Variation

Variation refers to the differences between individuals of the same species. No two individuals are exactly alike (except for identical twins, who still acquire differences). These differences can be sorted into two main types. Discontinuous variation is where characteristics fall into distinct, separate categories with no intermediates. Examples include human blood groups (A, B, AB, O) or the ability to roll your tongue. This type of variation is controlled by a single gene or a small number of genes and is not affected by the environment. In contrast, continuous variation is where a characteristic can take any value within a given range, showing a graduation from one extreme to the other. Examples include height, weight, and skin colour. When plotted on a graph, continuous variation typically produces a bell-shaped curve (a normal distribution). This type of variation is controlled by multiple genes (polygenic inheritance) and is often influenced by environmental factors.

To understand variation, we must distinguish between an organism's genetic makeup and its observable features. The genotype is the complete set of genes an organism possesses for a particular trait, inherited from its parents. It's the 'genetic blueprint'. The phenotype, however, is the organism's actual observable characteristics, such as its appearance, development, and behaviour. The phenotype is a result of the interaction between the genotype and the environment. This relationship can be summarised by a simple equation: Phenotype = Genotype + Environment. For example, a person may have the genetic potential (genotype) to be tall, but a poor diet during childhood (environment) could result in a shorter adult height (phenotype). Similarly, a plant may have the genes for making chlorophyll (genotype), but it will only turn green (phenotype) if it is exposed to light (environment).

The ultimate source of all new genetic variation is mutation. A mutation is a random and spontaneous change in the structure of a gene or a chromosome. Gene mutations are changes to the base sequence of DNA, which can lead to a change in the protein that is coded for. Chromosome mutations involve changes to the structure or number of whole chromosomes. Mutations can be harmful, beneficial, or neutral. Harmful mutations can cause genetic disorders like cystic fibrosis or sickle cell anaemia. Neutral mutations have no effect on the phenotype. Beneficial mutations, though rare, can give an organism a survival advantage. For example, a mutation leading to antibiotic resistance in bacteria is highly beneficial for the bacterium. The rate of mutation can be increased by exposure to certain factors called mutagens. These include high-energy radiation (like UV light and X-rays) and certain chemicals (like tar in tobacco smoke).

While mutation creates new alleles, sexual reproduction is a powerful engine for creating new combinations of existing alleles. This shuffling of genes leads to immense variation among offspring. There are three key mechanisms. First, during meiosis (the cell division that creates gametes), 'crossing over' occurs, where homologous chromosomes exchange sections of DNA, creating new combinations of alleles on a single chromosome. Second, also during meiosis, 'independent assortment' means that the pairs of homologous chromosomes line up randomly before being pulled apart, so each gamete receives a random mix of maternal and paternal chromosomes. Third, 'random fertilisation' means that any sperm can potentially fertilise any egg. Given the vast number of different gametes produced by each parent, the chance of two siblings (other than identical twins) having the exact same combination of alleles is practically zero. This genetic reshuffling ensures that a population has a wide range of phenotypes, increasing the chances that some individuals will survive if the environment changes.