Genetic Diversity and Mutation

Genetic diversity

Genetic diversity is the number of different alleles of genes in a population. Greater diversity means a population is more likely to contain individuals that can survive environmental change — it is the raw material for natural selection and evolution.

Sources of genetic variation

• Mutations — random changes to DNA base sequence; the ultimate source of new alleles.
• Meiosis — crossing over and independent segregation shuffle existing alleles into new combinations.
• Random fertilisation — any gamete can combine with any other.

Gene mutations

Changes to the base sequence within a gene:

• Substitution — one base replaced. May be silent (no change to amino acid, because the code is degenerate), missense (different amino acid) or nonsense (creates a stop codon).
• Deletion / insertion — cause a frame shift: every triplet after the mutation is read differently, usually drastically changing the protein.
• Mutagens (e.g. UV, X-rays, some chemicals) increase mutation rate.

Natural selection

1. Variation exists in a population (from mutation etc.).

2. Selection pressure (predation, disease, competition, climate) means not all survive.

3. Individuals with advantageous alleles are more likely to survive and reproduce (differential reproductive success).

4. They pass the advantageous alleles to offspring, so the allele frequency increases over generations.

Types of selection

• Directional selection: favours one extreme (e.g. antibiotic resistance) — the mean shifts.
• Stabilising selection: favours the average, against both extremes (e.g. human birth weight) — reduces variation.

Adaptations

Advantageous features may be anatomical (physical structures), physiological (internal processes) or behavioural. Over long timescales, natural selection can lead to speciation — the formation of new species when populations become reproductively isolated.

Worked example

Explain how a bacterial population becomes resistant to an antibiotic (a form of directional selection).

• A random mutation makes some bacteria resistant. When the antibiotic is applied, non-resistant bacteria die but resistant ones survive and reproduce, passing on the resistance allele — so its frequency increases and the population becomes resistant. ✓

Common mistakes

• Saying organisms mutate because they need to — mutations are random; the environment then selects.
• Forgetting a frame shift (deletion/insertion) affects all subsequent codons.
• Confusing directional (favours an extreme) with stabilising (favours the mean) selection.

Exam tips

• List the sources of variation: mutation, meiosis, random fertilisation.
• Explain natural selection as an ordered process (variation → selection pressure → differential survival → allele frequency change).
• Distinguish directional vs stabilising selection with examples.

Key facts to remember

• Genetic diversity = number of different alleles; increased by mutation (new alleles), meiosis and random fertilisation.
• Mutations: substitution (silent/missense/nonsense) and deletion/insertion (frame shift).
• Natural selection: advantageous alleles → greater reproductive success → allele frequency rises; directional (favours extreme) vs stabilising (favours mean).