Environmental Stability and Species Proliferation in Late Cambrian Trilobite Faunas: A Test of the Niche-Variation Hypothesis

Abstract

The “niche-variation” model predicts that increase in environmental stability should be accompanied by increase in homozygosity and reduction in morphological variability. All Late Cambrian trilobite biomeres show an increase in regional species diversity from low in the biomere toward the top. This change in diversity is believed to reflect increasing environmental stability and consequently affords the opportunity to indirectly test the “niche-variation” hypothesis in a paleontological context. Measurements were made of eight cranidial features of samples of 17 species populations from the Pterocephaliid Biomere of the Great Basin. Coefficients of variation and a multivariate analog of them failed to reveal a relationship between morphological variability and species diversity. Consideration of these data together with contradictory, but limited, informaton for other organisms suggests that the predicted decrease in genetic variability may either be absent or be readily masked in naturally occurring populations, which typically retain a high degree of genetic polymorphism. If heterozygosity is common, it would appear that accidents of geography, rather than the genetic consequences of stable or unstable environments, are among the primary factors controlling the probability of speciation.