Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 255

traits developed in response to stressful stimuli (high temperature or ether vapor,
both of which are not mutagenic) and produced phenotypic changes (ultrabithorax
and crossveinlessness) in a small number of individuals. By selecting and cross-
ing these changed individuals for many generations, he obtained individuals that
expressed the new traits even in the absence of the inducing environmental stimuli.
To explain the phenomenon, it is assumed that the evolution of the innate trait from
an acquired trait was enabled through the repetitive crosses between the individu-
als that showed the new trait with individuals that simply had the genes responsible
for the trait. Hence, the environmental stimuli merely exposed the existing genetic
variation. The fact that neither of the new traits seems to be adaptive makes it impos-
sible to qualify them as an evolutionary change that would survive under natural
conditions.
In modern biological literature, Pigliucci and Murren (2003), and Pigliucci et al.
(2006) not only revitalized Waddington's hypothesis, but also enriched it in one essen-
tial respect: they argued that the emergence of the developmentally induced new pheno-
type may not be related with/depend on the existing genetic variation in the population.
But it may also precede the evolution of the genetic determinants of the new phenotype.
For them, this renders the hypothesis to “be a particular case of the broader possibility
envisioned by West-Eberhard, specifically when the origin of the trait (step 1) is due to
an environmental, rather than a genetic, change” (Pigliucci et al., 2006). The evidence
to support the hypothesis is scarce at best. As for the possible role of genetic assimila-
tion in evolution, the proponents of the hypothesis believe it “is of course a matter for
empirical investigation” (Pigliucci et al., 2006).
The Hypothesis of the Genotype Network
Andreas Wagner's hypothesis of the origins of evolutionary innovations relies more
on a computational approach to the origins of evolutionary innovations than the pre-
ceding theories. The theory envisages populations as collections of genotypes in gen-
otypic spaces within which genotype networks of individuals of the same phenotype
exist. Mutations can change the position of individuals within the genotype space. A
change in the environment may make the phenotype of an individual better adapted
to the changed environment. The population can find this existing to superior geno-
type within the genotype network via several small mutations with little phenotypic
change. Within the genotype space, genotype neighborhoods also exist, which allow
that genotype to be reached by any other genotype in the genotype space via one or
a number of mutations. The genotype networks thus allow small step changes in the
genotype that can be reached by any genotype via small step mutations and genotype
neighborhoods allow different genotypes to search for different new phenotypes in
the neighborhood.
According to the theory, there are three classes of systems that bring about novel-
ties in the living world, all of which exist in respective genotype spaces of possi-
ble metabolic networks. The first class comprises hundreds to thousands of chemical
reactions catalyzed by specific enzymes in metabolic networks. According to the