Parasites can cause selection against migrants following dispersal between environments

1. The potential for selection against migrants to promote population divergence and speciation is well established in theory, yet there has been relatively little empirical work that has explicitly considered selection against migrants as a form of reproductive barrier, and its importance in the accumulation of reproductive isolation between populations has been overlooked until recently. 2. Parasites often differ between environments and can be an important source of selection on hosts, yet their contribution to population divergence in general, and selection against migrants in particular, is poorly understood. 3. Selection against migrants might be reduced if organisms escape parasitism when they disperse (natural enemy release). Alternatively, parasites could increase selection against migrants if, when they disperse, organisms encounter parasites to which they are poorly adapted. 4. Here we test experimentally the contribution that parasites could make to selection against migrants in the adaptive radiation of three-spined sticklebacks. These fish have repeatedly colonized freshwater environments from marine ones, and this has repeatedly lead to rapid speciation. 5. We use transplant experiments of lab-raised fish to simulate dispersal, and antihelminthic treatment to show that ancestral-type marine sticklebacks contract higher burdens of novel parasites when introduced to freshwater, than in saltwater, and suffer a growth cost as a direct result. 6. Susceptibility to parasites and their detrimental effect is less in derived, freshwater fish from evolutionarily young populations, possibly as a result of selection for resistance. 7. Our results support a role for parasites in selection against migrants and population diversification. They do not support the hypothesis of ‘natural enemy release’.