Variation

Variation refers to the differences that exist between individuals of the same species. For example, humans vary in height, eye colour, and blood type. No two individuals are exactly alike, except for identical twins. This variation is crucial for the survival and evolution of a species. Variation arises from two main sources: genetic factors inherited from parents, and environmental factors that influence how genes are expressed.

Discontinuous variation is where individuals fall into a limited number of distinct, separate categories, with no intermediate forms. Think of it as an 'either/or' situation. A classic example is human ABO blood groups; you are either group A, B, AB, or O, with nothing in between. This type of variation is controlled by a single gene or a small number of genes, and the environment has little to no effect on the phenotype. When plotted on a graph, discontinuous variation is shown using a bar chart with separate bars for each category.

Continuous variation is where a characteristic shows a complete range of values from one extreme to another, with many intermediates. There are no distinct categories. Human height is a perfect example: people are not just 'tall' or 'short'; there is a whole spectrum of heights in between. This type of variation is caused by a combination of factors: it is polygenic (controlled by multiple genes) and is also significantly influenced by the environment (e.g., diet and health). When plotted, continuous variation typically produces a bell-shaped curve known as a normal distribution.

A mutation is a random change in an organism's genetic material. This can be a change in the base sequence of a gene (a gene mutation) or a change in the overall structure or number of chromosomes (a chromosome mutation). Mutations are the ultimate source of all new genetic variation; they are how new alleles are created. While most mutations are neutral or harmful, occasionally a mutation can be beneficial, giving an organism a survival advantage. The rate of mutation can be increased by exposure to certain factors called mutagens. These include ionising radiation (like UV light from the sun and X-rays) and some chemicals (like tar in tobacco smoke).

Sickle-cell anaemia is a classic example of a gene mutation. A single change in the base sequence of the gene that codes for haemoglobin results in the production of abnormal haemoglobin (HbS). When oxygen levels are low, this abnormal haemoglobin causes red blood cells to deform into a rigid sickle or crescent shape. These sickled cells can block small blood vessels, causing pain, organ damage, and severe anaemia. The allele for normal haemoglobin is HbA and the sickle-cell allele is HbS. People with genotype HbS HbS have sickle-cell anaemia. Interestingly, people with genotype HbA HbS (heterozygous) have the 'sickle-cell trait'. They are generally healthy and, crucially, show increased resistance to malaria. This is because the malaria parasite has difficulty surviving in their red blood cells. This 'heterozygous advantage' explains why the HbS allele is more frequent in populations where malaria is common.

Natural selection is the process by which species evolve over time. It can only happen because of variation within a population. The process can be summarised in steps: 1. Variation: A population shows genetic variation. 2. Overproduction: Most organisms produce more offspring than can survive. 3. Struggle for Existence: The offspring compete for resources, and face 'selection pressures' like predation, disease, or climate change. 4. Survival of the Fittest: Individuals with alleles that give them advantageous features (adaptations) are better suited to the environment. They are more likely to survive and reproduce. 5. Inheritance: These successful individuals pass on their advantageous alleles to their offspring. 6. Adaptation: Over many generations, the frequency of these advantageous alleles increases in the population, and the population becomes better adapted to its environment.