Barrier Loci and Evolution

Abstract

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‐wide molecular methods and analytical tools. Barrier loci give important insights into the genetic basis of species differences and the process and rates of speciation.

Conservation Mitonuclear Replacement: Facilitated mitochondrial adaptation for a changing world

“…It has been argued that mitonuclear discordance, either natural or anthropogenic, does not disrupt the identity of a species and the integrity of its lineage for the purposes of conservation (Novak, 2018 ). However, barrier loci—those that determine reproductive isolation between populations—have an outsized role to play in evolutionary processes (Elmer, 2019 ; Ravinet et al., 2017 ). While the impact on the exterior phenotype from changing a mitonuclear locus may be small, the loss of reproductive isolation from the removal of a barrier locus might have profound evolutionary consequences down the line.…”

The role of recombination landscape in species hybridisation and speciation

“…Selection against introgression in these regions could also stem from linkage dragthe segregation of weakly deleterious mutations in populations that have smaller population sizes (Harris and Nielsen, 2016;Juric et al, 2016). Introgression is likely suppressed around loci responsible for reproductive isolation (Nosil, 2012;Ravinet et al, 2017;Elmer, 2019), as well as islands of differentiation (Feder and Nosil, 2010;Flaxman et al, 2012;Flaxman et al, 2013). Introgression levels could also be impeded due to nucleo-cytoplasmic interactions.…”

Patterns of genomic divergence and introgression between Japanese stickleback species with overlapping breeding habitats

“…What are the potential consequences of the breakdown of habitat isolation in this scenario? One possible outcome would be speciation reversal, where two species collapse into a single species by extensive hybridization (Elmer, 2019; Seehausen, Takimoto, Roy, & Jokela, 2008; Taylor et al., 2006). A second possibility is that a level of reproductive isolation is maintained in the face of ongoing gene flow.…”

“…A second possibility is that a level of reproductive isolation is maintained in the face of ongoing gene flow. In this case, introgression is likely to be suppressed near loci involved in reproductive isolation (Elmer, 2019; Nosil, 2012; Ravinet et al., 2017), whereas alleles that are beneficial to both populations (Anderson et al., 2009; Pardo‐Diaz et al., 2012; Whitney, Randell, & Rieseberg, 2006) or those with heterozygote advantage are likely to introgress elsewhere in the genome (Kim, Huber, & Lohmueller, 2018; Nadachowska‐Brzyska, ZieliŃSki, Radwan, & Babik, 2012; Wolfe et al., 2019). Finally, if reproductive isolation is strong enough, breakdown of habitat isolation would neither influence the hybridization rates nor erode overall genomic divergence between species.…”