Punctuated Equilibrium

∞ generated and posted on 2016.11.05 ∞

Idea that populations followed through the fossil record tend to remain morphologically stable over long periods, but with these periods of stability bracketed by relatively short spans during which substantial morphological change is observed.

Punctuated equilibrium is in essence the fossilized manifestation of allopatric speciation. That is, the idea that large populations tend to remain somewhat genetically stable through time while substantial change occurs in what are mostly short-lived and isolated peripheral populations. The latter, because they are small and ephemeral, tend to be poorly represented in the fossil record.

If these peripheral isolates come to replace their parental species then what will tend to be seen among fossils is a long stretch of relative lack of morphological variation (equilibrium) that "suddenly" (punctuation) is followed by substantial morphological change.

Punctuated equilibrium serves as a means of explaining this feature of the fossil record, i.e., long-term persistence of morphological types combined with their rapid replacement. It is based on an assumption of poor sampling of past populations in the fossil record and a resulting biased sampling, that is, towards those individuals that tend to be more numerous both across space and through time. Even so, it is important to realize that the punctuations still take place over what are likely thousands of years, versus millions of years for equilibrium periods. Nevertheless, since there are 1000 years in a million, and if the main population were 100-times more prevalent at any given time than the peripheral isolate, then we are talking about ~100,000-fold greater likelihood of sampling from main populations versus peripheral populations.

For the sake of context, it is important to keep in mind that what punctuated equilibrium is contrasting is so-called gradual change (phyletic gradualism), and also to some degree the concept of anagenesis, though not in a way that actually contradicts gradual change. Instead, this gradual change is occurring primarily in small populations and, rather than occurring over millions of years for noticeable change to occur, instead it is occurring over thousands of years. That noticeable change can occur over thousands rather than millions of years is a function, at least in part, of this change occurring in small rather than larger populations.