The Evolution of Philopatry and Dispersal Homing Versus Straying in Salmonids

Abstract

Abstract Many animals reproduce at the same site as their parents, a phenomenon called philopatry. In some cases, philopatry can be the simple result of geographic isolation or limited movement ability. In other cases, philopatry can be strong even when migratory movements take individuals far from their natal site. We must therefore seek explanations for philopatry through selective factors that favor return to natal sites rather than non-natal sites. The tendency for philopatry in any population or species is balanced to varying degrees by some dispersal to non-natal sites, with some groups showing high philopatry and low dispersal and others low philopatry and high dispersal. Again, some of this variation may be the result of varying isolation or movement ability, whereas some can only be explained through natural selection. Our goal in the present chapter is to use data from salmonids to evaluate theories for how selection might influence the evolution of philopatry and dispersal. Philopatry can be evaluated at a variety of spatial scales, ranging from precise nest locations to broad geographic areas. We focus on philopatry to spatially discrete locations that are predictably occupied by breeding aggregations. These aggregations often correspond to “populations”: groups of individuals that breed together and are at least partially reproductively isolated from other such groups. By “reproductive isolation,” we here mean that the different groups are not freely interbreeding within or between generations (i.e., not panmictic). Many factors can contribute to reproductive isolation (Dobzhansky 1937; Mayr 1963; Schluter 2000). “Extrinsic” isolation is caused by geographic barriers that severely limit movement between populations. This category does not apply to populations in separate physical locations as long as movement among them remains possible. Extrinsic isolation is of limited interest here because philopatry has been imposed on the organisms rather than evolving in response to selection. “Intrinsic” isolation is caused by features of organisms that limit gene flow between aggregations, even in the absence of strict geo graphic barriers.