Darwin's theory of evolution is a theory that explains(1) the history of life through geologic time; the diversity of living organisms, including their genetic, molecular, and physical similarities and differences; and the geographic distribution of organisms.
Once life began, (Darwin did not presume to know HOW life began), Darwin argued, organisms would slowly begin to change and diversify through a completely natural process. Simply, those individuals with trait variants that are favorable in the environment they inhabit (2) will thrive and produce more offspring than individuals with unfavorable variants.
Essence of the theory
1. Variation exists among individuals of a species (3).
We have seen evidence of variation earlier! Genetic mutations - remember?
2. Individuals having advantageous variations are more likely to survive and reproduce than those without the advantageous variations.
Darwin coined the term natural selection to describe the process by which organisms with favorable variations survive and reproduce at a higher rate. An inherited variation that increases an organism's chance of survival in a particular environment is called an adaptation. Over many generations, an adaptation could spread throughout the entire species. In this way, according to Darwin, evolution by natural selection would occur.
As an example Darwin noted that the ptarmigan turns white in winter. This color change, he inferred, helped protect it from predators, which would have a hard time spotting the bird in snow. Ptarmigans that didn't change color in winter would be spotted easily and eaten. In this way, Darwin implied, ptarmigans that turned white in winter would be more likely to survive, reproduce, and pass this adaptation to future generations.
He did not presume to know how life itself first arose.
The theory of evolution was originally developed to explain the evidence that was observed from the fossil record. So in this respect, every significant aspect of evolution has been exhaustively observed and documented, many times over.
Evolution explains the vast diversity of life on earth, with single species becoming many as they adapt to different environments.
1. "... explains..." That's all the theory of evolution does. It EXPLAINS completely, the vast diversity of life on earth. It is not an anti-god treatise.
2. "...in the environment..." Note that the environment of the organism determines what traits are beneficial!
Members of populations that actually or potentially interbreed.
In this sense, a species is the largest gene pool possible under natural conditions.
For a more detailed explanation, see http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_41
See "What Darwin Never Knew", a PBS special that shows what we now know in addition to what Darwin knew. http://www.pbs.org/wgbh/nova/beta/evolution/darwin-never-knew.html