The Evolution of Evolution 111change in the Hox genes can multiply the number of segments or reduce them and switch
one appendage (e.g., a leg) on each segment with another (e.g., a crab claw or an antenna
or mouth parts). Arthropods are a classic example of this modular development with inter-
changeable parts; with a small change in Hox genes, whole new body plans can easily
evolve to exploit new resources.
All of these ideas are part of the exciting new research field known as “evolutionary
development” (nicknamed “evo/devo”), and it is now the hottest topic in evolution. From
the neo-Darwinian insistence on every gene gradually changing to make a new species, we
now realize that only a few key regulatory genes need to change to make a big difference,
often in a single generation. This circumvents many of the earlier problems with ideas
about macroevolution and makes it entirely possible that the processes that build new
body plans and allow organisms to develop new ecologies are not the small-scale micro-
evolutional changes extrapolated upward. Some evolutionary biologists still see evo/devo
as just an extension of the neo-Darwinian synthesis (e.g., Carroll 2005), but others argue
that it is an entirely different type of process than neo-Darwinists envisioned in the 1950s
(e.g., Gould 1980a, 2002).
When Gould wrote his 1980 article, “Is a New and More General Theory of Evolution
Emerging?,” he was pointing to these provocative new ideas (heterochrony, regulatory
genes, homeotic mutants, and the stability of species through time as shown by punctu-
ated equilibrium). He argued that the neo-Darwinian synthesis, with its emphasis on tiny
changes in the genotype adding up to new species by microevolutionary change, was not
sufficient to explain macroevolution but that these new developments showed how macro-
evolution could occur.
There are many hard-core neo-Darwinians who do not agree, of course, so evolution-
ary biology is in an interesting, controversial time where new ideas are being intensely
debated. It may turn out that we understand less about how evolution works than we
thought we did back during the heyday of the synthesis in the 1950s and 1960s. But the
important point is that this is how normal science operates. Even if we knew nothing about
the mechanisms that drive evolution, it would not change the factual data that show it
has occurred and still is occurring (as discussed in the next section). We still don’t know
exactly how gravity works, but it does not change the fact that objects still fall to the
ground. We may never know completely how evolution works, but life keeps evolving.
And to repeat our earlier point: even if “neo-Darwinism is dead” (as the creationists
like to misquote Gould), that’s only one possible explanatory mechanism for evolution.
Neo-Darwinism is not all there is to evolution. Evolution happened in the past and is still hap-
pening right now.
The Evidence of Evolution
Well, evolution is a theory. It is also a fact. And facts and theories are different things,
not rungs in a hierarchy of increasing certainty. Facts are the world’s data. Theories
are structures of ideas that explain and interpret facts. Facts do not go away when
scientists debate rival theories to explain them. Einstein’s theory of gravitation
replaced Newton’s, but apples did not suspend themselves in mid-air, pending
the outcome. And humans evolved from apelike ancestors whether they did so by