Theory of Evolution
The theory of evolution is a shortened form
of the term “theory of evolution by natural
selection,” which was proposed by Charles
Darwin and Alfred Russel Wallace in the nineteenth
century. Ideas aimed at explaining how organisms
change, or evolve, over time date back to
Anaximander of Miletus, a Greek philosopher
who lived in the 500s B.C.E. Noting that human
babies are born helpless, Anaximander speculated
that humans must have descended from some
other type of creature whose young could survive
without any help. He concluded that those
ancestors must be fish, since fish hatch from
eggs and immediately begin living with no
help from their parents. From this reasoning,
he proposed that all life began in the sea.
Anaximander was correct; humans can indeed
trace our ancestry back to fish. His idea,
however, was not a theory in the scientific
meaning of the word, because it could not
be subjected to testing that might support
it or prove it wrong. In science, the word
“theory” indicates a very high level of
certainty. Scientists talk about evolution as
a theory, for instance, just as they talk
about Einstein’s explanation of gravity
as a theory.
A theory is an idea about how something in
nature works that has gone through rigorous
testing through observations and experiments
designed to prove the idea right or wrong.
When it comes to the evolution of life, various
philosophers and scientists, including an
eighteenth-century English doctor named Erasmus
Darwin, proposed different aspects of what
later would become evolutionary theory. But
evolution did not reach the status of being
a scientific theory until Darwin’s grandson,
the more famous Charles Darwin, published
his famous book On the Origin of Species.
Darwin and a scientific contemporary of his,
Alfred Russel Wallace, proposed that evolution
occurs because of a phenomenon called natural
selection.
In the theory of natural selection, organisms
produce more offspring that are able to survive
in their environment. Those that are better
physically equipped to survive, grow to maturity,
and reproduce. Those that are lacking in such
fitness, on the other hand, either do not
reach an age when they can reproduce or produce
fewer offspring than their counterparts. Natural
selection is sometimes summed up as “survival
of the fittest” because the “fittest”
organisms—those most suited to their environment—are
the ones that reproduce most successfully,
and are most likely to pass on their traits
to the next generation.
This means that if an environment changes,
the traits that enhance survival in that environment
will also gradually change, or evolve. Natural
selection was such a powerful idea in explaining
the evolution of life that it became established
as a scientific theory. Biologists have since
observed numerous examples of natural selection
influencing evolution. Today, it is known
to be just one of several mechanisms by which
life evolves. For example, a phenomenon known
as genetic drift can also cause species
to evolve. In genetic drift, some organisms—purely
by chance—produce more offspring than would
be expected. Those organisms are not necessarily
the fittest of their species, but it is their
genes that get passed on to the next generation.
Adaptation
Evolutionary adaptation, or simply adaptation,
is the adjustment of organisms to their environment
in order to improve their chances at survival
in that environment.
In evolutionary theory, adaptation is the
biological mechanism by which organisms adjust
to new environments or to changes in their
current environment. Although scientists discussed
adaptation prior to the 1800s, it was not
until then that Charles Darwin and Alfred
Russel Wallace developed the theory of natural
selection.
Wallace believed that the evolution of organisms
was connected in some way with adaptation
of organisms to changing environmental conditions.
In developing the theory of evolution by natural
selection, Wallace and Darwin both went beyond
simple adaptation by explaining how organisms
adapt and evolve. The idea of natural selection
is that traits that can be passed down allow
organisms to adapt to the environment better
than other organisms of the same species.
This enables better survival and reproduction
compared with other members of the species,
leading to evolution.
Organisms can adapt to an environment in different
ways. They can adapt biologically, meaning
they alter body functions. An example of biological
adaptation can be seen in the bodies of people
living at high altitudes, such as Tibet. Tibetans
thrive at altitudes where oxygen levels are
up to 40 percent lower than at sea level.
Breathing air that thin would cause most people
to get sick, but Tibetans’ bodies have evolved
changes in their body chemistry. Most people
can survive at high altitudes for a short
time because their bodies raise their levels
of hemoglobin, a protein that transports oxygen
in the blood. However, continuously high levels
of hemoglobin are dangerous, so increased
hemoglobin levels are not a good solution
to high-altitude survival in the long term.
Tibetans seemed to have evolved genetic mutations
that allow them to use oxygen far more efficently
without the need for extra hemoglobin. 
Organisms can also exhibit behavioral adaptation.
One example of behavioral adaptation is how
emperor penguins in Antarctica crowd together
to share their warmth in the middle of winter.
Scientists who studied adaptation prior to
the development of evolutionary theory included
Georges Louis Leclerc Comte de Buffon. He
was a French mathematician who believed that
organisms changed over time by adapting to
the environments of their geographical locations.
Another French thinker, Jean Baptiste Lamarck,
proposed that animals could adapt, pass on
their adaptations to their offspring, and
therefore evolve. The example he gave stated
the ancestors of giraffes might have adapted
to a shortage of food from short trees by
stretching their necks to reach higher branches.
In Lamarck’s thinking, the offspring of
a giraffe that stretched its neck would then
inherit a slightly longer neck. Lamarck theorized
that behaviors aquired in a giraffe's lifetime
would affect its offspring. However, it was
Darwin’s concept of natural selection, wherein
favorable traits like a long neck in giraffes
suvived not because of aquired skills, but
because only giraffes that had long enough
necks to feed themselves survived long enough
to reproduce. Natural selection, then, provides
a more compelling mechanism for adaptation
and evolution than Lamarck's theories.
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