CB4.1 — Evolution (Edexcel 1SC0)
Natural selection — the mechanism
- Organisms show variation in their characteristics (due to mutation and sexual reproduction).
- Organisms produce more offspring than the environment can support → competition.
- Individuals with characteristics better suited to the environment survive and reproduce (survival of the fittest).
- Those individuals pass their advantageous alleles to offspring.
- Over generations, the frequency of advantageous alleles increases → population changes.
Evidence for evolution
- Fossil record: shows gradual changes in species over geological time.
- Antibiotic resistance in bacteria: bacteria with resistant mutations survive antibiotic treatment → rapid evolution observed in real time.
- Comparative anatomy: homologous structures (e.g. human hand, whale flipper, bat wing) show common ancestry.
- DNA comparison: similarities in DNA sequences between species indicate common ancestors.
Speciation
Speciation occurs when a population is split (e.g. by a geographical barrier) and evolves separately over time until members can no longer interbreed.
Common misconceptions
- Evolution is not "directed" — mutations are random; selection acts on the results.
- Individuals don't evolve — populations do, over many generations.
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