Nonheterochronic developmental changes underlie morphological heterochronyin the evolution of the Ardeidae

Evolutionary changes in developmental timing and rates (heterochrony) are a source of morphological variation. Here we explore a central issue in hete rochronic analysis: are the alterations in developmental timing and rates t he only factor underlying morphological heterochrony? Tarsometatarsal growt h through endochondral ossification in Ardeidae evolution has been taken as a case study. Evolutionary changes in bone growth rate (morphological hete rochrony) might be either (a) the result of alterations in the mitotic freq uency of epiphyseal chondrocytes (process-heterochrony hypothesis), or (b) the outcome of alterations in the number of proliferating cells or in the s ize of hypertrophic chondrocytes (structural hypothesis). No correlation wa s found between tarsometatarsal growth rates and the frequency of cell divi sion. However, bone growth rates were significantly correlated with the num ber of proliferating cells. These results support the structural hypothesis : morphological acceleration and deceleration are the outcome of evolutiona ry changes in one structural variable, the number of proliferating cells.