Evolution
Evolution is the gradual, cumulative change over time of the characteristics of groups of organisms in a heritable manner. Eventually, these minute changes add up to produce an individual that is markedly different from its distant ancestors, but almost indistinguishable from its most immediate ancestors. These changes are brought about by the organism's genetic response to the environment, and, over the entire course of history, evolution has given rise to all different forms of life on Earth.
Evolution does not occur rapidly on the individual unit of life; changes are too small and slow to be effective at the individual level. In fact, evolution is more efficient at the population
level among groups of organisms that are capable of successfully breeding with each other. With organisms that do not breed with other individuals, the rate of evolutionary change is slower than it is among outbreeding organisms.
Evolution leads to increasing complexity and, eventually, to the production of new species, which survive or become extinct depending upon their reaction to the environment and its continuing changes. Evidence for evolution comes from the fossil record, genetics, and comparative studies.
The mechanism behind evolution is natural selection. Small, individual changes that arise by chance can confer an advantage to those possessing them; this group then has better success at breeding, and successful genes are consequently spread further throughout the population. The theory of evolution is now widely accepted, but when it was first put forward in the nineteenth century by English naturalist Charles Darwin there was much opposition, particularly from religious quarters. Opponents to the theory of evolution often argue for special creation, which states that each type of species was created in the form in which it currently exists, and that no two species are related, by descent, to any other. Most scientists now accept the theory of evolution, as the concept of evolution fits available evidence. There exist some gaps in scientific knowledge of evolution, such as the discovery of the common ancestor for both apes and humans, often referred to as the missing link, but, with time, these knowledge gaps have become smaller.
Evolution does not proceed at a constant rate. At times, a gradual change occurs that allows for a good reconstruction of the process from the fossil record. This is known as phyletic gradualism. The other method of evolution, which can leave gaps in the fossil record is the quicker and more explosive form, called punctuated equilibrium.
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