Historical Constraints and the Evolution of Development 1039
may yield extensive consequences for entire organismal phenotypes, as suites of
correlated characters change in concert with altered rates of development. Thus, with
strongly allometric ontogenies as favored channels, and with heterochrony available
as a mechanism to move sets of characters quickly along these channels, organisms
often meet both classical criteria—channeling and speed—for utilizing constraint as a
positive accelerator of evolutionary change.
For these and other reasons, the subject of heterochrony has generated a long,
memorable and voluminous literature (see De Beer, 1930 for the classic 20th century
statement; Gould, 1977b, for a historical and then-current summary; and McKinney,
1988, McKinney and McNamara, 1991, and McNamara, 1997, for subsequent
views). In a later summary, McKinney (1999) notes that three major themes have
marked the fruitful use of heterochrony within macroevolutionary studies in recent
years: heterochronoclines (or trends caused by temporal displacements of
developmental rates), heterochronic biases within clades, and the origin of novelties.
In summarizing the extensive literature on heterochronoclines, McKinney
emphasizes the same point stressed here—heterochrony and allometry as convenient
and available mechanisms, whereby selection can accelerate and intensify adaptive
change (making positive constraint a "partner," not an "antagonist," of selection in
many cases). He writes (1999, p. 150): "Heterochronic variation is a very rapid, easy
way to produce coadapted suites of traits. It makes sense that simple extrapolations
(or truncations) of major environmental parameters (such as water depth, sediment
size and temperature) could select for relatively simple extrapolations (or truncations)
along the ontogenetic trajectory of a population (= cladogenesis) or species (=
anagenesis)."
The frequency of neoteny in salamanders, potentiated by unusual ease in
dissociation of sexual maturation from somatic development, represents the classic
case of heterochronic biases. In another sensible correlation of positive constraints in
heterochrony with adaptive utility, Whiteman (1994) demonstrates that amphibian
paedomorphs generally arise when the aquatic habitat of larvae becomes more
productive, or more stable, than the terrestrial environment of adults. At a larger
scale, and in an intriguing macroevolutionary speculation, McNamara (1997) surveys
the known examples of heterochrony among trilobites, and finds that paedomorphosis
predominates in Cambrian lineages, while the opposite processes of peramorphosis
seems to gain the higher relative frequency in later Paleozoic lines. McNamara
wonders if this pattern might not reflect changes in the organization and activity of
homeotic genes in times of evolutionary turmoil in and after the Cambrian explosion
vs. the relative "calm" of the later Paleozoic evolutionary world.
For the third theme of evolutionary novelties, the classic literature has stressed
the role of global paedomorphosis, usually in the progenetic mode (as noted in Gould,
1977b), in shedding the "excess baggage" of adult complexity and reverting to the
more labile phenotypes that often characterize juvenile forms—an "escape from
specialization" in the classic description. Such