Charles Darwin's theory of natural selection was the first plausible mechanism to explain the change of species over time, however, in it's original form it did not explain how new traits could form, or how traits that had formed could be passed on to successive generations. The rise in modern genetics helped to modify biologists understanding of evolution by attributing the origin of new traits in a species to random genetic processes of mutation and sexual recombination, with the survivability of species with the new traits subject to natural selection. This combination of random mutation and natural selection is often referred to as Neodarwinism.
Modern models of evolution stress the relative importance of competition and survival, competition within a species for limited resources and the ability to mate, and survival from predators. As random changes develop within a species, the ability for the change to continue on to successive generations depends upon the ability of the mutated subject to survive long enough to produce viable offspring which are also capable of reproducing. Adaptation is the process of random mutations, subject to selection, accumulating over time to make a species better suited to survive and reproduce in their environment.