Phyletic gradualism

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Phyletic gradualism is a macroevolutionary hypothesis rooted in uniformitarianism. The hypothesis states that species continue to adapt to new challenges over the course of their history, gradually becoming new species. Gradualism holds that every individual is the same species as its parents, and that there is no clear line of demarcation between the old species and the new species. It holds that the species is not a fixed type, and that the population, not the individual, evolves. During this process, evolution occurs at a slow and smooth (but not necessarily constant) rate, even on a geological timescale. (cf. punctuated equilibrium)^[1]

Phyletic gradualism has been largely deprecated as the exclusive pattern of evolution by modern evolutionary biologists in favor of the acceptation of occurrence of patterns such as those described on punctuated equilibrium, quantum evolution, and punctuated gradualism.

Authors such as Richard Dawkins argue that such constant-rate gradualism is not present in academic literature, serving only as a straw-man for punctuated equilibrium advocates. He refutes the idea that Charles Darwin himself was a constant-rate gradualist, as suggested by Stephen Jay Gould. In the first edition of On the Origin of Species Darwin clearly stated that "Species of different genera and classes have not changed at the same rate, or in the same degree. In the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms... The Silurian Lingula differs but little from the living species of this genus".^[2] Lingula is among the few brachiopods surviving today but also known from fossils over 500 million years old. In the fifth edition of On the Origin of Species Darwin wrote that "the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form."^[3]

[edit] Sources

• The distinction between phyletic gradualism and punctuated equilibrium models
• DAWKINS, Richard. The Blind Watchmaker - Why the evidence of evolution reveals a universe without design. W. W. Norton & Company. New York, 1996

[edit] References