Situations in which the chance of traits arising depends on their embryonic development have been termed developmental constraints or developmental contingencies. One kind of mutation could occur and be unlikely to seriously harm the phenotype is a mutation that changes the rate at which the body, or a structure in the body, develops. Such a mutation would keep the same basic developmental steps, so that the overall developmental process would be less likely to be harmed, but they would occur either faster or slower than they did in individuals without the mutations. Heterochrony refers to a change in the rate of development of an organism or a structure of that organism. Mutations that cause heterochrony may be more likely to evolve since they can produce new forms that may turn out to have high fitness without disrupting the process of development.
Terminal addition is the name for heterochrony that speeds up development.
Paedomorphosis is the name for development that slows down development. Developmental contingencies, and heterochrony, can result in relationships between the process of embryonic development and evolutionary history. Interestingly, relationships between either organismic diversity or evolutionary history and embryonic development had already been observed, but not clearly explained, long before people knew enough about causes of embryonic development to propose the hypotheses of developmental contingency and heterochrony to explain them. We will first describe the proposed relationships between development and evolution, or organismic diversity, and then look at how they might be explained by developmental contingency and heterochrony. Then we will consider how to test for the phylogenetic patterns predicted by hypotheses of heterochrony.