Punctuated Equilibrium and the Validation of Macroevolutionary Theory 941
recent volumes have explored the growing power and prestige of this argument, as
provided by breakthroughs in unraveling the genetics of development (see Chapter
10), combined with classical data of allometry and heterochrony (Raff, 1996;
Schwartz, 1999; McNamara, 1997; McKinney and McNamara, 1991; McKinney,
1988).
Wray (1995) has recently summarized an emerging generality that integrates
all components of the argument across a wide variety of organisms. His chosen
title—"Punctuated evolution of embryos"—underscores the putative generality of
change in this mode, with punctuated equilibrium as its major expression at the
level of ordinary speciation, and his proposed linkage of development and ecology
as its hypothesized primary source for the rarer, but highly consequential,
phenomenon of the origin of morphotypic novelty.
Nearly two centuries of tradition proclaim the conservatism of early larval
and embryonic phases of the life cycles—from von Baer's enunciation of his
celebrated laws (1828, see discussion in Gould, 1977b) to the standard
evolutionary rationale that formative stages of early ontogeny become virtually
impervious to change because cascading consequences, even of apparently minor
alterations, would discombobulate the subtle complexities of development. Recent
discoveries of "deep" genetic homologies and developmental pathways among
animal phyla separated for more than 500 million years (see Chapter 10) have
tended to highlight this conventional view.
But Wray (1995) summarizes several recent studies of broad taxonomic
scope—with best examples from the sea urchin Heliocidaris, the frogs
Eleutherodactylus and Gastrotheca, and the tunicate Mogula—all showing that
"similar species have... modifications in a variety of crucial developmental
processes... that have traditionally been viewed as invariant within particular
classes or phyla" (Wray, 1995, p. 1115). These substantial changes in the
development of closely related forms exhibit three common properties: (1) They
usually affect traits of timing and regulation in early development, including
specification of cell fates and movement of cells during gastrulation. (2) They yield
substantial changes in larval forms and modes of life, but often leave the adult
phenotype largely unaltered. (3) They are associated with major changes in the
ecology and life history strategies of larval or early developmental forms, and
involve such major changes as loss of larval feeding ability (the echinoderm and
frog examples) or capacity to disperse (tunicates).
Wray presents evidence that alterations in larval ecology "drive changes in
development," not vice versa. Moreover, and most importantly, comparison of
molecular and phenotypic modification shows that these "functionally profound
changes in developmental mechanics can evolve quite rapidly" (ibid., p. 1116). For
species of Heliocidaris with strikingly different developmental mechanisms, fewer
than 10 million years have elapsed since divergence from common ancestry. Wray
draws a general and punctuational conclusion from this evidence for ecologically
driven change in mechanisms of early development—events that can occur very
rapidly and do not compromise the conserved life styles of later development due
to greater dissociation