Stasis is an important observation. Here's how Gould describes it in The Structure of Evolutionary Theory (p. 759).
Abrupt appearance may record an absence of information but stasis is data. Eldredge and I became so frustrated by the failure of may colleagues to grasp this evident point—though a quarter century of subsequent debate has finally propelled our claim to general acceptance (while much about punctuated equilibrium remains controversial)—that we ureged the incorporation of this little phrase as a mantra or motto. Say it ten times before breakfast every day for a week, and the argument will surely seep in by osmosis: "stasis is data; stasis is data ..."Okay, we get it.
What causes stasis? There are many possible explanations. One of the most common is stabilizing selection or the idea that a species is so well-adapted to its environment that any change will be detrimental. According to this view, evolution only occurs when the environment changes.
Developmental constraint was another explanation for stasis. The idea here is that morphological change is difficult because there aren't many ways of achieving it. This was the explanation preferred originally by Eldredge and Gould but they have now abandoned it as the chief mechanism of stasis.
The third explanation has nothing to do with adaptation. There's no evidence that the new morphological change is adaptive—maybe its just due to neutral alleles. If much of the morphological variation in a population is neutral then it will take a long time to become fixed in a population, especially one with a large effective population size. By chance, a small segment of the population may split off from the main body and become reproductively isolated. Due to the founder effect, this small population may accidentally contain morphological variants that rapidly become fixed in the new population by random genetic drift. When the daughter population comes back in contact with the parent population it looks different.
This explanation of stasis suggests that most morphological variation, of the sort that distinguishes closely related species, is neutral and stasis simply represent the slowness of change by drift.
I favor the last explanation as most Sandwalk readers would have guessed. I don't expect that every evolutionary biologist will prefer this explanation but I do expect most of them to be aware of all the various explanations. That's why I was surprised when I went to a recent talk at Evolutin Ottawa 2012.
The next day I heard a plenary session talk by Stevan Arnold, the incoming President of the the American Society of naturalists. He claimed that stasis was due to "bounded evolution" which I think is similar to some form of balancing selection—an idea that I thought had died in the 1960s (Uyeda et al., 2011). Jerry Coyne blogged about it last year: Want evolutionary change? Wait a million years.
Here's what I think Arnold and his colleagues are saying. Morphological change is under selective pressure but only small subsets of the population are experiencing adaptation at any one time. Those small populations are constantly exchanging alleles with the main population and that tends to dampen out the short-term selection that was occurring. Over time, the local adaptations tend to revert back to the standing variation in the main population. Thus, large scale deviations in morphology only happen within strict bounds giving rise to the appearance of stasis.
I don't see that this is any different than the idea of "brownian motion" involving neutral alleles and drift but the authors claim to have ruled this out using their mathematical models.
Here's the problem. Many evolutionary biologists are convinced that morphological change has to be due to selection. According to this view, the most likely way for new species to adapt is by gradual selection for new traits over time. This makes it difficult to explain millions of years of stasis where different traits (alleles) coexist without obvious selection.
That's the "paradox" of stasis. It's not consistent with adaptive evolution of morphological traits. Balancing selection of some sort is a way out of the problem but there aren't many examples of balancing selection (sickle cell alleles in countries with malaria is one).
I didn't realize until last month that stasis was such a problem for most evolutionary biologists.
With natural selection operating in a changing environment as an agent of adaptation, we expect to see changes at the organismal, ultimately physiological and morphological level. How, though, can we explain the paradoxical situation in which environments change, even dramatically, but organisms do not? (Wake et al., 1883, quoted in Gould, 2002, p. 857).Maybe the premise is wrong?
Uyeda JC, Hansen TF, Arnold SJ, Pienaar J. (2011) The million-year wait for macroevolutionary bursts. Proc. Natl. Acad. Sci. (USA) 108:15908-15913. [PubMed] [doi: 10.1073/pnas.1014503108]