"In biology, evolution is the process of change in the inherited
traits of a population of organisms from one generation to the next.
The genes that are passed on to an organism's offspring produce the
inherited traits that are the basis of evolution. Mutations in genes
can produce new or altered traits in individuals, resulting in the
appearance of heritable differences between organisms, but new traits
also come from the transfer of genes between populations, as in
migration, or between species, in horizontal gene transfer. In species
that reproduce sexually, new combinations of genes are produced by
genetic recombination, which can increase the variation in traits
between organisms. Evolution occurs when these heritable differences
become more common or rare in a population.

There are two major mechanisms that drive evolution. The first is
natural selection, a process causing heritable traits that are helpful
for survival and reproduction to become more common in a population,
and harmful traits to become more rare. This occurs because
individuals with advantageous traits are more likely to reproduce, so
that more individuals in the next generation inherit these traits.
Over many generations, adaptations occur through a combination of
successive, small, random changes in traits, and natural selection of
those variants best-suited for their environment. The second is
genetic drift, an independent process that produces random changes in
the frequency of traits in a population. Genetic drift results from
the role probability plays in whether a given trait will be passed on
as individuals survive and reproduce. Though the changes produced in
any one generation by drift and selection are small, differences
accumulate with each subsequent generation and can, over time, cause
substantial changes in the organisms.

One definition of a species is a group of organisms that can reproduce
with one another and produce fertile offspring. When a species is
separated into populations that are prevented from interbreeding,
mutations, genetic drift, and natural selection cause the accumulation
of differences over generations and the emergence of new species. The
similarities between organisms suggest that all known species are
descended from a common ancestor (or ancestral gene pool) through this
process of gradual divergence.

Evolutionary biology documents the fact that evolution occurs, and
also develops and tests theories that explain its causes. Studies of
the fossil record and the diversity of living organisms had convinced
most scientists by the mid-nineteenth century that species changed
over time. However, the mechanism driving these changes remained
unclear until the 1859 publication of Charles Darwin's On the Origin
of Species, detailing the theory of evolution by natural selection.
Darwin's work soon led to overwhelming acceptance of evolution within
the scientific community. In the 1930s, Darwinian natural selection
was combined with Mendelian inheritance to form the modern
evolutionary synthesis, in which the connection between the units of
evolution (genes) and the mechanism of evolution (natural selection)
was made. This powerful explanatory and predictive theory directs
research by constantly raising new questions, and it has become the
central organizing principle of modern biology, providing a unifying
explanation for the diversity of life on Earth."

http://en.wikipedia.org/wiki/Evolution