Causes of variation

Variation refers to the differences observed among individuals within any given population. Despite the high degree of similarity, especially in genetically identical organisms such as twins, each individual is distinct.

Variations can arise due to variety of factors.

Genetic factors that cause variation

Genetic variation is variation due to the genes and alleles an individual possesses.

Variations in alleles can lead to different phenotypes, such as the diversity seen in human blood groups.

Sources of genetic variation:

1. Mutations - Changes to genes and chromosomes that may be passed on to the next generation.
2. Meiosis - New combinations of alleles are present in the gametes formed, produced by independent assortment of chromosomes and crossing over between chromatids.
3. Random fertilisation - Random fertilisation of gametes produces new combinations of alleles in a zygote.
4. Random mating

Environmental factors that cause variation

Environmental variation is variation caused by the environment in which an organism lives.

Differences in environmental factors like climate or diet can cause variations among individuals, such as variations in accents or the presence of pierced ears.

Environmental factors that can cause variation include:

1. Light
2. Nutrient and food availability
3. Temperature
4. Rainfall
5. Soil conditions
6. pH

Environmental influences can also affect the way an organism's genes are expressed.

The combined effect of genetic and environmental factors on variation

While genetic makeup sets the potential for variation, environmental conditions can influence the actual outcome, such as with human height.

It is hard to distinguish between the effects of genetic and environmental influences on variation as many combine to produce differences between individuals.

Polygenes:

• These are different genes at different loci that all contribute to a particular aspect of phenotype.
• Their individual effects on a phenotype are too small to observe, but they can act together to produce observable variation.
• This combined effect of multiple genes is common in continuous variation.