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he thought that it could yield relatively large morphological changes by simple modifica-
tions of development, yielding large bursts of change in a short amount of time (
Gould,
1977, 1982; Maderson et al., 1982
). Second, heterochrony was thought to occur by selection
on size and/or life-history parameters, leading to predictable changes in morphology as a
by-product (
Gould, 1977, 1982, 1988; Maderson et al., 1982
). In regarding morphological
evolution as a by-product of selection on size or life-history, Gould followed
Huxley's
(1932)
reasoning that shape evolves by selection on size. The morphological changes might
have no selective value; they are simple correlates of selection on size and/or life-history
(
Maderson et al., 1982; Gould, 1988
). This connection between heterochrony and evolution-
ary allometry is seemingly obvious, but Gould was the first to recognize and emphasize it.
The idea that morphology evolves in a specific direction due to selection on size or life-
history is why Gould thought that heterochrony provides the best empirical data for the
study of developmental constraints (
Gould, 1988
). The third reason why Gould found
heterochrony to be so interesting is that it linked ecology to morphology; he argued, for
instance, that progenesis (truncated development, see below) would be causally linked
to unstable (r-selected) environments that promote rapid maturation whereas neoteny
(slow development) would be linked to stable environments (
Gould, 1977, 1988
). Under
contrasting ecological conditions, heterochrony would yield the same morphological
outcomes
descendant adults that resemble ancestral
juveniles, under contrasting
ecological conditions.
As well as regarding heterochrony as interesting, and even as especially informative
about developmental constraints, Gould also aimed to rehabilitate the concept of recapitu-
lation. He denied that recapitulation is a general rule, but he, nevertheless, argued that the
idea had been unfairly dismissed and for reasons unrelated to the failure of the theory.
Certainly, he was not alone in attempting to rescue the idea of recapitulation; others,
especially
Cope (1887)
also tried to do so, but by applying the concept to individual parts
(or measurements). Unlike them, Gould took an organismal, multivariate view of parallel-
ism. He strongly opposed the trait-by-trait approach to morphology, whereby each
individual organ (or measurement) is accorded its own explanation. Instead of that
approach, which he called “atomistic”, he favored viewing organisms as integrated enti-
ties, bound together by developmental correlations. Formal models predicting the evolu-
tionary response to selection on size (or any other trait) for the bivariate (
Lande, 1979
) and
more general multivariate case (
Lande and Arnold, 1983
) confirm Gould's intuition
that selection on body size or life-history trait can indirectly affect morphology, just not
necessarily in the direction of ontogenetic scaling or heterochrony.
The fundamental idea underlying all these implications of heterochrony is that growth,
morphogenesis and maturation can be dissociated from each other. Growth refers to an
increase in size (with size being equated to geometric scale), morphogenesis refers to the
process that alters shape, and maturation to the attainment of sexual maturity. Although
this separation, especially between size and shape, has sometimes been viewed as justified
solely on operational grounds, i.e. as necessary for the construction of Gould's clock
model, Gould justified separating them on the grounds that their dissociability is neces-
sary for heterochrony. Of course, they are correlated within any ontogeny, but they are
potentially dissociable in their evolution. When growth and morphogenesis are dissociated
from age, the descendant has the ancestral shape and size at a different developmental
