1146 THE STRUCTURE OF EVOLUTIONARY THEORY
Akam envisages the gradual evolution of different enhancers (or different levels
and mixtures of the same enhancers) in various arthropod segments, leading to a
phyletically diverging regulation of the same Hox gene down the body axis, but with
continued expression in a segment-specific manner. " Ubx for example is regulated
by the 'abx' enhancers in parasegment 5, which integrate patterning information in
one way, but by the 'bxd' enhancers in parasegment 6, which specify a different
within-segment pattern." As these alterations in expression evolved gradually within
different segments, "the change would not necessarily be recognized as a 'homeotic
mutation'" (p. 448). These and other models reinforce the important principle that
extensive and discontinuous phenotypic effects in the development of modern
organisms do not imply the saltational origin of these features in phylogeny.
Other models, however, permit more space for an important frequency of
saltational shifts in evolution. Duboule and Wilkins (1998), for example, tie an
increasing propensity for saltational change to functional recruitment of genes for
multiple tasks: "Transitions from gradual to discontinuous rates of evolutionary
change are an inevitable consequence of the multiple use of genes through
evolutionary tinkering, given appropriate selective pressures" (1998, p. 54).
Interestingly, in the context of this chapter and the history of this subject, their model
explicitly links this increasing frequency of saltation to the "hardening" of internal
constraints that arise as a consequence of incorporating key genes into multiple
networks of regulation. They write (p. 58): "The greater the number of networks that
a gene product is involved in, the smaller the scope for new variations to be offered to
natural selection. The idea of internal constraints leading to restrictions in the
production of evolutionary novelties is not new. However, we would like to argue
that internal constraints result, indirectly but inevitably, from the increasing work
load imposed by successive recruitment of genes to new functions." In networks of
such complexity, they conclude, any "novel equilibrium will have to be established as
a one-step event and not through the accumulation, in time and space, of many
mutations of small effect, or gradualistic change" (p. 58).
In any case, and however important such saltational changes may be in
establishing fundamental evolutionary novelties (my own betting money goes on a
minor and infrequent role), phyletic discontinuity at lower taxonomic levels, based on
small genetic changes with large regulatory effects, has been documented in several
cases. In a fascinating example, Ford and Gottlieb (1992) found that about 20- 30
percent of several hundred Clarkia concinna plants growing in a single locality at
Point Reyes, California displayed the bicalyx mutant, a homeotic variation that
replaces the usual circlet of four bright pink petals with a second circlet of sepals.
By Mendelian analysis of ratios in cross breeding between normal and bicalyx
plants, Ford and Gottlieb established that a single point mutation produces the bicalyx
phenotype. Moreover, and in contrast to many homeotic mutations, the bicalyx plants
show no developmental abnormalities and no apparent fitness depressions; insect
pollinators continue to visit bicalyx flowers with no apparent reduction in frequency.
Ford and Gottlieb note (1992, p. 673) that "homeotic mutants have been found and
propagated in