Overview
Darwin's theory is based on the notion of variation. It
argues that the numerous traits and adaptations that differentiate
species from each other also explain how species evolved over time
and gradually diverged. Variations in organisms are apparent both
within domesticated species and within species throughout the natural
world. Variations in colors, structures, organs, and physical traits
differentiate a multitude of species from one another. Heredity
is the mechanism that perpetuates variations, Darwin argues, as
traits are passed from parents to offspring. What is important about
these variations to Darwin, though, is the way they allow species
to adapt and survive in the natural world. He gives numerous examples
of variations that illustrate the wondrous adaptations that allow
species to survive in their natural environments: the beak that
allows the woodpecker to gather insects, the wings that allow the
bat to fly, the paddles that allow the porpoise to swim, and so
on. Darwin hypothesizes that the minor variations we see within
a single species—such as variations in size, shape, and color of
organisms—are related to the more distinct variations seen across
different species. His theory of evolution explains how variations
cause the origin of species.
Natural selection is the key component of Darwin's theory,
as it explains the relationship between variation and the eventual
evolution of a species. Borrowing from Thomas Malthus's principle
of exponential population growth, Darwin argues that the possibility of
infinite growth of population sizes is checked by the limits of geography
and natural resources, which will not allow an infinite number of
beings to survive. As a result of limited food, water, shelter,
and so on, species must engage in a “struggle for existence,” creating
competition for survival. What decides, then, which species will
survive and which will become extinct? Here is where “natural selection”
comes in. Darwin argues that organisms exhibiting “advantageous
variations”—variations that will allow them to adapt to their environment
better than other organisms do—will be more likely to survive. Through
heredity, these advantageous variations will be passed on to the
organisms' offspring. Eventually, natural selection will allow those
species best adapted to their environments to survive and prosper,
while species without these advantageous adaptations will lose the
struggle for existence and become extinct.
Natural selection is the mechanism that leads to “descent
with modification,” Darwin's term for the process of evolution.
Organisms will continually give birth to offspring that carry variations, some
of which are advantageous and some of which are not. As advantageous
variations are naturally selected and become perpetuated through
successive generations, organisms carrying these advantageous variations
will diverge from the original species, eventually becoming a species
of their own. Continual modification and divergence, then, create
a branching scheme of evolution, in which new species continually
branch off from old ones. The “branches” help biologists link later
species back to an original parent species, identifying the point
at which different species are related to one another. Darwin notes
that existing classification systems developed by naturalists already
show these relationships between species. Darwin's theory of descent
with modification, then, simply provides an explanation for why
many species seem so similar: Either they evolved from one another,
or they both evolved from a common parent species.
After laying out the main principles of his theory in
the early chapters of Origin of the Species, Darwin
devotes much of the rest of the book to defending his theory against
criticisms and presenting detailed examples of how natural selection
occurs. The geological record is a formidable impediment to Darwin's
theory, as the existing fossil record does not provide the “missing
links” in the chains of descent that Darwin proposes. In response,
Darwin argues that the geological record is imperfect and that many
fossil remains have been destroyed by changes in the earth or have
yet to be discovered.
Darwin also attempts to explain how variations occur in
species, driving natural selection and the creation of new species.
Geographical isolation is a key component of Darwin's theory. Darwin hypothesizes
that because all species originated from one or a few original beings,
species needed modes of transportation to migrate between geographical
areas throughout the world. Barriers such as oceans and mountain
ranges restrict the ability of organisms to migrate, and the few
that manage to do so play a large role in shaping the evolution
of species on islands and in geographically isolated areas. Geographical
isolation accounts for the plethora of unique species on islands,
as well as the wider distribution of species across continents.
Darwin's theory challenged not only the prevailing view
of the independent creation of species but also larger claims of
religion and science. Darwin explicitly denied the validity of natural
theology, which posited that species' adaptations to their environments
was proof of their “intelligent design” by a creator. It was natural
selection, not independent creation, that resulted in these adaptations, Darwin
argued. Moreover, Darwin's use of scientific methodology to prove
his theory amounted to an explicit critique of naturalists who would
attempt to ignore the scientific validity of his theory because
of its controversial nature. While the text of The Origin
of Species did leave room for religious theology, Darwin's
overall commitment to scientific rationale rather than theological
reasoning pitted him against religious doctrine. Darwin's text was
controversial when it was published, and it remains controversial
today. However, his theory of natural selection has stood the test
of time in scientific circles, and it remains the leading scientific
explanation for the origin of species.
