Variation

Genetic variation is caused by:

• independent assortment of chromosomes, and therefore alleles, during meiosis
• crossing over between chromatids of homologous chromosomes during meiosis
• random mating between organisms within a species
• mutation

The first four of these reshuffle existing alleles in the population. Offspring have combinations of alleles which differ from those of their parents and from each other. This genetic variation produces phenotypic variation.

Mutation however, does not reshuffle already existing alleles but produces completely new alleles. This may happen, for example, if a mistake occurs in DNA replication, so that a new base sequence occurs in a gene  leading to gene mutation. The new allele is often recessive, so it frequently does not show up in population until some generations after the mutation actually occured, when by chance two decendants of organisms in which the mutation happened mate and produce offspring.

Mutations that occur in body cells, or somatic cells, ofen have no effects at all on the organism. Somatic mutations cannot be passed on to offspring by sexual reproduction. However, mutations in cells in ovaries or testes of an animal, or ovaries or anthers of a plant, may be inherited by offspring. If such a gamete is one of the two which fuse to form a zygote, then the mutated gene will also be in zygote, which divides repeatedly to form a new organism, in which all thhe cells will contain the mutated gene.

Genetic variation whichever way it is caused can be passed on by parents to their offspring, giving differences in phenotype. Genetic variation provides the raw material on which natural selection can act.

Variation within a population means that some individuals have features that give them an advantage over other members of that population.

Variation in phenotype is also caused by the environment in which organisms live. Variation caused by the environment is not passed on by parents to their offspring

Continuous and Discontinuous variation

The variation that exists within a species is categorised as continuous and discontinuous. Phenotypic differences include qualitative differences, such as blood groups, and quantitative differences such as height and mass.

Qualitative differences fall into clearly distinguishable categories, with no intermediates -  e.g. you have one of four possible ABO blood groups: A, B, AB or O. This is discontinuous variation

In contrast, the quantitative differences between individual heights or masses may be small and difficult to distinguish e.g when the heights of a large number of people are measured, there are no distinguishable height classes. Instead there is a range of heights between two extremes. This is continuous variation.

The environment has considerable influence on the expression of features that show continuous (or quantitative) variation.

The genetic basis of continuous and discontinuous variation

Both qualitative and quantitative differences in phenotype may be inherited. Both may involve several genes. However, there are more important differences between them

In discontinuous (qualitative) variation:

• different alleles at a single gene locus have large effects on the phenotype
• different genes have different effects…