic relatedness. Using data on 1683 human couples from two data sources, we find evidence for both assortative and disassortative mating at specific, discernible loci throughout the entire genome. Then, using the composite of multiple signals (CMS) score, we also show that the group of SNPs exhibiting the most assortativity has been under stronger recent positive selection. Simulations using realistic inputs confirm that assortative mating might indeed affect changes in allele frequency over time. These results suggest that genetic assortative mating may be speeding up evolution in humans.

With a little help from my friends: cooperation can accelerate the rate of adaptive valley crossing

Background

Natural selection favors changes that lead to genotypes possessing high fitness. A conflict arises when several mutations are required for adaptation, but each mutation is separately deleterious. The process of a population evolving from a genotype encoding for a local fitness maximum to a higher fitness genotype is termed an adaptive peak shift.

Results

Here we suggest cooperative behavior as a factor that can facilitate adaptive peak shifts. We model cooperation in a public goods scenario, wherein each individual contributes resources that are later equally redistributed among all cooperating individuals. We use mathematical modeling and stochastic simulations to study the effect of cooperation on peak shifts in both panmictic and structured populations. Our results show that cooperation can substantially affect the rate of complex adaptation. Furthermore, we show that cooperation increases the population diversity throughout the peak shift process, thus increasing the robustness of the population to sudden environmental changes.

Conclusions

We provide a new explanation to adaptive valley crossing in natural populations and suggest that the long term evolution of a species depends on its social behavior.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0983-2) contains supplementary material, which is available to authorized users.

Assortative pairing in Ixodes ricinus (Acari: Ixodidae), the European vector of Lyme borreliosis

In sexual organisms, the way in which gametes associate can greatly influence the maintenance of genetic variation, the structure of this variation in space, and ultimately organismal evolution. Based on patterns of genetic structure previously found, we explicitly tested whether adults of the sheep tick Ixodes ricinus pair according to their genetic relatedness. We sampled tick pairs from the vegetation in four natural populations and genotyped individual ticks at seven microsatellite loci. Based on this data, we observed highly significant assortative mating in two of the four locations, a pattern that could not be accounted for by a spatial autocorrelation in the distribution of related ticks. One explanation for these observations may be the existence of local host associations that develop independently in different populations. Assortative mating in I. ricinus will have clear consequences for its population dynamics and, through processes of adaptation and transmission, may significantly alter the epidemiological patterns of the pathogens it carries, including the Lyme disease agent Borrelia burgdorferi s.l. Future tests will now be required to examine the mechanisms leading to this pattern and its epidemiological consequences.

No place like home: competition, dispersal and complex adaptation

In many groups of organisms the location of settling is determined by competition, and fitter individuals tend to settle closer to their natal territory than less fit ones. In this work we study the implications of this phenomenon to the problem of adaptation and speciation on a rugged adaptive landscape. One consequence of fitness-associated dispersal (FAD) is that individuals with high fitness are more likely to experience inbreeding, especially with other fit individuals. Another consequence is that when dispersal is costly, the less fit individuals are more likely to pay the cost. When a rare and advantageous allelic combination appears, FAD may increase its chances to spread in the population. In a two-locus two-alleles model with negative epistasis, we find that FAD significantly shortens the waiting time for an adaptive peak shift in comparison with random dispersal.