Natural selection is a process in which the best adapted individual survives. In Darwin's studies on the Galapagos islands, he found 4 different bird species on 4 different islands. The islands were not that far from each other but it still took a while to get from one to the other. He discovered that the species were different because of their environment. Therefore, each specie was adapted to its environment. On the other hand, evolution is a long term process that adapts an individual to it's habitat. Therefore, by eliminating all individuals who are not able to survive, we are left with the ones who were able to live and reproduce and have children who will have changed a bit and will continue changing until the specie is completly adapted. That is the link between natural selection and evolution.