Gradualism vs. Punctuated Equilibrium

Evolution takes a very long time to become visible. Generation after generation can come and go before any changes in a species are observed. There is some debate in the scientific community as to how quickly evolution occurs. The two generally accepted ideas for rates of evolution are called gradualism and punctuated equilibrium.

Gradualism

Based on geology and the findings of James Hutton and Charles Lyell, gradualism states that large changes are actually the culmination very small changes that build up over time. Scientists have found evidence of gradualism in geologic processes, which the Prince Edward Island Department of education describes as the

"...processes at work in the earth's landforms and surfaces. The mechanisms involved, weathering, erosion, and plate tectonics, combine processes that are in some respects destructive and in others constructive."

Geologic processes are long, slow changes that occur over thousands or even millions of years. When Charles Darwin first began formulating his theory of evolution, he adopted this idea. The fossil record is evidence that supports this view. There are many transitional fossils that show structural adaptations of species as they transform into new species. Proponents of gradualism say that the geologic time scale helps show how species have changed over the different eras since life began on Earth.

Punctuated Equilibrium

Punctuated equilibrium, by contrast, is based on the idea that since you cannot see changes in a species, there must be very long periods when no changes occur. Punctuated equilibrium asserts that evolution occurs in short bursts followed long periods of equilibrium. Put another way, long periods of equilibrium (no change) are "punctuated" by short periods of rapid change.

Proponents of punctuated equilibrium included such scientists as William Bateson, a strong opponent of Darwin's views, who argued that species do not evolve gradually. This camp of scientists believes that change happens very rapidly with long periods of stability and no change in between. Usually, the driving force of evolution is some sort of change in the environment that necessitates a need for quick change, they argue.

Fossils Key to Both Views

Strangely enough, scientists in both camps cite the fossil record as evidence to support their views. Proponents of punctuated equilibrium point out that there are many missing links in the fossil record. If gradualism is the correct model for the rate of evolution, they argue, there should be fossil records that show evidence of slow, gradual change. Those links never really existed, to begin with, say the proponents of punctuated equilibrium, so that removes the issue of missing links in evolution.

Darwin also pointed to fossil evidence that showed slight changes in the body structure of the species over time, often leading to vestigial structures. Of course, the fossil record is incomplete, leading to the problem of the missing links.

Currently, neither hypothesis is considered more accurate. More evidence will be needed before gradualism or punctuated equilibrium is declared the actual mechanism for the rate of evolution.