Monday, October 5, 2009

Gradualism and Punctuated Equilibrium

Gradualism and Punctuated Equilibrium

I started researching this topic by skimming through my old (2003) textbook from my Human Evolution class that I took as an undergraduate. The authors make no mention of these two terms within the text. They do acknowledge that although slow rates of change are the typical units observed within the fossil record, rapid evolutionary events most likely happened, and the lack of evidence is a result of an incomplete fossil record ( Boyd and Silk 2003:22). The textbook for our seminar is a 2004 edition and dedicates several pages to gradualism and punctuated equilibrium. To me, this represents the variation in academia when it comes to challenging or accepting new concepts.

Gradualism may be defined as the process in evolution that accumulates small units of change at a steady rate, over long temporal periods. The gradual accumulation of new adaptations causes a genetic divergence of offspring from the parent, or ancestral species (Lewin and Foley 2004:52). Gradualism is therefore characterized as being a slow process that remains consistent and constant where change is cumulative within a species. Gradualism is essentially a fundamental part of the theory of Modern Synthesis (1942) that is a union of ideas which resulted from the differences that remained between strict Darwinism and evolutionary theory. Modern Synthesis has three principal components of which the first is gradualism. Although gradualism was originated by James Hutton in 1795, it traveled into Charles Lyell’s repertoire in the form of Unitarianism. It then influenced Charles Darwin and his theory of evolution. It is important to point out that Darwin rejected the concept of saltation; saltation describes evolution as occurring in jumps from one generation to the next in a rapid fashion.

Punctuated Equilibrium essentially describes that species go through static periods with relatively little change that are accentuated by rapidly occurring modifications resulting in speciation. The species then return to a static period. Lewin and Foley (2004:52) indicate that separation of a daughter species from the ancestral species may still occur under punctuated equilibrium, but mainly it occurs through drifts in smaller, isolated populations. Steven Jay Gould and Niles Eldredge published their theory in 1972. Their work was influenced by Ernest Mayr, who also was a key contributor to Modern Synthesis, and Michael Lerner. One of the important differences between punctuated equilibrium and gradualism is how speciation is viewed. Punctuated equilibrium views adaptation as a possible result of speciation, while gradualism views it as a cause of speciation. In addition Gould and Eldredge view is similar to saltation only in the idea that change occurs rapidly. For Gould and Eldredge, adaptation occurs as result of speciation, but follows Darwinian fundamentals in doing so.

Gradualism and punctuated equilibrium may represent two different gears of the same mechanism; gradualism is the hour hand and punctuated equilibrium is the minute hand in which both are enclosed in a mechanism designed (or evolved) to measure time.

Gould and Eldredge (1993) celebrated the acceptance of their concept of punctuated equilibrium into the realm of theory. Can both theories actually coincide if some of the principal components to each contradict each other?


Boyd, Robert and Joan B. Silk

2003 How Humans Evolved. 3rd Edition. New York: W. W. Norton and Company Inc.

Gould, Stephen J. and Niles Eldredge

1993 Punctuated Equilibrium Comes of Age. Nature. 18 November 1993 (366) :


Lewin, Roger and Robert A. Foley

2004 Principles of Human Evolution. 2nd Edition. Victoria, Australia: Blackwell









1 comment:

  1. Good writing and, as far as it goes, pretty accurate. Some important details are, however, missing from this brief discussion. PG and Punk Eq are essentially two different theoretical models proposed to explain the tempo and mode of evolutionary change. PG suggests that evolution occurs at a very slow tempo as the accumulation of tiny genetic variants is filtered thru natural selection, and that the mode of evolutionary change is typically anagenetic change (change in an evolving lineage, as one species slowly turns into a descendant species). Obviously, anagenesis can't be the whole story since it doesn't account for increases in species diversity: but the proponents of PG feel that it is the most typical kind of evolutionary change.

    Puck Eq proponents suggest that most species don't change much over most of their histories... it suggests that the typical pattern seen in the fossil record is of stasis, punctuated by rare but rapid speciation events, during which most evolutionary change actually occurs. These speciation events are typically cladogenetic, or splitting events, in which one species splits into two descendant or daughter species. Thus the Punk Eq model can account for the increase in taxonomic diversity seen in the world and in the geological record. Gould and Eldredge relied heavily on Ernst Mayr's allopatric speciation model, and suggested that speciation events typically occur in small, isolated populations separated from the main body of the species. They suggest that small, peripheral populations would be more likely to undergo speciation, and would then often spread and replace the ancestral species from which it originated.

    While some have said that the Punk Eq model resembles saltational evolution, Gould and Eldredge emphasize that their model involves rapid speciation only in the geological sense of the word "rapid": they do not advocate the origin of new species from single mutational events like earlier evolutionary theorists did (the "hopeful monster" scenario). Darwin, of course, was a committed gradualist, who thought that evolutionary change was typically very slow and gradual, but remember, Darwin was not a paleontologist. he evidence for Punk Eq is very extensive in todays paleontological record, and these data were unknown to Darwin and his contemporaries.


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