Abstract
Evaluations of human impacts on Earth's ecosystems often ignore evolutionary changes in response to altered selective regimes. Freshwater habitats for Snake River fall Chinook salmon (SRFCS), a threatened species in the US, have been dramatically changed by hydropower development and other watershed modifications. Associated biological changes include a shift in juvenile life history: Historically essentially 100% of juveniles migrated to sea as subyearlings, but a substantial fraction have migrated as yearlings in recent years. In contemplating future management actions for this species should major Snake River dams ever be removed (as many have proposed), it will be important to understand whether evolution is at least partially responsible for this life‐history change. We hypothesized that if this trait is genetically based, parents who migrated to sea as subyearlings should produce faster‐growing offspring that would be more likely to reach a size threshold to migrate to sea in their first year. We tested this with phenotypic data for over 2,600 juvenile SRFCS that were genetically matched to parents of hatchery and natural origin. Three lines of evidence supported our hypothesis: (i) the animal model estimated substantial heritability for juvenile growth rate for three consecutive cohorts; (ii) linear modeling showed an association between juvenile life history of parents and offspring growth rate; and (iii) faster‐growing juveniles migrated at greater speeds, as expected if they were more likely to be heading to sea. Surprisingly, we also found that parents reared a full year in a hatchery produced the fastest growing offspring of all—apparently an example of cross‐generational plasticity associated with artificial propagation. We suggest that SRFCS is an example of a potentially large class of species that can be considered to be “anthro‐evolutionary”—signifying those whose evolutionary trajectories have been profoundly shaped by altered selective regimes in human‐dominated landscapes.
Highlights
As a consequence of major anthropogenic changes to all of the planet’s ecosystems (e.g., Vitousek, Mooney, Lubchenco, & Melillo, 1997), it has been suggested that we are facing a biodiversity extinction crisis to rival the most extreme in the planet’s history (Barnosky et al, 2011)
From one perspective it is important to try to minimize human influences; the other perspective might embrace human-mediated changes to biodiversity, provided the changes allow organisms to better cope with their strongly altered environments. These issues are germane to species covered by national protected-species legislation, such as the U.S Endangered Species Act (ESA), Canada’s Species at Risk Act (SARA), or Australia’s Endangered Species Protection Act (ESPA)
We report surprising results for a group of parents that were reared captively for a full year before release as yearling smolts: On average, these parents produced the fastest growing offspring of all! This could represent an example of transgenerational phenotypic plasticity, in which the environment the parent experiences early in its life influences life-h istory traits of its offspring
Summary
As a consequence of major anthropogenic changes to all of the planet’s ecosystems (e.g., Vitousek, Mooney, Lubchenco, & Melillo, 1997), it has been suggested that we are facing a biodiversity extinction crisis to rival the most extreme in the planet’s history (Barnosky et al, 2011). Environmental factors likely to have influenced this life-history change include colder water and less favorable growing conditions in the remnant habitat below Hells Canyon Dam (Dauble, Hanrahan, Geist, & Parsley, 2003), as well as availability of large reservoirs behind the lower Snake River and Columbia River dams, which provide convenient holding and overwintering habitat for juveniles that did not exist. If the population has genetically adapted its life history and enhanced the fraction of selectively favored yearling migrants, its fitness could drop suddenly when more natural habitat conditions are restored, especially if the restored free-flowing river no longer provides suitable overwintering habitat for individuals that do not migrate as subyearlings. We suggest that SRFCS is an example of a potentially large class of “anthro-evolutionary” species— those whose evolutionary trajectories have been profoundly shaped by altered selective regimes in human-dominated landscapes
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