Population differentiation in Pacific salmons: local adaptation genetic drift, or the environment?

Abstract

Morphological, behavioral, and life-history differences between Pacific salmon (Oncorhynchus spp.) populations are commonly thought to reflect local adaptation, and it is likewise common to assume that salmon populations separated by small distances are locally adapted. Two alternatives to local adaptation exist: random genetic differentiation owing to genetic drift and founder events, and genetic homogeneity among populations, in which differences reflect differential trait expression in differing environments. Population genetics theory and simulations suggest that both alternatives are possible. With selectively neutral alleles, genetic drift can result in random differentiation despite many strays per generation. Even weak selection can prevent genetic drift in stable populations; however, founder effects can result in random differentiation despite selective pressures. Overlapping generations reduce the potential for random differentiation. Genetic homogeneity can occur despite differences in selective regimes when straying rates are high. In sum, localized differences in selection should not always result in local adaptation. Local adaptation is favored when population sizes are large and stable, selection is consistent over large areas, selective differentials are large, and straying rates are neither too high nor too low. Consideration of alternatives to adaptation would improve both biological research and salmon conservation efforts.

Population specific smolt development, migration and maturity schedules in Atlantic salmon in a natural river environment

“…Differences were found 425 in smolt size, and duration of the smolt migration and in the overall lifetime success, 426 which was only 18% of the native wild population. Contrary to the suggestion of 427 Adkison (1994) that local adaptation would only theoretically operate at large 428 regional scale, the findings of these common garden experiments indicate that local 429 adaptation, with substantial fitness consequences, can occur between rivers separated 430 by no more than 60 km. The fact that this was a common garden experiment, and thus 431 the same environment, excludes the possibility that the observed differences are the 432 result of differential trait expression of similar genotypes.…”

“…It is also considered that salmon populations separated by small 64 distances are locally adapted (Youngson et al, 2003). Adkison (1994) suggests, 65 however, that there are definite limits to local adaptation, that local adaptation 66 operates over large rather than small areas and that that the trait differences observed, 67 particularly at local or regional scales, result from differential gene expression in 68 different environments. 69…”

Climate change and its implications for Australia's freshwater fish

“…Understanding the importance of genetic variability in determining how species will respond to climate change is critical to developing strategies to conserve the diversity of fish species in Australian freshwaters (sensu Adkison 1995). If trait variation is locally adaptive or dispersal potential low, then a range of populations would need to be protected to ensure that underlying genetic diversity is conserved (Adkison 1995;Hughes 2007).…”

“…If trait variation is locally adaptive or dispersal potential low, then a range of populations would need to be protected to ensure that underlying genetic diversity is conserved (Adkison 1995;Hughes 2007). Conversely, if trait variation is plastic, then conservation management should focus on preserving a variety of habitats to ensure that a range of life histories continues to be expressed in wild populations (Beechie et al 2006).…”

Disruption of historical patterns of isolation by distance in coastal steelhead

“…Concurrently, billions of hatchery-bred steelhead and rainbow trout have been released into watersheds throughout the native range as mitigation and to enhance fisheries. The importance of local adaptation in salmonid fishes is well established (Taylor 1991;Adkison 1995;Martínez et al in press) and such adaptation can be disrupted by dam construction or other habitat modifications. In addition, hatchery fish often have lower fitness than naturally spawning individuals (McLean et al 2003;Araki et al 2008;Caroffino et al 2008).…”