Abstract
Open ocean zooplankton often have been viewed as slowly evolving species that have limited capacity to respond adaptively to changing ocean conditions. Hence, attention has focused on the ecological responses of zooplankton to current global change, including range shifts and changing phenology. Here, we argue that zooplankton also are well poised for evolutionary responses to global change. We present theoretical arguments that suggest plankton species may respond rapidly to selection on mildly beneficial mutations due to exceptionally large population size, and consider the circumstantial evidence that supports our inference that selection may be particularly important for these species. We also review all primary population genetic studies of open ocean zooplankton and show that genetic isolation can be achieved at the scale of gyre systems in open ocean habitats (100s to 1000s of km). Furthermore, population genetic structure often varies across planktonic taxa, and appears to be linked to the particular ecological requirements of the organism. In combination, these characteristics should facilitate adaptive evolution to distinct oceanographic habitats in the plankton. We conclude that marine zooplankton may be capable of rapid evolutionary as well as ecological responses to changing ocean conditions, and discuss the implications of this view. We further suggest two priority areas for future research to test our hypothesis of high evolutionary potential in open ocean zooplankton, which will require (1) assessing how pervasive selection is in driving population divergence and (2) rigorously quantifying the spatial and temporal scales of population differentiation in the open ocean.Recent attention has focused on the ecological responses of open ocean zooplankton to current global change, including range shifts and changing phenology. Here, we argue that marine zooplankton also are well poised for evolutionary responses to global change.
Highlights
We review the 46 primary population genetic and phylogeographic studies that focus on population-level differentiation in marine zooplankton (55 taxa; Table 1), and show that a 2013 The Authors
Even though there is still active debate as to whether most of the genetic variation within and between species is selectively neutral (Kimura 1968, 1983), or whether a large proportion of the variation is subject to selection (Gillespie 1991; Hahn 2008; Wares 2010), we argue here that selection may be a dominant force in the evolution of open ocean plankton, because they are at the extreme end of the scale in terms of population size of marine organisms
A second line of evidence suggesting that selection may be an important driver of evolution in species with large populations is the finding of large discrepancies between census and effective population sizes, which are commonplace for marine species (Grant and Bowen 1998; Avise 2000; Hauser and Carvalho 2008; Portnoy et al 2009)
Summary
The oceans are changing on a global scale and, in some cases, at rates greatly exceeding those observed in the historical and recent geological record (e.g., Pelejero et al 2010). Three primary arguments have traditionally supported the perception that open ocean zooplankton have limited capacity to evolve in comparison to other terrestrial and marine species Plankton have been likened to airborne spores or wind-dispersed seeds that can drift almost anywhere in the ocean (Norris 2000), with their biogeographic ranges limited only by their ability to find suitable habitat for the establishment of new populations (Norris and de Vargas 2000; Sexton and Norris 2008) These three arguments have supported a persistent view of low evolutionary potential for marine zooplankton, in comparison to other marine and terrestrial groups.
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