NATURAL SELECTION

By 1844 Darwin had formulated, but not yet published, his ideas
on natural selection. The essence of his theory is as follows:
1. All organisms have a far greater reproductive potential than
is ever realized. For example, a female oyster releases about
100,000 eggs with each spawning, a female sea star releases
about 1 million eggs each season, and a female robin may
lay four fertile eggs each season. What if all of these eggs
were fertilized and developed to reproductive adults by the
following year? A half million female sea stars (half of the
million eggs would produce females and half would produce
males), each producing another million eggs, repeated over
just a few generations would soon fill up the oceans! Even
the adult female robins, each producing four more robins,
would result in unimaginable resource problems in just a
few years.
2. Inherited variations arise by random mutation. Seldom are
any two individuals exactly alike. Some of these genetic
variations may confer an advantage to the individual possessing
them. In other instances, variations may be harmful
to an individual. In still other instances, particular variations

may be neither helpful nor harmful. (These are said to be
neutral.) These variations can be passed on to offspring.
3. Because resources are limited, existence is a constant struggle.
Many more offspring are produced than resources can support;
therefore, many individuals die. Darwin reasoned that the individuals
that die are those with the traits (variations) that
make survival and successful reproduction less likely. Traits
that promote successful reproduction are said to be adaptive.
4. Adaptive traits are perpetuated in subsequent generations.
Because organisms with maladaptive traits are less likely to
reproduce, the maladaptive traits become less frequent in a
population and eventually are eliminated.

With these ideas, Darwin formulated a theory that explained
how the tortoises and finches of the Galápagos Islands
changed over time. In addition, Darwin’s theory explained
how some animals, such as the ancient South American
horses, could become extinct. What if a group of animals is faced
with a new environment to which it is ill-adapted? Climatic
changes, food shortages, and other environmental stressors could
lead to extinction.