Abstract
Many decades of scientific investigation have proved the role of selective pressure in Homo Sapiens at least at the level of individual genes or loci. Nevertheless, there are examples of polygenic traits that are bound to be under selection, but studies devoted to apply population genetics methods to unveil such occurrence are still lacking. Stature provides a relevant example of well-studied polygenic trait for which is now available a genome-wide association study which has identified the genes involved in this trait, and which is known to be under selection. We studied the behavior of FST in a simulated toy model to detect population differentiation on a generic polygenic phenotype under selection. The simulations showed that the set of alleles involved in the trait has a higher mean FST value than those undergoing genetic drift only. In view of this we looked for an increase in the mean FST value of the 180 variants associated to human height. For this set of alleles we found FST to be significantly higher than the genomic background (p = 0.0356). On the basis of a statistical analysis we excluded that the increase was just due to the presence of outliers. We also proved as marginal the role played by local adaptation phenomena, even on different phenotypes in linkage disequilibrium with genetic variants involved in height. The increase of FST for the set of alleles involved in a polygenic trait seems to provide an example of symmetry breaking phenomenon concerning the population differentiation. The splitting in the allele frequencies would be driven by the initial conditions in the population dynamics which are stochastically modified by events like drift, bottlenecks, etc, and other stochastic events like the born of new mutations.
Highlights
One of the greatest challenges in modern biology is to understand how selection has driven human evolution
To analyze the behavior of FST on a polygenic trait, we started by simulating the action of a polygenic stabilizing selection pressure in a very simple model
Such a model relies on the following simplifying assumptions: (i) each contributing allele has small and relatively equal additive effects, without neither environmental influences nor non-linear effects, (ii) individual fitness is given by a bell-shaped curve where the maximum is achieved by individual owning only a part of the advantageous alleles, (iii) no migration or demographic events affect the populations
Summary
One of the greatest challenges in modern biology is to understand how selection has driven human evolution. A worldwide distributed sexual selection seems to represent an overall reasonable scenario, even though local adaptation phenomena that could favor particular heights and body shapes in particular environmental conditions cannot be excluded. In this work we explored, in a simple simulated model, the behavior of FST, a widely used method for detecting selection based on populations differentiation, on a generic polygenic trait.
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