Dianne Ramage
Evolutionary history of Pacific salmon dynamic environments
Tue Jan 22, 2019 7:47am

As mentioned during Streamkeeper Training:

Evolutionary history of Pacific salmon in dynamic
Robin S. Waples,1 George R. Pess1,2 and Tim Beechie1,2
1 Northwest Fisheries Science Center, Seattle, WA, USA
2 Environmental Conservation Division

Full Paper

Note: *In this paper we consider steelhead (anadromous form of Oncorhynchus mykiss) to be honorary Pacific salmon.

Contemporary evolution of Pacific salmon (Oncorhynchus spp.) is best viewed in the context of the evolutionary history of the species and the dynamic ecosystems they inhabit. Speciation was complete by the late Miocene, leaving c.six million years for intraspecific diversification. Following the most recent glacial maximum, large areas became available for recolonization.
Current intraspecific diversity is thus the product of recent evolution overlaid onto divergent historical lineages forged during recurrent episodes of Pleistocene glaciation. In northwestern North America, dominant habitat features have been relatively stable for the past 5000 years, but salmon ecosystems remain dynamic
because of disturbance regimes (volcanic eruptions, landslides, wildfires, floods, variations in marine and freshwater productivity) that occur on a variety of
temporal and spatial scales. These disturbances both create selective pressures for adaptive responses by salmon and inhibit long-term divergence by periodically extirpating local populations and creating episodic dispersal events that erode emerging differences. Recent anthropogenic changes are replicated pervasively across the landscape and interrupt processes that allow natural habitat recovery.

If anthropogenic changes can be shaped to produce disturbance regimes that more closely mimic (in both space and time) those under which the species evolved, Pacific salmon should be well-equipped to deal with future challenges, just as they have throughout their evolutionary history.

Both Pacific salmon* (Oncorhynchus spp.) and Atlantic
salmon (Salmo salar) have experienced wide-spread population declines and extirpations (Jonsson et al. 1999; Gustafson et al. 2007), and both are strongly affected by anthropogenic changes to the freshwater and marine ecosystems they inhabit. These changes alter selective regimes the salmon experience and can be expected to elicit an evolutionary response a phenomenon considered by many of the papers in this special issue.

A fuller understanding of the nature and extent of these evolutionary changes can best be achieved in the context of the evolutionary history of salmon and the dynamic environments in which they evolved. The historical template describing the tempo and mode of dynamic changes to Pacific salmon habitats can be particularly useful in assessing likely consequences of future anthropogenic changes. In general, salmon populations should have sufficient evolutionary capital to respond to changes that fall within the historic temporal and spatial scales of natural disturbance regimes, but larger changes might require an evolutionary response that is outside the range of what salmon can muster.

In this paper, we attempt a synthesis of some of the
major features of Pacific salmon evolution, with particular emphasis on the interplay between variable environments and the evolutionary responses by salmon populations.
Because rates of phenotypic change (and perhaps evolutionary change) are higher in human-altered landscapes (Hendry et al. 2008), we also consider how anthropogenic changes affect contemporary evolution of salmon.

We illustrate with examples drawn primarily from Puget Sound and the interior Columbia River basin areas in the northwestern United States that have two attractive features for our analyses. First, these areas were at or near the southern extent of continental glaciation in the Pleistocene, so they have a rich and dynamic geologic history.

Second, in both areas the biological attributes of the populations are well characterized and detailed information about historical ecological/environmental conditions is available, which facilitates joint analysis of biological and physical processes.

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