Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 254

reorganization of the phenotype, which West-Eberhard terms “developmental recom-
bination,” resulting in a new phenotype.
If the new phenotype increases the fitness of the organism in the environment
and offers the carrier some advantage over other individuals, natural selection then
comes into play, favoring the spread of the phenotype in the population. From this
point onward, a process of “genetic accommodation” may begin. This process con-
sists in incorporating into the relevant developmental pathway a newly evolved gene
or an allele already present in the population. Because genetic accommodation fol-
lows the phenotypic change, West-Eberhard concludes that genes are followers
rather than leaders in evolution.
She supports her hypothesis with empirical evidence. For example, the recurrence
of two morphologically distinct species, the limnetic and benthic species pairs of the
three-spined stickleback ( Gasterosteus aculeatus ) complex of closely related fish
species, illustrates the developmental recombination; early on, some of these species
display limnetic morphology and feeding behavior (living in the water column and
feeding on plankton), but later in the life they appear benthic in both morphology
and feeding behavior, suggesting that the recurrent species pairs of the fish evolved
via a developmental switching (heterochrony) of an ancestral species that entertained
both limnetic and benthic morphology and behavior during its lifetime.
Empirical evidence on the role of hormone levels in expressing of parental care
by male birds with low testosterone levels and the incubation of eggs by males (like
sandpipers) also support her hypothesis of developmental recombination.
According to the hypothesis, evolution through developmental recombination
and its adaptive advantages counteract changes in genes and explain why the same
genetic toolkit is used recurrently in living organisms.
The hypothesis also articulates that speciation precedes reproductive isolation,
suggesting that speciation may occur in sympatry, whereas the neoDarwinian hypoth-
esis argues that allopatric speciation is almost the exclusive mode of speciation.
Developmental plasticity's great achievement is the first hypothesis to repre-
sent an empirically supported mechanism of production of evolutionary changes in
which epigenetic factors play the leading role. While changes in the developmental
pathways (e.g., in hormonal pathways) certainly produce phenotypic changes, the
hypothesis fails to demonstrate the ultimate source of information necessary to pro-
duce these adaptive, nonrandom, changes in developmental pathways.
The Genetic Assimilation Hypothesis
Essentially, this hypothesis dates back to the American psychologist James Mark
Baldwin (1861-1934), who argued theoretically that in a changed environment, a
new phenotype such as a learned behavior may help an organism survive or increase
its reproductive success. After a number of generations, inheritance mechanisms may
evolve making the behavior inherited even in the absence of a stimulus.
In the 1950s, Conrad Hal Waddington (1905-1975) demonstrated experimen-
tally in Drosophila his hypothesis of genetic assimilation where Drosophila acquired