Résumé
Speciation, the evolution of reproductive isolation amongst populations,
is continuous, complex, and involves multiple, interacting barriers. Until
it is complete, the effects of this process vary along the genome and can
lead to a heterogeneous genomic landscape with peaks and troughs of
differentiation and divergence. When gene flow occurs during speciation,
barriers restricting migration locally in the genome lead to patterns of
heterogeneity. However, genomic heterogeneity can also be produced or
modified by variation in factors such as background selection and
selective sweeps, recombination- and mutation-rate variation, and
heterogeneous gene density. Extracting the effects of gene flow, divergent
selection and reproductive isolation from such modifying factors presents
a major challenge to speciation genomics. We argue one of the principal
aims of the field is to identify the barrier loci involved in limiting
gene flow. We first summarise the expected signatures of selection at
barrier loci, at the genomic regions linked to them and across the entire
genome. We then discuss the modifying factors that complicate the
interpretation of the observed genomic landscape. Finally, we end with a
roadmap for future speciation research; a proposal for how to account for
these modifying factors and to progress towards understanding the nature
of barrier loci. Despite the difficulties of interpreting empirical data,
we argue that the availability of promising technical and analytical
methods will shed further light on the important roles that gene flow and
divergent selection have in shaping the genomic landscape of speciation.