15–3 Darwin Presents His Case

Darwin was hesitant to publish his ideas because they were so
extreme. When he learned that scientist Alfred Russel Wallace had
the same ideas, Darwin published On the Origin of Species in 1859.
In the book, Darwin supplied evidence that evolution has
occurred. He also explained his ideas about how evolution occurs.
Darwin’s theory was based on artificial selection. In artificial
selection, nature provided the variation, and humans selected
those variations that they found useful. For example, animal
breeders used only the largest hogs, fastest horses, or cows that
produced the most milk for breeding.

Darwin thought that a similar process occurs in nature. He
called this natural selection. This process can be summed up
as follows.
• Individuals differ, and some of the differences can be passed
on to their offspring.
• More offspring are produced than can survive and
reproduce.
• There is competition for limited resources, or a struggle
for existence.
• Individuals best suited to their environment survive and
reproduce most successfully. In other words, there is
survival of the fittest. Fitness is the ability to survive
and reproduce in a given environment. It results from
adaptations. Adaptations are inherited traits that increase
an organism’s chance of survival. Only the fittest organisms
pass on their traits. Because of this, a species changes
over time.

Darwin argued that species alive today descended with
modification from species of the past. Darwin also introduced the
principle of common descent. This principle holds that all species
come from common ancestors. The principle of common descent
links all organisms on Earth into a single tree of life.
Darwin argued that living things have been evolving on
Earth for millions of years. He presented four types of evidence
in support of evolution.

• The fossil record Comparing fossils from older and
younger rock layers provides evidence that evolution has
taken place.
• Geographic distribution of living species The presence
of similar but unrelated organisms in similar environments
suggests the action of natural selection.
• Homologous structures of living organisms Homologous
structures have different mature forms but develop from
the same embryonic tissues. They provide strong evidence
that organisms have descended, with modifications, from
common ancestors.
• Some homologous structures no longer serve major roles
in descendants. If the structures are greatly reduced in size,
they are called vestigial organs. For example, the appendix
in humans is a vestigial organ. It carries out no function
in digestion.
• Similarities in early development The early stages, or
embryos, of many animals are very similar. These similarities
are evidence that the animals share common ancestors.
Scientific advances have upheld most of Darwin’s hypotheses.
However, evolutionary theory continues to change as new data are
gathered and new ways of thinking arise.

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