Lecture Summary February 23: Gradualism vs. Punctuated Equilibrium

The fossil record shows many sudden spurs of change in morphology, and number of species. This fact led Gould and Eldrige (1972) to the theory of Punctuated equilbrium that contrasts the then current view of gradual change.

Phyletic Gradualism

The speciation process is a steady continous process. There are no episodes of fast or slow evolution through time. Random variation is due to mutation and selection is due to individual survival and reproduction. Changes happen in populations that will expand or go extinct.

Punctuated equilibrium

Genetic drift in peripheral isolated populations moves these populations to new adaptive peaks. These "better" species will replace or coexist (for a while) with its parental species (species selection). The change happens between species. Stanley (1975): Macroevolution is decoupled from Microrevolution.

Pro and Cons

Population geneticists refuted the need for species selection to explain the fossil record. Wright's shifting balance theory, where local small populations achieve a higher fitness peak and then successively export such allele combinations by migration to other populations would produce a similar result in the fossil record, without assuming that the populations have to be reproductively isolated. Speciation is not needed to explain the pattern in the fossil record.

The data of the fossil record permits different interpretation the same data was interpreted by different authors as gradual or punctuated. The distinction between gradualism and punctuated equilbrium is not well defined. The fossil record with its sparse data seems often punctuated, instantantenous rates for a paleontologist often covers many thousands of years. Even moderate or low individual selection can achieve huge (gradual) changes in only a few thousand generations.