MORPHOLOGICAL MODULARITY AND MACROEVOLUTION

Morphological modularity refers to the dissociability of phenotypic wholes into parts. It provides a basis for understanding biodiversity in combinatorial terms. Morphological modules have potentially causal roles in the structuring of ecological and genealogical systems, but the importance of modularity in macroevolution remains an open empirical issue. Here, the relationship between morphological modularity and diversity, size, evolvability, trends, and context-dependence is explored in echinoderms. Three candidate metrics of morphological modularity are used: a) number of characters or constructional elements; b) within- and among-module morphological integration; c) disparity.

Within echinoid higher taxa, the number of derived character states correlates with taxonomic diversity, but not with size variance. Derived regular and irregular echinoids have on average an equivalent number of derived character states, but the origin of the Irregularia appears to have involved a genuine increase in modularity.

Evolvability is thought to be promoted by parcellated phenotypes. Patterns of morphological integration and disparity in atelostomate echinoids suggest that more modular regions of the test indeed changed more readily, and contributed a larger proportion of the variance at the origin of holasteroids and spatangoids.

Based on the extraxial-axial theory of echinoderm homologies, the phylogenetic sequence of divergence of echinoderm classes suggests a trend towards reduction of modularity as inferred from skeletal types. It has been suggested that in metazoans parcellation should be more common, but this may more often involve the origin of modules, not their fate.

Inferences about morphological modularity depend on the context in which modules are defined. With plate columns as modules, Paleozoic echinoids are more modular than post-Paleozoic echinoids; with plates as modules, holasteroids and spatangoids are the least modular, and temnopleurids and phymosomatoids the most modular echinoids; with plate types as modules, irregular echinoids are more modular than regular echinoids. Modularity is relative, and its role in macroevolution multifactorial.