Genetic Change
What you must understand
Genetic change underpins evolution. Know the sources of genetic variation: mutation, meiosis (crossing over, independent assortment), and random fertilisation. Understand natural selection: variation exists, selection pressures act, favourable alleles increase in frequency over generations.
Know the types of mutation - point mutations (substitution, insertion, deletion) and chromosomal mutations - and their possible effects on protein structure and function. Understand how mutations can be neutral, harmful or beneficial depending on environment.
Understand speciation: geographic isolation, reduced gene flow, accumulation of differences, and reproductive isolation. Know the evidence for evolution: fossil record, comparative anatomy (homologous and analogous structures), embryology, and molecular evidence (DNA and protein sequences).
Traps in questions
- Individuals do not "mutate to suit" an environment; mutations arise randomly and selection acts on them.
- "Survival of the fittest" means reproductive fitness, not physical strength.
- Homologous structures share ancestry; analogous structures share function only.
- Evolution occurs in populations, not individuals.
- Insertions and deletions cause frameshifts, often more damaging than substitutions.
Rule of thumb
Use the variation → selection → inheritance chain: identify the variation, name the selection pressure, then explain how allele frequency changes over generations.
Worked example
A bacterium randomly acquires a mutation giving resistance to an antibiotic. When the antibiotic is applied, resistant bacteria survive and reproduce, so the resistance allele increases in frequency in the population.