How does natural selection work ?

A common misunderstanding is to think that evolution results from a species changing to be better adapted to its environment. Evolution is actually the result of environmental pressures selecting different characteristics of a species over time, ie natural selection. Every individual is genetically different, and some of these genetic differences lead to a visible (phenotypical) difference. If that gives them an advantage (eg makes them better hunters) then they will be able to survive longer and reproduce more often than other members of that species. So their genetic difference will spread within the population.