Encyclopedia of Evolution.pdf - Online Reading Center

Nariokotome skeleton See Homo ergaster.
natural selection Natural selection is the process by which
evolution produces adaptation. Other processes (such as
genetic drift) cause evolution to occur, but in a random direction.
Only natural selection guides the evolutionary process
in a direction that produces adaptation to environmental
and social conditions. Natural selection is the process that
was first clearly explained by Charles Darwin (see Darwin,
Charles; origin of species [book]), although some earlier
writers such as W. C. Wells, Edward Blyth, and Patrick
Mathew had presented fragments of the idea. Natural selection
was independently discovered by Wallace (see Wallace,
Alfred Russel).
Darwin not only demonstrated that evolution had
occurred but also explained how it occurred. Evolution became
believable for the first time. Darwin’s contemporaries largely
accepted his demonstration that evolution had occurred, but
the scientific community did not embrace natural selection for
another 80 years (see modern synthesis).
sexual selection, which was also first elucidated by
Charles Darwin, operates in a manner similar to natural
selection. Many evolutionary scientists consider it a subset of
natural selection. However, sexual selection usually produces
arbitrary and sometimes outlandish characteristics that are
not usually considered adaptation.
Both Darwin and Wallace alluded to artificial selection,
the process by which humans breed crops and livestock
in desired directions, in crafting the phrase natural selection.
In both cases, some individuals are selected to propagate, and
others are not selected to propagate, only in natural selection
it is nature rather than humans that is doing the selecting.
Both artificial and natural selection can produce genetic
changes within populations and can cause divergence of populations
into different varieties. Either could eventually cause
varieties to be different enough that they would constitute
N
different species. Artificial selection, however, has not gone
on long enough for this to happen.
Natural selection occurs within populations. Individual
organisms cannot evolve because they cannot undergo wholebody
genetic changes (see Mendelian genetics). If mutations
occur in somatic cells, which make up most of the
body of an animal, the cells that contain the mutations will
die when the organism dies. Only those mutations that occur
in germ line cells, either eggs or sperm or the cells that produce
them, can be passed on to the next generation. Therefore
the genetic effects of evolution can only be observed in
the next generation of the population.
Natural selection occurs because individuals within
populations differ from one another in how successfully
they reproduce. Evolutionary theories before Darwin (see
Lamarckism; Chambers, Robert) were incorrectly based
upon the transformation of an entire population or species.
Because some individuals in the population reproduce more
successfully than others, over time the population begins to
resemble the successful individuals. Ernst Mayr (see Mayr,
Ernst) said that by formulating the theory of natural selection,
Darwin introduced population thinking into evolution,
to replace typological thinking (see evolution)—that
is, evolution occurs in populations of diverse individuals,
rather than being the transformation of one type into
another.
Process of Natural Selection
Ernst Mayr summarized the process of natural selection as
five facts and three inferences that can be derived from those
facts. All of these facts are part of the science of ecology,
the study of the relationship between organisms and their
living and nonliving environments. Therefore by elucidating
the process of natural selection, Darwin was not only making
evolution believable for the first time but also inventing the
science of ecology while he was at it.