Darwinists have never been comfortable with reports of "stasis". They have never been reconciled to Gould's statement that "Stasis is data". Instead, stasis is regarded as a paradox and as a problem to be solved. Estes and Arnold have contributed a significant new paper on the subject. In one paragraph, they review the evidence that stasis might have a genetic explanation and conclude that this refutes "the notion of omnipresent, internal constraints on evolution." However, their perspective is entirely Darwinian, and there is no engagement with structuralist thinking, or bau-plans or basic types. Darwinian theory makes no distinction between adaptations of traits and the development of novel structures, so the fact that "populations are often well equipped genetically to respond to at least short-term ecological challenges" is sufficient to settle the matter. Then "stabilising selection emerges as the leading contender among explanations for stasis."
Estes and Arnold employ the adaptive landscape concept to "evaluate the degree to which six evolutionary models fit the observed data." Three of these models are based on random variation, and these do not fit at all. In a News and Views article in Nature, Hendry comments "This conclusion will be reassuring, or perhaps just obvious, to the innumerable evolutionary biologists who believe that adaptation plays a central role in evolution". The other three models all involve a shift in the fitness peak of the adaptive landscape. In one case, the peak moves continuously in a particular direction. In the second case, there are two peaks separated by a valley. The third case is called the "displaced optimum" model; it has a single peak which jumps in a generation to a new location and stays there. "In the authors' estimation, this last model fits the data quite well."
Approaching a conclusion, Hendry explains the title of his article: "For some, any report of the death of this paradox will probably evoke the same reaction as the death of Elvis, with a large number of fans reluctant to accept its passing. But in the end, evolutionary biologists will probably converge on more pertinent questions, such as 'What generates and maintains adaptive zones in the first place?', and 'How do some lineages ultimately bridge the gap between different adaptive zones?'. In other words, these biologists can get back to the same questions that they have been puzzling over for 60 years!
The analogy with the death of Elvis is most unfortunate, because there is no valid comparison. Stasis is the conclusion of extensive analysis of data by specialists, not the emotive response of bystanders. The adaptive landscape, on the other hand, is a theoretical construct offered to illustrate adaptive change, but never validated by empirical research. The paper by Estes and Arnold provides theoretical modelling of reported data, with no analysis of the environmental factors changing the adaptive landscape, and no attempt to explain the changes in terms of adaptive responses to environmental factors. There are really so many loose ends to this work that the authors should not be allowed to get away with the idea that they have confronted "the predictions of alternative evolutionary models with the reality of data".
The paradox of stasis is not dead, nor is it in its death throes!
Resolving the Paradox of Stasis: Models with Stabilizing Selection Explain Evolutionary Divergence on All Timescales
Suzanne Estes and Stevan J. Arnold
American Naturalist, February 2007. Vol. 169, pp. 227-244.
ABSTRACT: We tested the ability of six quantitative genetic models to explain the evolution of phenotypic means using an extensive database compiled by Gingerich. Our approach differs from past efforts in that we use explicit models of evolutionary process, with parameters estimated from contemporary populations, to analyze a large sample of divergence data on many different timescales. We show that one quantitative genetic model yields a good fit to data on phenotypic divergence across timescales ranging from a few generations to 10 million generations. The key feature of this model is a fitness optimum that moves within fixed limits. Conversely, a model of neutral evolution, models with a stationary optimum that undergoes Brownian or white noise motion, a model with a moving optimum, and a peak shift model all fail to account for the data on most or all timescales. We discuss our results within the framework of Simpson's concept of adaptive landscapes and zones. Our analysis suggests that the underlying process causing phenotypic stasis is adaptation to an optimum that moves within an adaptive zone with stable boundaries. We discuss the implication of our results for comparative studies and phylogeny inference based on phenotypic characters.
Hendry, A. Evolutionary biology: The Elvis paradox. Nature 446, 147-150, (8 March 2007) | doi:10.1038/446147a
Abstract: Evidence for a universal driver of evolution across all timescales could mean that the venerable paradox of stasis is dead. But even with such evidence, some biologists would be reluctant to accept its passing.
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