The Evolution of Evolution 125In Loberg Lake, Alaska, the change took only a dozen years, or just six generations. Stickle-
backs also change their spines in response to local conditions. In open water, longer spines
are an advantage because they protect against being swallowed by predators. But in shallow
water, the long spines are a liability because they make it easier to be captured by dragonfly
larvae, which have long pincers. A single Hox gene, Pitx1, turns off and on the switches that
regulate spine length. Other studies have shown that when captive sticklebacks are artificially
modified so they have unusual new combinations of spines, the females only mate with the
males that have new traits, so sexual selection is a driving force in their evolution of novelty.
Prior to these studies, ichthyologists would readily assign specimens with different spine
counts and different body armor to different species, but these studies show just how easy it
is for one stickleback population to transform to another species given the right conditions.
There are the classic cases of industrial melanism, so familiar from every textbook. The
peppered moth, Biston betularia, normally has a speckled appearance that blends in well with
mottled appearance of trunks and branches of trees. During the Industrial Revolution, soot
in the air made the tree trunks black, and the normal form was conspicuous. Instead, a dark-
colored mutant became dominant, because they were well camouflaged against the dark tree
trunks, while the birds picked off the normal speckled varieties. When environmental regu-
lations cleaned up the air and eliminated the sooty tree trunks, the normal speckled varieties
returned, and the dark mutants were again selected against.
Examples like these could be multiplied endlessly. In New England, the periwinkles
dramatically changed their shell shape and thickness in less than a century, probably due to
predation pressure by newly introduced crabs. In the Bahamas, the anole lizards (the com-
mon “chameleon” in the pet shops, which are not true chameleons) changed the proportions
of their hind limbs after people introduced them to new islands with different vegetation. In
Florida, the soapberry bug evolved a significantly longer beak in response to the invasion of
its habitat by a nonnative plant with larger fruits. In Hawaii, the honeycreeper birds evolved
shorter bills as their favorite food source, the native lobelloids, has disappeared, and the
birds switched to another source of nectar. In Nevada, the tiny mosquito fish that lives in
isolated desert water holes that were once connected during the last ice age quickly evolved
major differences in less than 20,000 years. And in Australia, the introduced wild rabbits
(brought by European settlers less than a century ago) modified their body weight and ear
size in response to the different conditions of the outback.
Humans are often the strongest agents of selection for many wild animals. In popula-
tions of bighorn sheep, trophy hunters killed off most of the rams with spectacular horns, so
the smaller males with reduced horns had a better chance of breeding, and the population
no longer has many large-horned rams. Rattlesnakes that are too nervous and buzz when
humans approach are quickly killed, so in many regions, the rattlers no longer give any
warning. Overfishing of the Atlantic cod led to a population crash during the 1980s, and
large cod nearly vanished; those that bred quickly while they were small and immature had
a better chance of survival.
But the most dramatic and rapid examples of evolution in action occur with microor-
ganisms, especially viruses and bacteria. Every year doctors have new flu strains to battle,
because last year’s cold and flu strains have evolved new protein coats that make them
unrecognizable to our immune systems and allow them to infect us again. This is why there
will never be a cure for the common cold—the virus evolves too fast for any drug to keep
up. The heavy use of antibiotics has selected for strains of bacteria that are resistant to every