NATURE VS. NURTURE: PARTITIONING THE PHENOTYPE AND ESTIMATES OF HERITABLE VARIATION IN FOSSIL STUDIES OF MICROEVOLUTION AND SPECIATION

Modularity in colonial organisms allows for partitioning of variation in skeletal phenotype into its genetic and environmental sources. In studies of microevolutionary patterns and processes relevant to speciation, the question of the degree of variation accounted for by an organism’s heritable genotype, relative to environmental effects on morphogenesis, have limited our reading of the fossil record, even with its unique temporal perspective for evolutionary biology.

Nested Analysis of Variance (ANOVA) allows for hypothesis testing of the significance of genotype vs. environment, and provides for direct calculation of the amount (percentage) of the total variance attributable to each factor. A review of our own and other’s previous work, based on ANOVA, shows that in experimental designs set up to constrain genotypic and environmental variation, 30% to 60% of morphological variation can be attributed to genotypic effects. Environmental effects typically account for 0% to 10% of the morphological variation, and the residual (variance not accounted for by the model), which is typically 50% to 70%. The residual can be reduced by accounting for module position or within-colony environmental variation.

With sophisticated experimental designs applied to modern organisms, the genotype can be further partitioned into heritable and non-heritable sources. The present study reports new results from analysis of field manipulations of multiple genotypes of the modern encrusting bryozoan Electra pilosa (L.). In the first half of the experiment, colonies were grown in a wild, high-energy tidal setting in Western Scotland. Live colonies were transplanted to other environments and collected after additional growth of the same colonies in a second environment. Genotype accounted for an average of 50.9% of the observed phenotype, whereas environment accounted for an average of 0.8%. Residuals averaged 43.9%.

These methods could be applied to any skeletal, colonial organism. We do not know how our results apply directly to solitary organisms, but we feel that a benchmark of 30% to 60% of morphological variation attributable to the genotype is a place to start for comparison in paleontological studies of microevolution and speciation.