Integrating developmental evolutionary patterns and mechanisms: A case study using the gastropod radula

Determining the connection between ontogeny and phylogeny continues to be a major theme in biology. However, few studies have combined dissection of p attern and process that lead to transformation of complex morphological str uctures. Here we examine the patterns and processes of shape change in a mo del system-the gastropod radula. This system is a simple one having only tw o processes: initial secretion and postsecretional movement of teeth. Howev er, it produces a tremendous amount of shape variability and fusion pattern s. To determine both pattern and mechanism of shape change in an evolutiona ry context, we use three complementary approaches and datasets. First, we u se a phylogenetic hypothesis to determine the polarity of developmental eve nts. Second, we perform a morphometric analysis of shape change using relat ive warp analysis that allows us to locate and compare the direction and ma gnitude of ontogenetic and phylogenetic shape divergence. These comparisons are the basis for testing hyptheses of heterochrony and heterotopy, and we show how our results do not conform to expectations of pure heterochrony. The rejection of heterochrony as a hypothesis is based on empirically demon strating (1) initial shape differs in each taxon; (2) a single dimension of shape variability does not simultaneously describe ontogenetic and evoluti onary shape changes; and (3) a significantly different shape and size covar iance between taxa. This rejection is probably based on spatial changes in initial conditions and not spatial changes caused by the process itself. Fi nally, we construct a mechanistic model that explains how shape change happ ens based on the sequence of events during ontogeny. By using the parameter s in the model as characters in the phylogenetic dataset, we show that diff erent parts of the system have arisen at different times and become co-opte d into the process. By integrating our analyses together we show that spati al process parameters can be responsible for our nonspatial patterns and th at different ontogenetic processes can create similar end morphologies.