title: When maladaptive gene flow does not increase selection.
abstract: Populations receiving high maladaptive gene flow are expected to experience strong directional selection-because gene flow pulls mean phenotypes away from local fitness peaks. We tested this prediction by means of a large and replicated mark-recapture study of threespine stickleback (Gasterosteus aculeatus) in two stream populations. One of the populations (outlet) experiences high gene flow from the lake population and its morphology is correspondingly poorly adapted. The other population (inlet) experiences very low gene flow from the lake population and its morphology is correspondingly well adapted. Contrary to the above prediction, selection was not stronger in the outlet than in the inlet, a result that forced us to consider potential reasons for why maladaptive gene flow might not increase selection. Of particular interest, we show by means of a simple population genetic model that maladaptive gene flow can-under reasonable conditions-decrease the strength of directional selection. This outcome occurs when immigrants decrease mean fitness in the resident population, which decreases the strength of selection against maladapted phenotypes. We argue that this previously unrecognized effect of gene flow deserves further attention in theoretical and empirical studies.
authors.affiliation: Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke St. W., Montreal, Quebec, H3A0C4, Canada. gregor.rolshausen@mcgill.ca.
authors.forename: Gregor
authors.initials: G
authors.lastname: Rolshausen
authors.affiliation: Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke St. W., Montreal, Quebec, H3A0C4, Canada.
authors.forename: Shahin
authors.initials: S
authors.lastname: Muttalib
authors.affiliation: Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke St. W., Montreal, Quebec, H3A0C4, Canada.
authors.forename: Renaud
authors.initials: R
authors.lastname: Kaeuffer
authors.affiliation: Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke St. W., Montreal, Quebec, H3A0C4, Canada.
authors.forename: Krista B
authors.initials: KB
authors.lastname: Oke
authors.affiliation: Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke St. W., Montreal, Quebec, H3A0C4, Canada.
authors.forename: Dieta
authors.initials: D
authors.lastname: Hanson
authors.affiliation: Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke St. W., Montreal, Quebec, H3A0C4, Canada.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Hendry
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics; D025041:Animal Migration; D000818:Animals; D001955:British Columbia; D051456:Gene Flow; D005828:Genetics, Population; D045483:Rivers; D012641:Selection, Genetic; D023701:Smegmamorpha / Q000033:anatomy & histology / Q000235:genetics*
keywords: Adaptation; fitness; gene flow; models/simulations; natural; selection