In modern biology, stasis refers primarily to a relative lack of evolutionary change over a long period during the history of a species. It is one of the key facets of macroevolution, or evolution that takes place at or above the level of the species. Stasis is frequently associated with the theory of punctuated equilibrium, in which most evolutionary change is concentrated during the phylogenetic branching of lineages in very rapid bursts of speciation. Much longer episodes of relative morphological invariance, or stasis, follow speciation events. Stasis is also contrasted with incremental directional change within and between related lineages (gradualism), and sometimes with lineage patterns that cannot be distinguished from random trends. The idea of stasis is hardly new. Notions of the fixity of species in space and time extend back well before biology became a recognized science, and these notions have influenced thinking about evolution from before Darwin until today. While most discussion begins with lineages of interrelated populations of organisms at or near species level, the idea of stasis has also been applied to other levels of biological hierarchy for entities other than species: morphological traits, highly conserved genes, ecological communities, and more. How species stasis is recognized as an evolutionary pattern depends, in part, upon how species are discriminated and which species concepts are used, which properties are measured, and methods of analysis. The dynamics and causes of macroevolution are crucial, yet still unsettled, problems in evolutionary biology, either invoking or questioning the extrapolation of microevolutionary mechanisms to explain protracted temporal patterns such as stasis.
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.
Eldredge, Niles, John N. Thompson, Paul M. Brakefield, et al. 2005. The dynamics of evolutionary stasis. Paleobiology 31.2 Suppl.: 133–145.
Reformulates basic research questions about stasis, focusing on geographic structure and the evolution and establishment of novelties. Argues that stasis is maintained by the geographic structure of genetic variation among different populations under heterogeneous selection and connected by gene flow. Also a good overview of other proposed mechanisms.
Erwin, Douglas H., and Robert L. Anstey. 1995. Speciation in the fossil record. In New approaches to speciation in the fossil record. Edited by Douglas H. Erwin and Robert L. Anstey, 11–38. New York: Columbia Univ. Press.
Reviews both assertions of the punctuated equilibrium model and proposed mechanisms for gradualism, rapid speciation, and stasis as fossil patterns. Shows that a wide variety of species-level evolutionary patterns are found in empirical fossil studies.
Gould, Stephen J. 2007. Punctuated equilibrium. Cambridge, MA: Belknap.
The development, precepts, and evidence for the theory, with discussion of possible causal mechanisms and importance of punctuated equilibrium in biology, from Gould’s perspective at the end of his career. Stasis and gradualism are examined. Excerpted from his book The Structure of Evolutionary Theory (Cambridge, MA: Belknap, 2002).
Gould, Stephen J., and Niles Eldredge. 1977. Punctuated equilibria: The tempo and mode of evolution reconsidered. Paleobiology 3.2: 115–151.
Revisits arguments for punctuated equilibrium in light of levied criticisms, five years after original publication of the idea (Punctuated Equilibrium). Reexamines classic cases of morphological evolution in the fossil record and shows how they could be reinterpreted.
Hunt, Gene. 2007. The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages. Proceedings of the National Academy of Sciences of the United States of America 104:18404–18408.
The first large quantitative analysis of empirical fossil studies showing different modes of evolution, conducted using a comparative model-fitting method. Stasis and random walks occurred equally frequently in fossil lineages, whereas gradualistic directional change was seldom a best-fit model.
Hunt, Gene, Melanie J. Hopkins, and Scott Lidgard. 2015. Simple versus complex models of trait evolution and stasis as a response to environmental change. Proceedings of the National Academy of Sciences of the United States of America 112:4885–4890.
Uses a comparative model fitting method similar to Hunt 2007, but in addition allows for shifts in evolutionary modes and for punctuations. Complex models with shifts between modes are often favored, but the overall dominance of stasis or random walks over gradualism in fossil sequences is confirmed.
Levinton, Jeffrey S. 1983. Stasis in progress: The empirical basis of macroevolution. Annual Review of Ecology and Systematics 14:103–137.
Primarily a critique of stasis, but some of the issues raised apply to estimating rates and modes of evolution in general. In particular, the distinction between “character stasis” and “species stasis” can also be stated as a distinction between “character change” and “species change.”
Sepkoski, David. 2014. Punctuated Equilibria. In Oxford Bibliographies in Evolutionary Biology. Edited by Jonathan Losos. New York: Oxford Univ. Press.
Reviews the development and consequences of Eldredge and Gould’s theory of punctuated equilibria in paleontology and evolutionary biology, with an annotated bibliography. Available online by subscription.
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