title: Evolutionary forces shaping genomic islands of population differentiation in humans.
abstract: BACKGROUND
  Levels of differentiation among populations depend both on demographic and selective factors: genetic drift and local adaptation increase population differentiation, which is eroded by gene flow and balancing selection. We describe here the genomic distribution and the properties of genomic regions with unusually high and low levels of population differentiation in humans to assess the influence of selective and neutral processes on human genetic structure.
  
  
  METHODS
  Individual SNPs of the Human Genome Diversity Panel (HGDP) showing significantly high or low levels of population differentiation were detected under a hierarchical-island model (HIM). A Hidden Markov Model allowed us to detect genomic regions or islands of high or low population differentiation.
  
  
  RESULTS
  Under the HIM, only 1.5% of all SNPs are significant at the 1% level, but their genomic spatial distribution is significantly non-random. We find evidence that local adaptation shaped high-differentiation islands, as they are enriched for non-synonymous SNPs and overlap with previously identified candidate regions for positive selection. Moreover there is a negative relationship between the size of islands and recombination rate, which is stronger for islands overlapping with genes. Gene ontology analysis supports the role of diet as a major selective pressure in those highly differentiated islands. Low-differentiation islands are also enriched for non-synonymous SNPs, and contain an overly high proportion of genes belonging to the 'Oncogenesis' biological process.
  
  
  CONCLUSIONS
  Even though selection seems to be acting in shaping islands of high population differentiation, neutral demographic processes might have promoted the appearance of some genomic islands since i) as much as 20% of islands are in non-genic regions ii) these non-genic islands are on average two times shorter than genic islands, suggesting a more rapid erosion by recombination, and iii) most loci are strongly differentiated between Africans and non-Africans, a result consistent with known human demographic history.
authors.affiliation: Computational and Molecular Population Genetics Lab, Institute of Ecology and Evolution, University of Bern, 3012 Bern, Switzerland. tamara.hofer@iee.unibe.ch
authors.forename: Tamara
authors.initials: T
authors.lastname: Hofer
authors.forename: Matthieu
authors.initials: M
authors.lastname: Foll
authors.forename: Laurent
authors.initials: L
authors.lastname: Excoffier
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics; D003198:Computer Simulation; D019143:Evolution, Molecular; D040961:Genetic Drift; D044404:Genomic Islands; D006801:Humans; D008957:Models, Genetic; D020641:Polymorphism, Single Nucleotide; D011995:Recombination, Genetic; D012641:Selection, Genetic