Continuous and discontinuous variation

Variation means differences between individuals of the same species. An observable characteristic, such as body length or seed colour, is called a phenotype. Phenotypes can vary continuously or discontinuously.

Continuous variation

In continuous variation, phenotypes form a range between two extremes, with many intermediate values. Body length and body mass are examples: individuals do not fall into just a few distinct categories, because many measurements are possible between the smallest and largest values.

Continuous variation is caused by both genes and the environment. For example, inherited genes can influence body mass, while environmental factors such as diet can also affect it.

Discontinuous variation

In discontinuous variation, phenotypes fall into a limited number of distinct categories, with no intermediate forms. For example, a person’s ABO blood group is A, B, AB or O; it is not a value between two of these groups. Pea seed shape and pea seed colour are further examples: seeds can be round or wrinkled, and yellow or green.

Discontinuous variation is usually caused by genes only.

Identifying the type of variation

To investigate variation, observe or measure the same phenotype in several individuals of a species. If the results form a range with intermediate values, the variation is continuous. If they fall into distinct categories with no intermediates, the variation is discontinuous.

Mutation

A mutation is a change in genetic material, such as a change in the DNA sequence. Mutations can occur spontaneously, without an identifiable external cause.

A gene can exist in different versions called alleles. When a mutation changes the DNA sequence of a gene, it can produce a new allele. Mutation is therefore the way new alleles are formed.

Ionising radiation and some chemicals increase the rate at which mutations occur. They make mutations more likely; they do not produce a mutation every time exposure occurs.

Sources of genetic variation in populations

Individuals in a population can differ genetically because they have different versions of genes and different combinations of those versions. A version of a gene is called an allele. Genetic variation arises through mutation, meiosis, random mating and random fertilisation.

Mutation: a source of new alleles

A gene mutation is a random change in the sequence of DNA bases within a gene. The changed sequence can form a new allele. If the new allele is passed on to offspring, it can become part of the genetic variation in the population.

New combinations of alleles

Meiosis produces gametes, or sex cells, with different combinations of alleles. During meiosis, matching chromosomes can exchange sections of DNA, and the chromosomes are distributed into gametes in different combinations. As a result, gametes from the same individual need not carry the same combination of alleles.

Random mating affects which individuals contribute alleles to offspring. Different pairings of individuals can bring together different sets of alleles.

Random fertilisation affects which gametes combine. When a sperm fertilises an egg, the offspring receives alleles from both gametes. Different sperm and egg combinations can therefore produce offspring with different combinations of alleles.

Mutation can introduce new alleles. Meiosis, random mating and random fertilisation produce different combinations of alleles among individuals.