Natural Selection

Darwin was influenced by his interest in what is now known as artificial selection
Artificial selection involves techniques that breeders use to breed animals with particular characteristics. E.g., greyhound breeders select the fastest animals to mate. Over successive generations this has created greyhounds with long legs, deep chests, slender build and small heads - these characteristics all contribute to speed (Badcock, 2000). Darwin realised that this could happen naturally. Cheetahs for example have a greater chance of catching prey if they are fast and catching prey gives the animal a greater chance of surviving for long enough to pass on its genes to a greater number of offspring than slower animals. Cheetahs have developed many of the same characteristics as greyhounds (long legs, slender build, deep chests, small heads) through this natural way of selecting which animals breed.

Mutations

There are natural variations in the heritable traits of all animals. This provides the differences that are selected by evolutionary pressures. These variations occur because during reproduction, genes do not always make exact copies of themselves. Often, a particular gene will have many different variations, these are called alleles.

Selection by death

Mutations that give animals a greater than average chance of survival are more likely to be passed on to the next generation. This is because animals that live longer tend to produce more offspring. Animals that have genetic mutations that reduce the chances of survival tend to produce less offspring.

This means that over successive generations, genes that give a survival advantage become more frequent in the population, while genes that do not become less frequent.

Selection is limited to genes that already exist in the population, so mutations are important because they provide the differences that natural selection can act upon. Nevertheless, most mutations are not adaptive and are not selected.