Variation
No two individuals (apart from identical twins) are exactly alike. The differences between organisms — height, weight, blood group, intelligence, susceptibility to disease — are called variation.
Three causes of variation
Variation has three sources:
- Genetic variation — differences in alleles inherited from parents (eye colour, blood group).
- Environmental variation — differences caused by surroundings (a plant grown in shade vs sun; bigger meal → bigger child).
- Combination — many traits result from genes and environment together (height, weight).
Examples to know:
- Eye colour is purely genetic.
- Language spoken is purely environmental.
- Height is a combination — your genes set a potential range, environment (nutrition, illness) determines where in that range you end up.
Genetic variation in detail
Every characteristic determined by genes can vary because:
- Different alleles exist in the population.
- Sexual reproduction shuffles alleles between parents (independent assortment, crossing over — HT).
- New alleles arise by mutation.
What is a mutation?
A mutation is a change in the base sequence of DNA. Mutations occur continuously and at random — they're the only source of new alleles.
Most mutations have no effect on the phenotype:
- They occur in non-coding DNA.
- They are silent (different codon, same amino acid).
- They affect a non-essential part of the protein.
Some mutations have a small effect — slightly alter a protein's shape and how well it works.
A few mutations have a large effect — significantly change a protein's shape, sometimes preventing it working entirely. If this affects survival or reproduction, natural selection acts on the mutation: it spreads if helpful, dies out if harmful.
Factors that increase mutation rate
- Ionising radiation (X-rays, gamma rays, UV light from the sun)
- Chemical mutagens (chemicals in tobacco smoke, some industrial chemicals)
- Viruses that integrate into the genome
This is why exposure to radiation and tobacco smoke increases cancer risk — both are caused by mutations in genes that control cell division.
Continuous vs discontinuous variation
- Continuous variation: many possible values along a range (height, weight, leaf length). Plotted as a histogram or smooth curve. Usually controlled by many genes (polygenic) and influenced by environment.
- Discontinuous variation: distinct categories with no in-betweens (blood group A/B/AB/O, attached/unattached earlobes). Usually controlled by one or a few genes.
A typical exam graph: heights of students plotted as a histogram showing a normal distribution — clear continuous variation.
Why variation matters
Variation provides the raw material for evolution by natural selection. If all individuals in a population were identical, the species could not adapt to change.
⚠Common mistakes
- Saying mutations are always harmful. Most are neutral; a few are beneficial; only a small minority are seriously harmful.
- Saying environment changes the genome. It does not — but it can affect whether a gene is expressed.
- Confusing variation in individuals with variation in populations. Genetic variation is best measured at the population level.
- Saying twins prove "everything is genetic". Identical twins separated at birth still differ slightly — environment matters.
Links
Sets up B6.6 (natural selection) and B6.9 (theories of evolution). Connects to B6.3 (mutations from a molecular angle).
AI-generated · claude-opus-4-7 · v3-deep-biology