Stasis - history of future e.p.


    Remarkably few papers define the term stasis . Even Eldredge and Gould, in their seminal paper Eldredge and Gould 1972 (cited under Punctuated Equilibrium ) introducing punctuated equilibria, did not explicitly define stasis aside from claiming that “no variation in the most important feature of discrimination . . . through long spans of time” (p. 106) is sufficient evidence of it. Their definition was contrasted with “phyletic gradualism,” which they characterized as evolution proceeding in an incremental manner both within a species’ duration and during speciation. A more explicit recent exception is Eldredge, et al. 2005 : “Stasis is generally defined as little or no net accrued species-wide morphological change during a species-lineage’s existence up to millions of years—instantly begging the question of the precise meaning of ‘little or no’ net evolutionary change” (p. 133). The question these authors posed within their definition hints at the variety of ways in which stasis could be defined quantitatively. Although typically defined to be “species-wide,” most evidence for morphological stasis comes from studies of one or a few characters, leading Levinton 1983 to distinguish between “character stasis” and “species stasis.” van Valen 1982 (cited under Proposed Causal Mechanisms or Processes ) points out that the concept of “stasis” was similar to that of “species integrity,” defined as uniformity in the most basic respects among geographically distributed populations, except for peripheral isolates. In doing so, van Valen defines stasis as “integration of species in time” rather than in space. Roopnarine 2001 (cited under Stasis in Species Lineages: Quantitative Approaches ) suggests that traditional definitions of stasis as “no net change” or “oscillatory variation” may be equivalent in pattern, if not process, to unbiased random walks. Often the application of different methods for quantifying stasis depends on a given species concept ( Species Concepts ) and assumptions made about the underlying processes ( Proposed Causal Mechanisms or Processes ). Despite several attempts, such as Erwin and Anstey 1995 and Gould 2007 , variation in definitions of stasis prevented unambiguous assessments of the frequency of stasis versus other modes of evolution. Without quantitative definitions, many workers have come to different conclusions about the same datasets, as demonstrated by examples in Gould and Eldredge 1977 . These views are also considered in Sepkoski 2014 in Oxford Bibliographies in Evolutionary Biology, and in the section Punctuated Equilibrium . Hunt 2007 provides the first large-scale quantitative assessment of different modes of evolution under a single definition of stasis. Hunt found that stasis and random walks occurred equally frequently in fossil lineages and that directional change occurred rarely in comparison. These findings are confirmed in Hunt, et al. 2015 , analyzing a greater number of empirical studies and using more complex models of evolutionary modes.


    Stasis - History Of Future E.P.Stasis - History Of Future E.P.Stasis - History Of Future E.P.Stasis - History Of Future E.P.

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