Phyletic gradualism is a speciation hypothesis rooted in uniformitarianism. The hypothesis states that species continue to adapt to new environmental and biological selection pressures over the course of their history, gradually becoming new species. Phyletic gradualism holds that a species population changes gradually and that there is no clear line of demarcation between an ancestral species, that is a descendant species unless a splitting (cladogenetic) event occurs or the gradually-changing lineage is divided arbitrarily. During this process, anagenetic evolution occurs at a smooth, steady, and incremental (but not necessarily constant or slow) rate, even on a geological timescale. In contrast to punctuated equilibrium, phyletic gradualism states that new species arise by the gradual transformation and ultimate gradual splitting of ancestral species into descendant species lineages rather than by the abrupt splitting of relatively stable and unchanging ancestral species lineages. With phyletic gradualism, the rate of evolution during speciation is similar to anagenetic change at other times, not concentrated in a speciation event as is the case with the punctuated equilibrium hypothesis.
Phyletic gradualism was long considered the dominant pattern of speciation and evolution, but it is deprecated by modern evolutionary biologists as the exclusive pattern today in favor of the acceptation of a variety of hypotheses such as 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".[1] 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."[2]
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
References
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