Natural selection can only occur if there is variation among members of the same species

Mutation, meiosis and sexual reproduction causes variation between individuals in a species

Natural selection requires variation among members of a species in order to differentiate survival (variation needed for selection)

• This variation can manifest as either discontinous (distinct classes) or continuous (range across a characteristic spectrum)

With dominant and recessive alleles of a single gene, the number of possible phenotypes is limited.

For example, a person either has cystic fibrosis or not. When multiple alleles are introduced, the number of possibilities for a single trait increases accordingly. For example, the ABO blood type has three alleles and four possible phenotypes. When a second gene is introduced, the number of possible genotypes increases dramatically. With three, four or five genes determining the phenotype, the number of possibilities is so big that it is impossible to see the difference between certain genotypes in the phenotype.

When an array of possible phenotypes can be produced, it is called continuous variation. The colour of skin in humans is an example of continuous variation and it is thought that the intensity of pigment in skin is the result of the interaction of multiple genes. In humans, continuous variation can also be seen in the genetic components of traits such as height, body shape, and intellectual aptitude. Each of these is also influenced by environmental components. A person’s height, for example, is determined by whether he or she inherits genes for tallness, but it also depends on the person’s nutrition as he or she is growing.

There are three main mechanisms by which genetic variation between individuals in a species may occur:

• Mutations – Changing the genetic composition of gametes (germline mutation) leads to changed characteristics in offspring
• Meiosis – Via either crossing over (prophase I) or independent assortment (metaphase I)
• Sexual reproduction – The combination of genetic material from two distinct sources creates new gene combinations in offspring

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait

• New alleles are formed by mutation

Gene mutations can be beneficial, detrimental or neutral

• Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
• Detrimental mutations truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait
• Neutral mutations have no effect on the functioning of the specific feature (silent mutations)