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Biology: Chapter 15 – Evolution
15-1: The puzzle of life’s diversity
Evolution, or change over time, is the process by which modern
organisms have descended from ancient organisms. The theory of
evolution can explain Earth’s biodiversity. A scientific theory is an
explanation of natural events that is supported by evidence and can be
tested with new evidence.
Charles Darwin added the most to our understanding of
evolution. In the 1830’s, Charles Darwin sailed around the world.
Darwin made many observations and collected evidence. Darwin
observed many organisms. He saw that many plants and animals were
very well suited to their environment. Darwin collected fossils, or the
preserved remains of ancient organisms. Some fossils were unlike any
creatures he had ever seen. He wondered why the species in fossils
had disappeared.
Darwin’s observations on the Galapagos Islands influenced him
most. The islands are near one another but have different climates.
Darwin saw that the characteristics of many animals and plants varied
noticeably among the different islands. He wondered whether animals
on different islands had once belonged to the same species. According
to this hypothesis, these separate species would have evolved from an
original ancestor species after becoming isolated from one another.
15-2 : Ideas that shaped Darwin’s thinking
In Darwin’s day, most European’s thought that the Earth and all of its
life-forms had existed for only a few thousand years. They also thought
that species did not change. Some scientists of Darwin’s time began
challenging these ideas. These scientists influenced the development
of Darwin’s theory of evolution.
 Hutton and Lyell helped scientists recognize that Earth is many
millions of years old, and that it had changed over time. These
ideas helped Darwin realize that life might change as well.
Knowing that Earth was very old convinced Darwin that there had
been enough time for life to evolve.
 Jean-Baptiste Lamarck was one of the first scientists to see that
evolution occurred. He also recognized that organisms adapt to
their environments. Lamarck proposed that by selective use or
disuse of organs, organisms acquired or lost certain traits during
their lifetime. These traits could then be passed on to their
offspring. Over time, this process led to change in a species.
Scientists now know that some of Lamarck’s ideas were wrong.
However, his general ideas about evolution and adaptation are
correct. These ideas influenced Darwin.
 Economist Thomas Malthus also influenced Darwin. Malthus
thought that if human population kept growing, sooner or later
there would be insufficient living space and food for everyone.
Darwin thought this was true for all organisms.
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 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 that can survive and reproduce
 There is competition for limited resources, or a struggle for
existence
 Individuals best suited to their environment survive and
reproduce 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 for 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 on 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.
o 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 the 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.