108 Evolution and the Fossil Record
range of variation and into new body plans. These changes were due to slight changes in
“controlling genes” (what we now call regulatory genes). According to Goldschmidt, specia-
tion was a discontinuous, rapid process that was caused by changes in controlling genes,
not by accumulation of small microevolutionary changes. If a new macromutation appeared
that gave the individual a big advantage, it might produce a “hopeful monster” that could
establish a new species or a new adaptive zone.
Naturally, such opinions were highly unorthodox with respect to the gradualistic ideas
of the newly dominant neo-Darwinians, and they subjected Goldschmidt to ridicule and
scorn. When I was taking evolution classes from hard-core neo-Darwinians in graduate
school, they would scoff “How does the hopeful monster find a mate?” Without more than
one hopeful monster, there is no possibility of breeding or establishing a new population,
and thus there would be no chance of a new species forming.
Ironically, the past 20 years have vindicated Goldschmidt to some degree. With the dis-
covery of the importance of regulatory genes, we realize that he was ahead of his time in
focusing on the importance of a few genes controlling big changes in the organism, not small-
scale changes in the entire genome as neo-Darwinians thought. In addition, the hopeful mon-
ster problem is not so insurmountable after all. Embryology has shown that if you affect an
entire population of developing embryos with a stress (such as heat shock), it can cause many
embryos to go through the same new pathway of embryonic development, and then they all
become hopeful monsters when they reach reproductive age (Rachootin and Thomson 1981).
The more we learn about regulatory genes, the more we realize their primary impor-
tance to evolution. A common example is the study of heterochrony, where organisms change
the sequence of their developmental timing. This allows evolution to take advantage of the
changes already encoded in our embryology and development. For example, nature fre-
quently makes changes through neoteny, where an organism retains its juvenile body form
while achieving reproductive maturity. The most famous case involves the salamanders
(such as the Mexican axolotl) that do not complete their metamorphosis into lunged sala-
manders but hold on to their juvenile gills and body form, yet they can breed like adults
(fig. 4.4). Whenever these salamanders are exposed to stagnant water conditions, they can
complete their metamorphosis into lunged adults and walk to the next fresh pool of water.
Thus, this ability to choose to breed either as the juvenile or adult body form gives them
great ecological flexibility, all with a few tiny changes in the regulation of their development.
As Stephen Jay Gould pointed out in his book Ontogeny and Phylogeny (1977), this mech-
anism is extremely common in nature, especially when the juvenile and adult body forms
have radical differences in shape and ecology, and allows the organism to “switch-hit” for
whatever works best. Those pesky aphids that invade your flowers each spring are a classic
example. When the food resources are abundant (in the spring and summer), they multiply
rapidly, with each female giving birth to immature daughters as asexual clones (no males are
born at all). Those offspring, in turn, also reproduce asexually as juveniles, so they can make
literally hundreds of daughters in a short period of time (which is why they can infest your
flowers so quickly). When the fall comes and the food resources dry up and cold weather
approaches, they switch to sexual reproduction. A few males are born and mature into adults
and then quickly mate with adult females. These lay normal eggs that can survive the winter
and hatch out next spring to start the process all over again. All of this evolutionary flexibil-
ity does not require big changes in the genome, just small changes in regulating the normal
sequence of embryonic development already encoded in the organism.