Heterchrony: Subtle changes over evolutionary time in an organism's developmental timetable are a potent source of overall evolutionary change. This is an idea with a history as long as the study of evolution.
Ernst Haeckel (1836 - 1919) Noted that the embryos of evolutionarily derived creatures may pass through stages in which they display ancestral traits. The human embryo, for instance, at certain stages has a tail and open gill slits. Proposed what he termed the biogenic law which maintains that "ontogeny recapitulates phylogeny." He attempted to integrate it with Darwinian evolution through the notion that evolutionary change is accomplished by the addition of new changes to the end of the embryological or developmental sequence. This idea seems simplistic when you consider what it implies about the evolution of butterflies, but it was eagerly embraced for the first half of the 20th century. (Note: Haeckel was an interesting originator of vocabulary. He gave us the terms "heterochrony" and "biology."
Karl Ernst von Baer (1792 - 1876) - Was an opponent of recapitulationism and set forth a series of developmental laws that held that:
Stephen J. Gould: (1941 - 2002) Accumulating evidence during the 20th century revealed many weaknesses in the "biogenic law." Von Baer's laws, while holding up intact, cried out for elaboration. In 1979, Stephen Gould stepped to the plate with the terminology and organization of heterchrony as we understand it. We have two basic forms of heterochrony, and three distinct parameters that can be tweeked to produce the change. The result is six distinct varieties. Study the diagram below for full illumination.
Paedomorphosis: Heterchronic change in which the adult of a derived organism resembles a juvenile of the ancestor.
Peramorphosis: Heterchronic change in which the juvenile of a derived organism resembles an adult of the ancestor.
A single orgamisn can incorporate both paedomorphic and peramorphic change. Consider humans:
If this is the ancestral life history:
Combining these developmental parameters gives us:
Postdisplacement: Postponed onset of development
Progenesis: Developmental offset occurs at an earlier time.
Predisplacement: Onset of development occurs at an earlier time
Hypermorphosis: Delayed developmental offset.
Cultural "evolution": Note that paedomorphosis and peramorphosis occur in popular cultural figures.
Fossil case study: Sabre-tooth cats - E.G.: North American Pleistocene Smilodon fatalis
Fossil case study: Blastoids - Pentremites
Blastoids were stalked echinoderms of the Paleozoic. Their thecae (main bodies) had upper portions, vaults and lower pelves. Blastoids tended to come in two general morphs:
In Pentremites pyriformis, a possible descendant of P. concoideus, the adult is also pyriform.
In Pentremites robustus, another possibile descendant of P. concoideus, the juvenile is godoniform.