Evolution by Natural Selection

This lesson covers evolution by natural selection — the mechanism by which species change over time — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand Darwin's theory, the process of natural selection, speciation and the evidence for evolution.

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What Is Evolution?

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

Evolution occurs because of natural selection — a process first described by Charles Darwin following his voyage on HMS Beagle and his observations of finches on the Galapagos Islands.

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Darwin's Theory of Natural Selection

Darwin's theory can be summarised in the following steps:

1. Variation — Within any population, individuals show variation in their characteristics (caused by genetic differences and mutations).
2. Competition — Organisms compete for limited resources (food, water, territory, mates).
3. Survival of the fittest — Individuals with characteristics that are best suited to the environment are more likely to survive and reproduce. ("Fittest" means best adapted, not physically strongest.)
4. Inheritance — The advantageous alleles are passed on to the next generation.
5. Accumulation — Over many generations, the frequency of the beneficial allele increases in the population, and the species gradually changes.

graph TD
    A["Variation exists in a population"] --> B["Competition for resources"]
    B --> C["Best-adapted individuals survive"]
    C --> D["Survivors reproduce and pass on alleles"]
    D --> E["Beneficial alleles become more common"]
    E --> F["Population changes over time (evolution)"]

Exam Tip: Never say organisms "choose" to evolve or "develop" a new characteristic because they "need" it. Evolution happens because individuals with beneficial variations are more likely to survive and reproduce — it is not a conscious process.

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Worked Example: Antibiotic Resistance in Bacteria

Antibiotic resistance is a modern example of evolution by natural selection:

1. A population of bacteria shows variation — some have a mutation that makes them resistant to an antibiotic.
2. When the antibiotic is used, most bacteria are killed.
3. The resistant bacteria survive and are not killed by the antibiotic.
4. The resistant bacteria reproduce rapidly (bacteria reproduce very quickly by binary fission).
5. The population of resistant bacteria increases.
6. The antibiotic is now ineffective against this population — this is called antibiotic resistance.

This is why doctors and scientists urge:

• Only use antibiotics when necessary (not for viral infections).
• Always complete the full course of antibiotics.
• Develop new antibiotics to stay ahead of resistant strains.

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Evidence for Evolution

Several types of evidence support the theory of evolution:

1. Fossil Record

Evidence	Explanation
Fossils show how organisms have changed over time	Fossils in deeper (older) rock layers are simpler; newer layers show more complex organisms
Transitional fossils show intermediate forms	E.g. Archaeopteryx — features of both reptiles and birds
Gaps in the fossil record exist	Many organisms did not fossilise (soft bodies, conditions not right)

2. Antibiotic Resistance

• Provides observable evidence of natural selection happening today.
• Bacteria evolve resistance within years — a rapid example of evolution.

3. Comparative Anatomy

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

4. DNA and Molecular Evidence

• Species that are closely related have more similar DNA sequences.
• Comparing genomes allows scientists to construct evolutionary trees (phylogenetic trees).

Exam Tip: If asked for evidence of evolution, give at least two different types of evidence and explain how each supports the theory. Don't just list them — explain the reasoning.

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Speciation

Speciation is the formation of a new species from an existing one. A species is a group of organisms that can interbreed to produce fertile offspring.

How Speciation Occurs

1. A population becomes geographically separated (e.g. by a river, mountain range or island formation) — this is called isolation.
2. The two separated groups experience different environmental conditions.
3. Natural selection acts differently on each group — different alleles are favoured.
4. Mutations occur independently in each group, adding to the genetic differences.
5. Over a long period of time, the two groups become so genetically different that they can no longer interbreed to produce fertile offspring.
6. They are now two separate species.

graph TD
    A["Original population"] --> B["Geographic separation (isolation)"]
    B --> C["Group A: different environment"]
    B --> D["Group B: different environment"]
    C --> E["Different natural selection pressures"]
    D --> F["Different natural selection pressures"]
    E --> G["Genetic differences accumulate"]
    F --> G
    G --> H["Can no longer interbreed"]
    H --> I["Two new species"]

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Darwin vs Lamarck

Before Darwin, Jean-Baptiste Lamarck proposed a different theory of evolution:

Lamarck's theory	Darwin's theory
Organisms can acquire characteristics during their lifetime and pass them on	Characteristics are determined by genes and passed on through reproduction
If an organ is used a lot, it gets stronger and is inherited	Variation already exists; natural selection acts on existing variation
Example: giraffes stretched their necks to reach food, and long necks were inherited	Example: giraffes with naturally longer necks survived better and passed on their alleles

Lamarck's theory is incorrect — acquired characteristics are not passed on. Only changes to DNA in gametes can be inherited.

Exam Tip: A common 6-mark question asks you to explain evolution using a specific example (peppered moths, antibiotic resistance, etc.). Always follow the steps: variation → competition → survival of fittest → inheritance → population changes over time.

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Summary