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Major Twentieth Century Theories of Evolution: The Neo-Darwinian Synthesis and Punctuated Equilibrium

by Richard Peachey

The two modern theories differ over the tempo (timing) and mode (mechanism) of evolution.

Natural selection is of primary importance for Neo-Darwinists, but not for those who hold to Punctuated Equilibrium.

The two views also disagree on the relationship of macroevolution to microevolution.

1. The Neo-Darwinian Synthesis (also called the "Synthetic Theory of Evolution")

• formulated during the decade 1937-1947.

• updated Darwin's ideas using new information from many scientific fields.

• main features of this view are mutation and natural selection.

• genetic mutations produce variation within a population (Darwin could not explain variation).

• natural selection preserves the most fit varieties within a species, as explained by Darwin.

• macroevolution is simply microevolution extrapolated.

• evolution is slow, gradual, and continuous, as held by Darwin.

• this view has difficulty explaining the fossil record with its lack of transitional forms.

In the 1930s and '40s evolutionists worked to incorporate new data from various subdisciplines of biology into a revised version of classical Darwinism. The primary focus on natural selection was maintained, but other aspects of Darwin's thinking were updated.

The geneticist Theodosius Dobzhansky (Genetics and the Origin of Species, 1937) presented his understanding of the significance of Mendelian genetics and mutations. (Darwin was unaware of Mendel's views, and had mistaken ideas on how inheritance worked; he thought ordinary "variation" would provide enough raw material for natural selection to work on.) The ornithologist and systematist Ernst Mayr (Systematics and the Origin of Species, 1942) defined "species" as "groups of actually or potentially interbreeding populations that are reproductively isolated from other groups" (biological species concept), and urged the central importance of geographic barriers as reproductive isolating mechanisms promoting the evolution of new species. (Darwin's concept of "species" was that they were simply "well-marked varieties;" he considered geographic isolation of limited importance.) The paleontologist George Gaylord Simpson (Tempo and Mode in Evolution, 1944) integrated the fossil record into the synthesis, proposing that mutations and natural selection were sufficient to account for both micro- and macro-evolution. Other contributions came from Bernhard Rensch (zoologist), G. Ledyard Stebbins (botanist), and Julian Huxley. A 1947 symposium at Princeton University consolidated the efforts of various evolutionists and confirmed that a new synthesis was being achieved.

Ernst Mayr summarized the position in 1963: "The proponents of the synthetic theory maintain that all evolution is due to accumulation of small genetic changes [mutations], guided by natural selection, and that transspecific evolution [macroevolution] is nothing but an extrapolation and magnification of the events that take place within populations and species [microevolution]" (quoted in Stephen Jay Gould. 1980. "Is a new and general theory of evolution emerging?" Paleobiology, Vol. 6 No.1, p. 120).

More recent adherents of strict neo-Darwinism include John Maynard Smith, Richard Dawkins, and Daniel Dennett. These people are insistent that natural selection is the overriding factor at work in evolution. Their opponents (especially Stephen Jay Gould) call them "selectionists," "ultra-Darwinians," or "Darwinian fundamentalists." Gould and others refer to themselves as "pluralists" because they acknowledge a variety of important factors affecting evolution, of which natural selection may not even be the most important.

2. Punctuated Equilibrium (originally called "Punctuated Equilibria")

• proposed in 1972 by two paleontologists (scientists who study the fossil record).

• attempts to explain how evolution could produce the fossil record as we know it, with its lack of transitional forms.

• main features of this view are stasis or equilibrium (essentially, "no evolution") and sudden appearance.

• most of a species' existence on Earth consists of little or no directional evolution, "punctuated" at rare intervals by a burst of relatively rapid change.

• natural selection operates at the level of microevolution; it is important but not considered as fundamental as in neo-Darwinism.

• macroevolution is not simply microevolution extrapolated, but must be explained by some other mechanism(s).

• when significant evolution occurs, it is rapid (compared to a neo-Darwinian time-frame).

• this view has difficulty offering an explanation for the rapid bursts of evolution.

The punctuated equilibrium view was proposed in 1972 by paleontologists Stephen Jay Gould and Niles Eldredge, based on three noteworthy features of the fossil record:

1. The "extreme rarity" of transitional forms ("missing links").

2. "Stasis" or "equilibrium" (meaning little or no change in a species throughout its time on Earth) as the most prominent feature of the fossil record.

3. "Sudden appearance" (new species appear "all at once and 'fully formed'").

Gould and Eldredge proposed that most of a species' existence is characterized by evolutionary inaction or stability ("stasis"), punctuated by relatively rare ''bursts'' of evolution. (A ''burst'' on the evolutionary time scale would consist of relatively rapid evolution over a few thousand to tens of thousands of years.) Some evolutionists feel the punctuated equilibrium view is diametrically opposed to Darwinism; others prefer to think Gould and Eldredge's points can be accommodated within the neo-Darwinian synthesis. Gould himself appears to have spoken with both of these attitudes at different times!

At the historic Chicago conference of 1980 on "Macroevolution," a "wide spectrum of researchers" (geologists, paleontologists, ecologists, population geneticists, embryologists, and molecular biologists) assembled to discuss the central question of "whether the mechanisms underlying microevolution can be extrapolated to explain the phenomena of macroevolution." The answer of the majority was "No" (Roger Lewin. 1980 [Nov 21]. "Evolutionary Theory Under Fire." Science 210:883).

Those who hold to punctuated equilibrium, however, must still provide an explanation for how it could actually work, "a topic about which 'there are a lot of hypotheses and not many facts'" (Richard A Kerr. 1995 [Mar 10]. "Did Darwin Get It All Right?" Science 267:35).



For further reading:

"Evolutionists and E x t r a p o l a t i o n" <>

"Do Examples of 'Microevolution' Provide Support for Macroevolution?" <>

"Personalities in the Evolution/Creation Conflict" <>