Critics of the concept of organic change through time have demanded proof not only of “transitional forms” but of specific transitions among higher taxonomic groups. Transitional forms among species and between a species of one genus and a species of another genus have been criticized because most demonstrated ancestor-descendant transitions are considered to occur within one “kind” of organism; the “kind” concept is bereft of biological meaning.

Natural selection acts upon organisms at all stages of ontogeny, and especially at larval-juvenile stages. Large shifts in the morphology of one or more features are common in groups of organisms that evolve by heterochrony. Because heterochrony involves a change in timing of the appearance or development of a particular feature, recognition of heterochrony requires a confident knowledge of ontogeny. The resulting increase in complexity (e.g. recapitulation) or decrease in complexity (e.g. paedomorphosis), well documented among living organisms, commonly excludes morphologic intermediates. Paedomorphosis is especially important in the evolution of progressively simplifying lineages and has been well documented from living plants and animals and fossil representatives of echinoderms (blastoids, crinoids), conodonts, arthropods, mollusks and vertebrates. Heterochrony characterizes the evolution of most metazoan organisms, occurs at all taxonomic levels and was probably responsible for major innovations by which higher taxonomic groups are recognized.