Evolution and Natural Selection

GCSE Biology · Inheritance

Evolution

Evolution is the gradual change in the inherited characteristics of a population over many generations. This may lead to the formation of new species.

Charles Darwin and Natural Selection

Charles Darwin (1809–1882) proposed the theory of evolution by natural selection after his voyage on HMS Beagle, where he studied variation in finches on the Galapagos Islands.

How Natural Selection Works

1. Within a population, there is genetic variation (caused by mutations and sexual reproduction)

2. Some individuals have characteristics that make them better adapted to the environment

3. These individuals are more likely to survive and reproduce (survival of the fittest)

4. They pass on their advantageous alleles to their offspring

5. Over many generations, the advantageous allele becomes more common in the population

6. The population evolves

Example — Antibiotic Resistance in Bacteria

1. A population of bacteria has natural variation — some have a mutation giving antibiotic resistance

2. An antibiotic is applied, killing non-resistant bacteria

3. The resistant bacteria survive and reproduce

4. The resistance allele is passed to offspring

5. Over time, the entire population becomes resistant

This is why antibiotic resistance is increasing — it is natural selection in action.

Example — Peppered Moths

Before the Industrial Revolution, most peppered moths were pale (camouflaged against lichen-covered trees). During industrialisation:

Pollution killed lichen and darkened tree trunks with soot
Dark moths were now better camouflaged → less likely to be eaten by birds
Dark moths survived and reproduced more → dark allele became more common
After clean air laws reduced pollution, pale moths increased again

Evidence for Evolution

1. Fossils

Fossils are the preserved remains or traces of organisms that lived millions of years ago
The fossil record shows how organisms have changed over time
Fossils can be formed by: mineralisation of hard parts (bones, shells), casts and moulds in rock, preservation in amber/ice/peat, traces (footprints, burrows)
Limitations: the fossil record is incomplete — many organisms did not fossilise (soft bodies decay), geological activity destroys fossils, and not all fossils have been found

2. Antibiotic resistance

Observable in real time — demonstrates natural selection happening today

3. DNA and protein analysis

Species with similar DNA sequences are more closely related
Comparing genomes reveals evolutionary relationships

4. Anatomy

Homologous structures — similar bone structures in different species (e.g. pentadactyl limb in humans, whales, bats) suggest a common ancestor

Extinction

A species becomes extinct when there are no remaining individuals of that species alive. Causes include:

Environmental changes — climate change, habitat destruction
New predators or competitors
New diseases
Catastrophic events — volcanic eruptions, asteroid impacts
Failure to adapt quickly enough to changing conditions

Speciation

Speciation is the formation of a new species. It usually occurs through geographical isolation:

1. A population is separated by a physical barrier (river, mountain, sea level change)

2. The two groups experience different environmental conditions

3. Natural selection acts differently on each group — different alleles are favoured

4. Over many generations, the two groups become so genetically different that they can no longer interbreed to produce fertile offspring

5. They are now two separate species

Alternative Theories

Jean-Baptiste Lamarck (1744–1829) proposed that organisms could pass on acquired characteristics — features developed during their lifetime. For example, he suggested giraffes stretched their necks to reach high leaves, and this longer neck was inherited by offspring.

This theory is incorrect — acquired characteristics do not change DNA and cannot be inherited. Only changes to gamete DNA (mutations) can be passed on.

Alfred Russel Wallace

Alfred Russel Wallace independently developed a theory of natural selection similar to Darwin's. He:

Conducted extensive fieldwork in South America and Southeast Asia
Presented his ideas jointly with Darwin in 1858
Contributed to the theory of warning colouration in animals
Helped drive Darwin to publish On the Origin of Species (1859)

Exam Tips

Natural selection requires variation, competition, survival of the fittest, and reproduction — include all four steps
Evolution is a change in a population over time, not in an individual — individuals do not evolve
Know the difference between Darwin's theory (natural selection of inherited variation) and Lamarck's (inheritance of acquired characteristics)
Speciation requires isolation so that two populations can no longer interbreed