Darwin used information from several disciplines in developing his theory of
evolution. He was particularly impressed by the amount of variation that occurs
within living species, especially in domestic animals, and he spent a great
deal of time studying breeding programs. Even in Darwin’s day, the human effort
in breeding variants of domestic animals had resulted in many breeds of dogs,
cats, horses, sheep, and cattle. As an example, consider the tremendous variation
in domestic dogs. The Chihuahua and the Saint Bernard are about as different
in size, shape, hair length, and other features as one could imagine; yet both
breeds are domestic dogs with the scientific name Canis familiaris. The differences
between them were produced by human-engineered selective breeding programs.
Artificial selection is the term for what we do when we choose plants and animals
with desirable features and breed them to produce or enhance these features
in their offspring. As different as they look, Chihuahuas and Saint Bernards
... and Poodles, Pomeranians, Pekinese...all domestic dogs share the same gene pool. This shared gene pool means that all dogs have the ability to interbreed,
and this is why all domestic dogs are placed in one species. The common gene
pool of dogs also allows for the great variation we see in “man’s best friend.”
A standard definition of species in animals is the ability to interbreed and
produce fertile offspring.
Darwin gathered data and honed his theory for 20 years before publishing his
well-known book in 1859, The Origin of Species by Means of Natural Selection,
or The Preservation of Favoured Races in the Struggle for Life. Darwin and his
fellow naturalist Alfred Wallace independently came to the conclusion that geologically
older species of life gave rise to geologically younger and different species
through the process of natural selection.
Darwin’s theory of evolution can be summarized in four statements.
Variation exists among individuals within species. Anyone who looks at their
friends and relatives, or their pets, can see variation. Breeders of animals
and plants use these diverse characteristics to establish new varieties of
dogs, cats, pigeons, wheat, cotton, corn, and other domesticated organisms.
Scientists who name and classify plants and animals are acutely aware of variation
in natural populations. For example, the level of resistance to insecticides
varies among individuals within species of insects. This variation enables
some individuals to survive application of insecticides and produce offspring
that inherit this resistance to these insecticides.
Organisms produce more offspring than the environment can support. All living
things produce more individuals than can survive to maturity. Think of the
thousands of acorns that one mature oak tree produces every year. A female
salmon produces about 28,000,000 eggs when spawning. One oyster can produce
114,000,000 eggs in a single spawning. Darwin calculated that in elephants,
which are among the slowest breeding land mammals, if all of the potential
young of a single female survived and reproduced at the same rate, after 750
years the descendants of this single mother could number 19,000,000! Clearly,
if all of these seeds, eggs, and young survived to become adults who also
reproduced, the world would soon be overrun with oak trees, salmon, oysters,
Competition exists among individuals. Regardless of the rate of reproduction
in a species, all of the young do not survive to become reproducing adults.
This fact indicates that large numbers of offspring somehow are eliminated
from the population. Some certainly die by accident. But most of them succumb
to competition with other individuals. The most intense competition may be
among individuals of the same species who compete for nearly identical environmental
requirements. Competition may be as simple as a race to get a rabbit — the
first fox there gets lunch; the others go hungry. Competition may involve
obtaining a choice nesting site, or being able to find the last available
hiding hole when a bigger fish comes looking for dinner. Those individuals
who catch the rabbit or find the hiding hole survive to pass on their genes
to the next generation.
The organisms whose variations best fit them to the environment are the
ones who are most likely to survive, reproduce, and pass those desirable variations
on to the next generation. Many of the natural variations we observe in species
do not seem to be either particularly helpful or particularly harmful to an
individual in its struggle for survival. Hair and eye color may be such neutral
variations in human beings. Some variations certainly lower the chances of
survival, such as hemophilia in mammals, albinism in many wild animals, or
an unusually thin shell in clams living where there are numerous hungry snails.
Some variations are helpful. For example, any variation that increases an antelope’s
speed may help it elude predators. Any variation that increases water retention
in a desert plant will favor survival of that plant to reach maturity. Those
animals and plants that survive to maturity and are able to reproduce become
the parents of the next generation, passing on the genes for the successful
Darwin called the process by which favorable variations are passed from generation
to generation natural selection. He made many important observations on the
relationship of individual variation to survival. During his stay in the Galapagos
Islands, Darwin noted that the populations of tortoises on each island had physical
features so distinctive that people could often tell from which island an animal
came simply by looking at it.
We commonly hear natural selection referred to as “survival of the fittest.”
This popular phrase has a very specific biological meaning. “Fittest” means
that organisms must not only survive to adulthood, they must actually reproduce.
If they do not reproduce, their genes are not passed on to the next generation.
Evolution occurs only when advantageous genetic variations are passed along
and become represented with increasing frequency in succeeding generations.