THE STORY OF EVOLUTION 31
occurred. For example, a fossil
record stretches back 60 million
years for ancestors of the horse.
The earliest of these were dog-
sized forest-dwelling animals with
several toes on each foot. Evolution
produced much larger horses with
just a single hoof on each foot,
adapted for life on open grasslands
where they would often have had
to outrun predators.
Peppered moths (biston
betularia) reveal change over a
shorter period. The moth is usually
pale, providing camouflage against
the bark of birch trees, but a
mutation produces some black
moths. Before the 19th century,
most peppered moths were pale.
During the Industrial Revolution
(1760–1840), however, smoky air left
deposits of soot on trees and
buildings in British cities, and the
black form became much commoner.
By 1895, 95 percent of peppered
moths in Britain’s cities were black,
as paler moths were eaten by birds
because their coloring provided no
camouflage. This phenomenon
continues to act as an example of
Darwin’s theory in action today, as
the pale moth becomes common
once more due to the declining soot
concentrations in Britain’s cities. ■Evolution in real time
Richard Lenski, a professor
at Michigan State University,
established the Long-term
Experimental Evolution
project in 1988. For more than
25 years, he studied 59,000
generations of the E. coli
bacterium. During this time,
he observed that the species
used the glucose solution
it lived in more efficiently,
increasing in size but also
growing faster. Also, a new
species had evolved that was
able to use a compound in the
solution called citrate, which
the parent bacterium could
not. Evolving bacteria can
pose a potential threat to
humans. Increasing antibiotic
use destroys many disease-
causing bacteria, but not those
with mutations that make
them resistant to the drugs.
As the non-resistant bacteria
are killed off, the resistant
strains become more
dominant, multiplying and
passing on their mutations
to future generations. That
is natural selection at work.Two peppered moths exhibit
evolution at work, the lower one an
example of industrial melanism. The
dark variety began to appear in British
cities in the early 1800s.Escherichia (E.) coli bacteria
can cause serious gut and other
infections that will be increasingly
difficult to treat as drug-resistant
strains of E. coli multiply.Seen in the light of evolution,
biology is, perhaps,
intellectually the
most satisfying and
inspiring science.
Theodosius DobzhanskyIndividuals within
a species have
a variety of forms
of a characteristic.The individuals with
the characteristic best
suited to the environment
are more likely to survive
and breed.These
characteristics
are passed on
to the next
generation.US_024-031_Evolution_by_Natural_Selection.indd 31 12/11/18 6:24 PM