Why salmon return (or don’t return) to their home stream?

Is it adaptation or natural selection? The evolutionary biologists S. J. Gould and R. C. Lewontin (1979) proposed a very plain definition for the term “adaptation”: “a good fit of organisms to their environment”. Although simple, the concept says enough and their assay became an influential and controversial work. Gould and Lewontin introduced the analogy of the architectural spandrel to a “biological spandrel” meaning a phenotypic characteristic that is considered to have developed during evolution as a side-effect of an adaptation, rather than arising from natural selection. The central idea is that organisms not only occupy an environment because (Darwinian) natural selection allows then to do so, but they can also develop the traits, as by-products of environmental demands, that will make it possible for them to fit and occupy that environment. Because fitness of an individual and a population is a function of phenotypes matching a given environment, adaptation to local environments, within the margins of an adaptive zone, are possible. At the same time, the local adaptation of a population to a particular environment is revealed when the average performance, or fitness of the local individuals, exceeds that of immigrants or those individuals originating from foreign environments. This in turn results in reproductive isolation of populations that live (and fit) different environments and are locally adapted to differential selective demands. In salmonid population genetics, local adaptations, along with restricted gene flow, provide the opportunities for population-specific differences to evolve. This would be the reason for poor performance of transplants, the difference in performance of wild fish relative to hatchery fish, the degree of homing accuracy in salmonids, and in some cases the resistance to pathogenic challenges, such as the inherited resistance of certain Baltic salmon populations to the external parasite Gyrodactilus salaris (Mo 1994). The fact that most attempts to transplant salmonid species to other locations have failed, seems to support the notion of strong local adaptation of salmonids to specific environmental conditions. However, numerous successful translocations of salmonid populations outside their native habitat challenge the strength of the local adaptation hypothesis. Some examples are: the introduction of non-anadromous sockeye salmon to many lakes in Canada, the wide transplantation of rainbow trout all over the continents, the establishment of pink salmon in the Great Lakes and of chinook salmon in New Zealand and certain rivers in southern Chile. However, Taylor (1991) suggests that the apparent flexibility of such introduced populations may be the result of adaptation to highly variable environments through natural selection for phenotypic plasticity. Certainly, Gould and Lewontin’s variation to the traditionally accepted concept of Darwinian natural selection as the main evolutionary mechanism is not exempt from criticism and controversy. However, while proposing an adaptacionist perspective, apparently contrary to Darwin’s selectionism, they state in their paper that they “support Darwin's own pluralistic approach to identifying the agents of evolutionary change”. References: Gould SJ, Lewontin RC (1979). The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proc. R. Soc. Lond., B, Biol. Sci. 205 (1161): 581–598. Mo TA (1994) Status of Gyrodactylus salaris problems and research in Norway. In: Pike AW, Lewis JW (eds) Parasitic diseases of fish, pp 43–58. Samara Publishing, London. Taylor EB (1991) A review of local adaptation in Salmonidae, with particular reference to Pacific and Atlantic salmon. Aquaculture 98:185-207.