Evolution is among the most substantiated concepts in science and is the unifying theory of biological science. Charles Darwin co-originated, with Alfred Russel Wallace, the theory of evolution by natural selection. His masterwork, the 1859 "Origin of Species," offered ample evidence for evolution having occurred, as well as the first strong explanation for its mechanism, natural selection. Modern evolutionary theory incorporates these concepts: species change over time; genetic mutations are responsible for the changes; individuals with beneficial genetic mutations will survive preferentially compared with their competitors, in a process known as natural selection; those successful individuals' more numerous offspring will spread the beneficial genetic constructs throughout the population; when enough genetic changes reproductively isolate a population, that population has become a new species.
Biological evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.
Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't involve descent through genetic inheritance.
The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother.
Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we're all distant cousins: humans and oak trees, hummingbirds and whales.