Progenesis and Neoteny

In this chapter, I plan to test the hypothesis that progenesis and neoteny—despite their common consequence of paedomorphosis—evolve as adaptations to strikingly different ecological conditions: progenesis, with its acceleration of maturity, is a life-history strategy for r selection; while neoteny tends to evolve in the stable, favorable, and "crowded" situations that specify K selection.

For a primary test, I shall contrast the most famous and unambiguous cases of paedomorphosis in modern organisms: progenesis in various insects (including wingless aphids and parthenogenetic larval gall-midges), and neoteny in ambystomatid salamanders. These cases are especially favorable for several reasons:

1. Their fame has led to an extensive, recent literature.

2. Heterochrony can be induced experimentally by disturbing an endocrine mechanism that probably regulates the occurrence of paedomorphosis in nature as well.

3. Both involve a profound metamorphosis in ontogeny, providing good markers for the diagnosis of heterochrony.

4. Paedomorphosis is not genetically fixed in either case. It is one of several ontogenetic paths potentially available to each member of a population. Its regulation by the immediate environment can be easily tested. Moreover, in deciding whether a paedomorphic form is progenetic or neotenic, comparisons can be made with genetically similar, conspecific, normal forms, not with inferred generation times in hypothetical ancestors.