1144 THE STRUCTURE OF EVOLUTIONARY THEORY
and discontinuous phenotypic shifts in mutant organisms, or the conventional basis
assigned to them: small genetic alterations with major developmental consequences.
For example, a single base substitution in bicoid, the maternal gene product that sets
the AP axis by supplying positional information within the Drosophila larva, can
reverse the axes of symmetry (Frohnhofer and Nusslein-Volhard, 1986; Struhl et al,
1989). Of this and other cases, Akam et al., in the introduction to their 1994 book on
The Evolution of Developmental Mechanics write (1994, p. ii): "It is a commonplace
of developmental genetics that minimal genetic change can lead to the most dramatic
morphological effect."
Second, we can also posit believable mechanisms that avoid the classical
problems of specifying how genes with such disruptive effects could ever be
integrated into the intricate and finely tuned development of a complex metazoan
ontogeny and, even if viable, how such saltations could spread through populations
following their mutational origin in single individuals, given the almost inevitable
fitness depression that must accompany any interbreeding with modal individuals.
Schwartz (1999), for example, presents a modern version of the old argument for
origin in a nonlethal recessive state, followed by accumulation without expression in
heterozygotes until the achievement of a critical frequency permits an effectively
simultaneous overt appearance of the phenotype in numerous homozygotes.
Addressing the second aspect of this problem, and basing their case on a viable
homeotic mutant in a homonomous species of centipedes, Kettle et al. (1999, p. 393)
argue that most workable mutations of such large effect may arise in homonomous
ancestors of more specialized groups: "Perhaps the severe fitness depression
accompanying homeotic transformation would have been less pronounced, even
absent, in a primitive arthropod with many similar segments."
But mere plausibility doesn't imply likelihood, and two strong arguments would
seem to indicate a minimal role for the evolutionary efficacy of such developmental
saltations: first, a negative statement based on the fallacy of usual sources of
inference; and, second, a positive argument about more plausible alternatives for the
same sources of inference.
For the negative statement, both common arguments for inferring saltational
origins from modern developmental patterns falter upon the general fallacy
(discussed in detail in Chapter 11) of invoking current circumstances to make
unwarranted inferences about historical origins:
- The fact that major phenotypic effects accompany the repression or alteration
of key developmental switches in modern organisms does not imply a saltational
origin either for the switch itself, or for any extensive consequences of its mutational
variations. For example, the fact that Hox genes now repress the expression of Dll in
the insect abdomen, thus suppressing the development of appendages—and that a
single mutation (albeit ultimately lethal) in one Hox gene can reverse this effect and
emplace leg rudiments on each larval segment (Lewis, 1978)—does not permit the
inference that insects lost their abdominal legs in one phylogenetic saltation. - Reasonable inferences about saltational losses cannot be theoretically inverted