University of Vermont

Evolution 101

plas

Gradual Change Vs. Punctuated Equilibrium

models of change
    Charles Darwin believed that evolution was a slow and gradual process.  He did not believe this process to be "perfectly smooth," but rather, "stepwise," with a species evolving and accumulating small variations over long periods of time.  Darwin assumed that if evolution is gradual then there should be a record in fossils of small incremental change within a species.  But in many cases, Darwin, and scientists today, are unable to find most of these intermediate forms.  Darwin blamed lack of transitional forms on gaps in the fossil record, a good assertion, because the chances of each of those critical changing forms having been preserved as fossils are very small.  However in 1972, evolutionary scientists Stephen Jay Gould and Niles Eldredge proposed another explanation for the numerous gaps in the fossil record. They suggested that the "gaps" were real, representing periods of stasis in morphology.  They termed this mode of evolution "punctuated equilibrium." This means that species are generally morpholgically stable, changing little for millions of years.  This leisurely pace is "punctuated" by a rapid burst of change that results in a new species.  According to this idea, the changes leading to a new species don't usually occur from slow incremental change in the mainstream population of a species,  but occur in those populations living in the periphery, or in small geographically isolated populations where their gene pools vary more widely due to the slightly different environmental conditions where they dwell.  When the environment changes, these "peripheral" or "geographic isolates" possess variation in morphology which might enable them to have an adaptive advantage, leading to greater reproductive success.  These new successful morphotypes spread through the geographic range of the ancestral species, appearing as a new morphology where once the older forms were present. 

Content (http://www.pbs.org/wgbh/evolution/library/03/5/l_035_01.html)
Photo (http://evolution.berkeley.edu/)


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