Variation, Mutation and Evolution

Evolution means "change" - in biology, evolution simply refers to the changes that can occur in a species over time. A species has certain characteristics, which are usually defined based on the majority of members in that species having that characteristic. If, over time, the proportion of members of that species who have this characteristic change, the species is said to be evolving. Change the population enough (so that it is distinctly different from its predecessor), and there may be a new species.Within any population, there is some variety - this is variation. Some people are tall, some short. Variations can occur from a number of sources, but one in particular is mutation. Mutation is a defect or error in the DNA that controls inheritance. The offspring is not exactly like its parents. The great majority of the time (99.9 %+), the mutation is useless or unhealthy, and the organism dies. Rarely (very rarely), the mutation makes a change that is not instantly deadly (note that it does not have to be a positive change). This increases variation. Mutation, which is determined by random chance, is the engine that drives variation.

If an organism is born with a new, or altered characteristic (from whatever cause), it will still need to live in its environment. It will compete with others of its species for food and mates, it will need to escape predators and disease, and it will need to live long enough for it to mate and have offspring. While variation is driven by chance, survival is not. Those organisms best suited to survive and breed within a certain environment will. Having offspring, also referred to as "reproductive success" is the only criteria that matters. In biology, nothing else matters.

Who decides which organism has "reproductive success"? Since each organism in a population is slightly different (variation), they each have benefits and handicaps when dealing with their environment. Those organisms that have more benefits than handicaps will succeed more often - this is "natural selection", also referred to as "survival of the fittest". Whether a characteristic is a benefit or a handicap is determined by the environment - change the environment, and the species must evolve to adapt (or die - extinction is part of evolution, too) 

Note that "survival of the fittest" does not mean that the species is getting bigger, stronger or smarter. Evolution doesn't have a direction. There is no drive to "improve" the species. The only thing that matters is breeding the next generation - if the species breeds more by becoming smaller, stupider and slower, that's what will happen.