The idea of gradualism is one of the key facets of macroevolution, and mostly concerns lineages of interrelated populations of organisms at or near the species level. In modern biology, gradualism, or “phyletic gradualism,” refers primarily to a pattern of sustained, directional, and incremental evolutionary change over a long period during the history of a species. With the introduction of the theory of punctuated equilibrium, gradualism has been contrasted with stasis (species’ relative morphological invariance over significant time intervals), and less frequently with lineage patterns that cannot be distinguished from random walks. The idea of gradualism is hardly new. Notions of continuity and transitional intermediate forms between related species extend back well before biology became a recognized science, and these notions have influenced thinking about evolution from prior to Darwin until today. How lineage gradualism 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 which methods of analysis are used. 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 gradualism.
Gradualism in biology and geology refers most broadly to a theory that changes of organic life and of the Earth itself occur through gradual increments, and often that transitions between different states are more or less continual and slow rather than periodic and rapid. Gradualism has been ascribed to the manner in which Darwin inferred the origin of one species from another over geologic time, or the “transmutation” of species. Eldredge and Gould 1972 (cited under History of the Concept: Punctuated Equilibrium) contrasts the new theory of punctuated equilibrium—long intervals of species’ relative morphological invariance, or stasis, “punctuated” by rapid bursts of speciation—with what Eldredge and Gould term “phyletic gradualism.” In this article and later papers by Gould, gradualism is a uniformitarianist view of speciation, in which evolution proceeds in a measured, variable, yet roughly steady manner both within a species’ duration and during speciation, such that species formation need not involve discrete episodes of phylogenetic branching or sudden, major morphological change. The long history and modern research controversies surrounding stasis are considered in Lidgard and Hopkins 2015. The dominant gradualist view of speciation eventually adopted by Darwin and revised during the Modern Evolutionary Synthesis is discussed in Sepkoski 2014 and Eldredge 2015 (cited under Books). Several overviews such as Erwin and Anstey 1995 provide evidence for gradualism versus stasis as empirical patterns in the fossil record. Geary 1995, Sheldon 2001, and Bell 2009 describe hypothesized mechanisms or explanations for the empirical patterns, including gradualism. Paleontologists have long argued about what the fossil record can tell us about how species evolve over periods of geologic time ranging from thousands to millions of years. However, large-scale comparative studies of morphological change in fossil lineages under a single analytical framework have begun only recently with the work of Hunt 2007. This study and Hunt, et al. 2015 indicate that stasis and random walks are about equally common in the fossil record, and that the recorded incidence of gradual, directional evolution is relatively low.
Bell, Michael A. 2009. Implications of a fossil stickleback assemblage for Darwinian gradualism. Journal of Fish Biology 75:1977–1999.
Summarizes several decades of work on phenotypic evolution within fossil stickleback lineages sampled from lake deposits. Studied traits show gradual change but also conflict with some expectations of gradual evolution, including the role of genes and associated genetic evolution. Outlines criteria necessary for identifying gradualism in the fossil record.
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 the 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 patterns are found in empirical fossil studies.
Geary, Dana H. 1995. The importance of gradual change in species-level transition. In New approaches to speciation in the fossil record. Edited by Douglas H. Erwin and Robert L. Anstey, 67–86. Columbia Univ. Press.
Defines gradualism as net morphological change in a consistent direction, thus accommodating problems of recognizing gradualism under different scales of temporal sampling. Cites examples of gradual change within various gastropod lineages. Describes several possible mechanisms for gradual change and argues that knowledge of geographic variation is critical for pattern interpretation.
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 104:18404–18408.
The first large, quantitative analysis of empirical fossil studies showing different modes of evolution, conducted by using a comparative model-fitting method. Finds that stasis and random walks occur equally frequently in fossil lineages, whereas gradualistic directional change is 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 that in Hunt 2007, but in addition allows for shifts in evolutionary modes and for punctuations. Finds that complex models with shifts between modes are often favored, but the overall dominance of stasis or random walks over directional change in fossil sequences is confirmed.
Lidgard, Scott, and Melanie Hopkins. 2015. Stasis. In Oxford Bibliographies in Evolutionary Biology. Edited by Jonathan Losos. New York: Oxford Univ. Press.
Reviews the history, analyses, and implications of the concept of evolutionary stasis, with an annotated bibliography. Available online by subscription.
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.
Sheldon, Peter R. 2001. Punctuated equilibrium and phyletic gradualism. eLS.
Notes that species-level patterns of gradualism, stasis, and random trends with fluctuating reversals have been demonstrated in the fossil record. Describes the “plus ça change” model, which predicts that environments where gradualism is more likely to prevail are less well represented in the fossil record and thus documented less frequently.
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