Evolution occurs through changes in genes, the "recipes" for constructing the organism. When an organism reproduces, small random changes in the genes make the offspring different from the parent. Sometimes these changes help the offspring survive to reproduce. When this happens, the genes for the beneficial traits are passed on to the organism's own offspring, becoming more common in the next generation. Genes that do not help organisms reproduce may become rarer or be completely eliminated from the population. This is called natural selection, a major part of evolution. Through natural selection, populations of organisms slowly change over time as they adapt to changes in their environments.

The scientific theory of evolution—the explanation for how evolution occurs—states that all living things are descended from a single common ancestor at some point in the distant past. This idea is called common descent. Since the beginning of life, evolution has transformed the first species into more and more different species as life has found a variety of ways to survive and flourish. This has resulted in the many diverse forms of life that exist today."  — Wikipedia
 

  
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"In biology, evolution is the change in a population's inherited traits from generation to generation. These traits are the expression of genes that are copied and passed on to offspring during reproduction. Mutations and other random changes in these genes can produce new or altered traits, resulting in heritable differences (genetic variation) between organisms. New traits can also come from transfer of genes between populations, as in horizontal gene transfer or breeding between species. Evolution occurs when these differences become more common or rare in a population, either randomly through genetic drift or nonrandomly through natural selection.

Natural selection is a process that causes heritable traits that are helpful for survival and reproduction to become more common and harmful traits to become more rare. This occurs because organisms with these advantageous traits produce more offspring, thus passing more copies of the traits on to the next generation. Over very long periods of time, adaptations are produced by a combination of the continuous production of small, random changes in traits, followed by natural selection of the variants best-suited for their environment.

A species is a group of animals that can reproduce with one another. However, when a species is separated into different populations that are prevented from interbreeding, mutations and genetic drift, combined with different environments selecting for and against different traits, result in these populations accumulating differences over time and eventually becoming two separate new species. The similarities between organisms suggest that all known species are descended from a single ancestral species, through this process of gradual divergence.

The theory of evolution by natural selection was first proposed by Charles Darwin and Alfred Russel Wallace and put forth in detail in Darwin's 1859 book On the Origin of Species. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis.  With its enormous explanatory and predictive power, this theory has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.^"  — Wikipedia
 

  
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Origin Views — Creation/Evolution Comparison Chart