6.1 - Variation

Types and causes of variation

• Variation is the differences in characteristics between individuals of the same species.
• Members of one species resemble each other but usually differ in many ways - for example, in a room full of people you'd see different hair colour, eye colour, and a range of heights.
• There are two underlying causes of variation:
  1. The genes an organism inherits
  2. The environment it lives in
• Most characteristics come from a combination of the two.

Genetic variation

• An organism's characteristics are determined by the genes inherited from its parents.
• In a sexually reproducing species, genes are carried in the gametes, and most offspring receive some genes from one parent and some from the other.
• This combining of genes from two parents means no two members of a species are genetically identical - apart from identical twins.
• The different alleles that distinguish individuals ultimately arise from mutation, the original source of all new genetic variation.
  • Most mutations have no effect.
  • Occasionally one produces a new allele that affects the phenotype, which may be harmful, beneficial, or neutral.
• Some characteristics are determined by genes alone, e.g. eye colour, blood group, and inherited disorders such as cystic fibrosis.

Environmental variation

• The conditions an organism lives and grows in also cause differences between members of the same species.
• Examples:
  • A plant grown on a sunny windowsill grows lush and green, while a genetically identical plant grown in darkness grows tall and spindly with yellow leaves.
  • In people, getting a suntan is an environmental difference.
• Features acquired during an organism's lifetime are called acquired characteristics.

A mixture of genetic and environmental factors

• Most characteristics are determined by a mixture of genetic and environmental factors rather than by one alone, e.g. body weight, height, and skin colour.
• The maximum height an organism could reach is set by its genes, but whether it actually grows that tall depends on its environment, such as how much food it gets.
• So the phenotype - the characteristics an organism actually shows - is the result of its genes interacting with its environment.

Continuous and discontinuous variation

Variation within a species falls into two patterns: continuous variation and discontinuous variation.

Continuous variation

• Individuals vary across a range between two extremes, with no distinct categories - a characteristic can take any value within the range.
• Examples:
  • Height - The classic example: people can be any height in a range, not simply tall or short
  • Body mass
  • Number of leaves on a tree
• Caused by both genes and the environment.
• Usually involves several genes acting together (polygenic inheritance) rather than a single gene.
• When plotted as a histogram (number of individuals against the measured value), it tends to form a smooth bell-shaped curve.

Discontinuous variation

• Also called discrete variation.
• There are two or more distinct categories, and each individual falls into exactly one of them, with no intermediates in between.
• Examples:
  • Human ABO blood group - A person can only be group A, B, AB or O, with nothing in between
  • Pea seed shape - Round or wrinkled
  • Tongue rolling - Able or not able
• Usually controlled by a single gene and not affected by the environment, which is why the categories stay distinct.
• When plotted it forms a bar chart with separate bars and gaps between the categories, because the groups do not blend into one another.