What is Cope's rule in simple terms?

It is the generalization that groups of organisms, starting from small ancestors, tend on average to become larger as they evolve over long periods of time.

Does Cope's rule apply to all organisms?

No. The trend is found across mammals and marine animals as a whole, but groups such as Cretaceous molluscs show no clear trend, so it is treated as a statistical tendency rather than a universal law.

Cope's Rule

🔬 Natural Sciences By CampusDown Wiki Editorial Team Last updated October 26, 2026

Cope's rule is the paleontological generalization that, as lineages evolve, average body size tends to increase over time. It is named after the nineteenth-century American paleontologist Edward Drinker Cope, although the formulation owes more to the French paleontologist Charles Depéret (1907) and to later authors.

Contents

1. Origins and naming
2. Theoretical interpretations
3. Tests in the fossil record
4. FAQ
5. References

Origins and naming

In The Primary Factors of Organic Evolution (1896), the American paleontologist Edward Drinker Cope set out the law of the unspecialized, holding that new lineages usually arise from small, unspecialized forms. The proposition that body size increases over time was stated more explicitly by the French paleontologist Charles Depéret (1907), but the name Cope's rule became established as authors such as Rensch (1948) and Newell (1949) used it for this trend. Fossil series in which large descendants evolved from small ancestors, such as horses, elephants and dinosaurs, are frequently cited as examples.

Theoretical interpretations

Stanley (1973) argued that because new lineages usually begin at small body sizes, random diversification alone will raise average size when there is a lower limit on size. Gould (1988) developed this point, arguing that many evolutionary trends may be increases in variance away from a wall at the small end rather than a directed push. Kingsolver and Pfennig (2004), by contrast, noted that directional selection favoring larger bodies for survival, fecundity and mating success is common in living organisms and suggested that selection on individuals could be the cause. Hone and Benton (2005) summarized the trade-off: large size helps with avoiding predators, competing and regulating body temperature, but it also raises extinction risk through smaller populations and longer generation times. Kleiber's law, which relates body size to metabolic rate, connects to these discussions.

Tests in the fossil record

Polly (1998) reviewed the history and issues surrounding Cope's rule. Alroy (1998) analyzed some 1,500 species of North American fossil mammals and found that new species were on average slightly larger than their ancestors. Jablonski (1997), however, reported that among Cretaceous molluscs, size increases, decreases and stasis occurred at similar rates, so Cope's rule was not general. Clauset and Erwin (2008) proposed a model in which body size distributions are explained by a balance between a lower size limit and extinction risk, and Heim et al. (2015), analyzing data on about 17,000 genera of marine animals over 542 million years, reported a large increase in mean body size driven mainly by the diversification of larger-bodied groups.

FAQ

What is Cope's rule in simple terms?: It is the generalization that groups of organisms, starting from small ancestors, tend on average to become larger as they evolve over long periods of time.
Does Cope's rule apply to all organisms?: No. The trend is found across mammals and marine animals as a whole, but groups such as Cretaceous molluscs show no clear trend, so it is treated as a statistical tendency rather than a universal law.

Sources reviewed: 2026-10-26

References

Cope, E. D. (1896). The Primary Factors of Organic Evolution. Chicago: Open Court.
Depéret, C. (1907). Les transformations du monde animal. Paris: Flammarion.
Rensch, B. (1948). Histological changes correlated with evolutionary changes of body size. Evolution, 2(3), 218–230.
Newell, N. D. (1949). Phyletic size increase, an important trend illustrated by fossil invertebrates. Evolution, 3(2), 103–124.
Stanley, S. M. (1973). An explanation for Cope's rule. Evolution, 27(1), 1–26.
Gould, S. J. (1988). Trends as changes in variance: A new slant on progress and directionality in evolution. Journal of Paleontology, 62(3), 319–329.
Polly, P. D. (1998). Cope's rule. Science, 282(5386), 47–51.
Jablonski, D. (1997). Body-size evolution in Cretaceous molluscs and the status of Cope's rule. Nature, 385(6613), 250–252.
Alroy, J. (1998). Cope's rule and the dynamics of body mass evolution in North American fossil mammals. Science, 280(5364), 731–734.
Kingsolver, J. G., & Pfennig, D. W. (2004). Individual-level selection as a cause of Cope's rule of phyletic size increase. Evolution, 58(7), 1608–1612.
Hone, D. W. E., & Benton, M. J. (2005). The evolution of large size: How does Cope's rule work? Trends in Ecology & Evolution, 20(1), 4–6.
Clauset, A., & Erwin, D. H. (2008). The evolution and distribution of species body size. Science, 321(5887), 399–401.
Heim, N. A., Knope, M. L., Schaal, E. K., Wang, S. C., & Payne, J. L. (2015). Cope's rule in the evolution of marine animals. Science, 347(6224), 867–870.

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This article was last updated on October 26, 2026. It is based on widely recognized original works and textbooks; when citing it in a paper, please check the original sources listed in the references.