Barrier Loci and Evolution


The evolution and maintenance of new species is a major question in evolutionary biology. During speciation, the genomes of individuals in the splitting populations do not diverge at an equal rate: some parts diverge more quickly while other parts remain more similar. The organisation of genetic diversity and differentiation in the genome is important to how speciation happens and reflective of speciation processes. Barrier loci are parts of the genome that contribute to a restriction of gene flow at that genomic region and are most diverged between young species. The size and distribution of barrier loci is influenced by evolutionary processes such as demography, selection, and gene flow, and by properties of the genome such as mutation rate, gene density, and recombination. Speciation research is in an exciting new era of identifying barrier loci and their distribution in the genome and testing what they reveal about the processes of evolution.

Key Concepts

  • As populations become more different with divergence and speciation, the rate and pattern at which genetic differences accumulate are not the same across the genome.
  • Barrier loci are those loci that resist homogenisation with the other genome when gene flow occurs between diverging species, and consequently, they are the most different parts of the genome between divergent populations.
  • Genomic divergence is affected by many properties of the genome and of the species being studied, and therefore the process of divergence and the effects on barrier loci are complex and variable.
  • Barrier loci form due to a reduction in gene flow at particular genomic regions. Therefore, they are a function of both population factors (demography, time since divergence, and strength and target of selection) and properties of the genome (recombination rate, mutation rate, gene density, and the organisation of relevant loci).
  • Barrier loci are inferred with genome‚Äźwide molecular methods and analytical tools.
  • Barrier loci give important insights into the genetic basis of species differences and the process and rates of speciation.

Keywords: speciation; genomics; population genetics; gene flow; reproductive isolation

Figure 1. The genomic landscape of barrier loci: expectations under sympatric divergence (a) vs secondary contact (b). These are only schematics. As divergence happens with high levels of gene flow (a) or with periods of isolation (b) (gene flow shown in blue arrows), speciation occurs for example from dark grey medium‐sized ancestral fish species to light grey thin‐bodied and black high‐bodied contemporary fish species. The background genome will be more divergent (higher Fst or genetic differentiation) under secondary contact and therefore make it more difficult to accurately infer functional barrier loci.


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Elmer, Kathryn R(Feb 2019) Barrier Loci and Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0028138]