Wikipedia: Evolution

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Generally speaking, evolution is any process of growth, change or development. The word stems from the Latin evolutio meaning "unfolding" and prior to the late 1800s was confined to referring to goal-directed, pre-programmed processes such as embryological development. A pre-programmed task, as in a military maneuver, may be termed an "evolution." While one can also speak of stellar evolution, [cultural evolution]? or the evolution of an idea, the concept is nowadays largely used in the sense of biological evolution, and refers to the change in the bodyplans of species over time and the appearance of new species. The remainder of this article discusses this concept of biological evolution, which has also been called transmutation in earlier times.

The evolutionary timeline outlines the major steps of evolution on Earth. An explanation as to how this evolution occurs is referred to as a theory of evolution, and while transmutation was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species which provided the first cogent mechanism by which evolutionary change could persist: his mechanism of Natural selection. It is worth noting that the mechanism (natural selection) is logically independent of the observation that evolution does indeed occur. Thus, a disproof of Darwinism does not in itself disprove the occurrence of evolution which is an observable fact based on evidence from many fields (e.g. embryology?, paleontology, genetics).

Biological evolution results in populations of organisms which are more adapted? to their current environment than previous populations. Since, in the long run, environments always change, if successive generations did not develop adaptations which allowed them to survive and reproduce, species would simply die out. Evolution therefore allows life, as expressed in a given genetic line, to persist over greater spans of time.

Following the dawn of molecular biology, it became clear that the mechanism for evolutionary change was through the mutagenesis of DNA. During the cell cycle, DNA is copied fairly, but not entirely, faithfully. These copying errors introduce genetic mutations of three general consequences relative to the current environment: good, bad, or neutral. As long as the environment remains stable, individuals with "good" mutations will have an a stronger propensity to propagate, individuals with "bad" mutations will have less of a chance at successful reproduction, and those carrying "neutral" mutations will have neither an advantage nor a disadvantage. Considered at the level of a single gene, these variations just described represent different genetic alleles. Following environmental change, alleles may retain their classification of good, bad, or neutral, or shift into one of the other categories. Individuals carrying alleles formerly neutral but now "good" bear adaptive mutations. Since neutral alleles can accumulate in the population without consequence while an environment is stable, they create a considerable reservoir for adaptability.

Books:

The Origin of Species (entire text is in Wikipedia)
Darwin's Dangerous Idea