Explain the theory of evolution by natural selection as presented by darwin.?

Darwin explained that the best adapted organisms are those which are able to reproduce. These organisms may have beneficial characteristics which enable them to survive in their surroundings. By reproducing, the characteristics are passed onto their offspring while other, less beneficial characteristics are not passed on. Eventually, the only organisms which survive are those best adapted and so all the organisms are well adapted. This allows the species to adapt to a new set of surroundings. New characteristics may develop because of a mutation in the DNA sequence. The new gene would spread quickly if it was beneficial, such as resistance to a disease, better protection or camouflage, ability to run faster etc. The fossil record provides proof of this as we can see how organisms have changed over time by looking at fossils from different times. We can also see some of the conditions at the time and show how the organisms characteristics might have benefited the organism at the time.