Key facts & rules

• Continuous variation: range of values (e.g. height, mass) — controlled by many genes + environment.
• Discontinuous variation: distinct categories (e.g. blood group, tongue rolling) — usually one gene.
• Natural selection: heritable variation + struggle for existence + differential reproduction = evolution.
• Darwin's theory: individuals with favourable variations survive and reproduce more → those alleles become more common.
• Selective breeding: humans select parents with desirable traits → breed together over generations.
• Evidence for evolution: fossil record, comparative anatomy, DNA comparisons.
• Antibiotic resistance: bacteria with resistance allele survive treatment → reproduce → resistance spreads (natural selection).