Recent studies of Paleozoic benthic marine invertebrate communities and ecological investigations of modern marine and terrestrial environments have provided information on patterns of faunal density and diversity, patterns which are commonly inferred to be controlled primarily by the physical milieu. Furthermore, there is some evidence indicating that genetic (chromosomal) variability in some invertebrates is correlated with the stability of the physical environment. Previously, these three areas of investigation had not been brought together in consideration of the evolutionary implications of long-term (tens or hundreds of millions of years) environmental stability. Three simple relationships are herein formulated from Recent and fossil data: (l) Environmental stability is positively correlated with diversity; (2) Environmental stability is negatively correlated with long-term biotic (taxonomic) stability; and (3) Therefore diversity is negatively correlated with biotic stability. To explain these observed correlations, we propose an evolutionary model based on genetic-adaptive strategies.

We contend that the genetic-adaptive strategy employed by a species population depends in large part on the regularity, direction, and rate of change in environmental stability. A prolonged period of stability could significantly reduce a species' potential for survival in a more rigorous environment, and hence might be expected to precede a time of widespread or mass extinction. A homozygotic genome would be selected for (homoselection) by species subjected to long-term environmental stability, and it is this selection that could ultimately prove disastrous for survival in a later, only slightly less stable regime.

Recognizing the teleological pitfalls that appear in any evolutionary model hinting at predictability, we nevertheless believe that attempts at the development of hypotheses based on adaptive patterns or strategies have a very definite heuristic value, if only in preventing the facile recourse to exotic environmental events as an explanation for major evolutionary events in the history of life. The proposed hypothesis of genetic-adaptive strategies should be viewed as a first approximation, perhaps, at best, leading to empirical research, which for now is introduced in an attempt to encourage investigation of environmental events leading up to periods of mass extinction.

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