How important is intraspecific genetic admixture to the success of colonising populations?

Genetic admixture of divergent intraspecific lineages is increasingly suspected to have an important role in the success of colonising populations. However, admixture is not a universally beneficial genetic phenomenon. Selection is typically expected to favour locally adapted genotypes and can act against admixed individuals, suggesting that there are some conditions under which admixture will have negative impacts on population fitness. Therefore, it remains unclear how often admixture acts as a true driver of colonisation success. Here, we review the population consequences of admixture and discuss its costs and benefits across a broad spectrum of ecological contexts. We critically evaluate the evidence for a causal role of admixture in successful colonisation, and consider that role more generally in driving population range expansion.