Table 8. The macroevolutionary significance of heterochrony.

Less common
Usual heterochronicheterochronic event
event with strongwith great macro-
immediate advantagesevolutionary potential

r selectionProgenesis, withProgenesis, with an
specialization andexperimental set of
simplificationjuvenile and adult
accompanying pre-characters freed from
cocious maturationrigid selection upon
morphology (exceedingly
K selectionHypermorphosis, withNeoteny, with delayed
extended differentiationmaturation linked to
and increase in specializedretarded differentiation
complexity accompanyingand retention of flexible
delayed maturationjuvenile morphology
(moderately common)

permorphosis only because evolutionary trends usually lead to increasing differentiation of ancestral structures (see, for example, M. Williams, 1976, on how, under regimes of sexual selection in vertebrates, the advantages to males of appearing older will yield greater differentiation in phylogeny along ancestral ontogenetic pathways).

I envision two distinct macroevolutionary roles for the two forms of paedomorphosis—linked in a complex way with their differing immediate significances as r strategies for precocious maturation and K strategies for juvenile morphology. Progenesis may lead to the origin of higher taxa in a rare and serendipitous fashion when morphology is released from rigorous control by selection. Neoteny provides evolutionary flexibility as a moderately common pathway to adaptation in K environments; its occurrence is promoted by the common developmental correlation of delayed maturation with retarded somatic development. I believe that the key to human evolution lies in such a correlation (Chapter 10).

The Social Correlates of Neoteny
in Higher Vertebrates

Delayed maturation has been correlated with crowding and intense intraspecific competition in several groups of birds. Ashmole (1963) proposed an individual advantage for delayed maturation in tropical