Natural selection and evolution

The core mechanism

Charles Darwin's theory of evolution by natural selection (1859) explains how species change over many generations. The argument rests on five linked observations:

1. Variation within a species — caused mainly by mutation, also by sexual reproduction.
2. Overproduction — more offspring are born than the environment can support.
3. Competition for finite resources (food, mates, space).
4. Survival of the fittest — variants best adapted to the environment are more likely to survive.
5. Inheritance — survivors pass advantageous alleles to offspring; over many generations the population changes.

Random variation → non-random survival → directional change.

Evidence for evolution

Fossils

Fossils form when hard parts of dead organisms (bones, shells, teeth) are replaced by minerals over millions of years, or when softer remains leave impressions in rock. The fossil record shows:

• Older rocks contain simpler organisms.
• Gradual change of forms through layers.
• Transitional fossils — e.g. Archaeopteryx (feathers + bony tail and teeth: bridges reptiles and birds).

Limitations: soft tissues rarely fossilise; fossils destroyed by geological activity; record is incomplete.

Antibiotic-resistant bacteria

A clear, observable, modern example:

1. A random mutation produces a bacterium with resistance.
2. The antibiotic kills non-resistant (susceptible) bacteria.
3. Resistant bacteria survive and reproduce rapidly (every 20 min).
4. Resistance alleles dominate the population — e.g. MRSA.

Slowing resistance: don't over-prescribe antibiotics; complete every course; use the right antibiotic for the right pathogen.

Other evidence

• Comparative anatomy (pentadactyl limb).
• Comparative DNA / protein sequences — closely related species have more similar sequences.

Speciation (Higher)

Two populations become geographically isolated → no gene flow between them → different selection pressures and different mutations → genetic differences accumulate → eventually they cannot interbreed to produce fertile offspring → two species.

OCR exam tip

For a 4-mark "explain antibiotic resistance" answer, the order matters: mutation → resistance → antibiotic kills the rest → resistant ones reproduce → population becomes resistant. Skipping mutation loses M1.