Variation

Variation refers to the differences in characteristics between individuals of the same species. No two individuals (except for identical twins) are exactly alike. These differences can be obvious, like eye colour, or more subtle, like blood type. Variation is crucial for the survival and evolution of a species. It arises from two main sources: the genetic information inherited from parents (genotype) and the influence of the environment. The observable characteristics of an organism are called its phenotype. The relationship can be summed up as: Phenotype = Genotype + Environment.

Discontinuous variation describes characteristics that fall into distinct, separate categories with no in-between values. You either have the trait or you don't. These characteristics are typically controlled by a single gene or a small number of genes and are not influenced by the environment. Because they fall into clear-cut groups, we can easily count individuals in each category and represent the data using a bar chart. Human blood groups (A, B, AB, O) are a perfect example; you cannot have a blood group that is 'in between' A and B.

Continuous variation describes characteristics that show a full range of values from one extreme to another, with many intermediates in between. Think of human height: people are not just 'tall' or 'short', but can be any height within a certain range. This type of variation is caused by the combined effect of many genes (this is called polygenic inheritance) and is often significantly influenced by environmental factors. For example, your genes determine your potential height, but your diet and health during childhood affect whether you reach that potential. When data for a continuous variable is plotted as a histogram, it often forms a 'bell-shaped' curve, known as a normal distribution curve. The majority of individuals are clustered around the average (mean) value, with fewer individuals at the extremes.

Mutation is the ultimate source of all genetic variation. A mutation is a rare, random change in the genetic material of an organism. This can be a change in the DNA base sequence of a single gene (a gene mutation) or a change in the structure or number of chromosomes (a chromosome mutation). Mutations can be harmful (e.g., sickle-cell anaemia, which is caused by a single gene mutation), neutral (have no observable effect), or, very rarely, beneficial. A beneficial mutation gives an organism a survival advantage, such as antibiotic resistance in bacteria. While mutations happen spontaneously, their rate can be increased by exposure to mutagens, which include high-energy radiation (like UV light and X-rays) and certain chemicals (like tar in tobacco smoke). Chemicals that cause cancer are called carcinogens.

While mutation creates new alleles, sexual reproduction is the process that shuffles existing alleles into new combinations. This is why siblings (except identical twins) look different from each other and from their parents. This genetic shuffling is a major driver of variation within a population and occurs in three key ways. First, during meiosis (the cell division that produces gametes), 'crossing over' occurs, where homologous chromosomes exchange sections of DNA. Second, 'independent assortment' means the maternal and paternal chromosomes are sorted into the gametes randomly. Third, 'random fertilisation' means that any one of the millions of unique sperm has an equal chance of fertilising the unique egg. The combination of these processes creates a vast number of potential genetic combinations in the offspring.