Evolution by Natural Selection

Evolution by natural selection is one of the most important ideas in biology. This lesson covers Darwin's theory, the evidence for evolution, and how natural selection leads to the development of new species. This is a core topic in the AQA GCSE Combined Science Trilogy specification (8464).

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What Is Evolution?

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

Evolution occurs because of natural selection — organisms with characteristics best suited to their environment are more likely to survive, reproduce and pass on their alleles to the next generation.

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Darwin's Theory of Natural Selection

Charles Darwin (1809–1882) developed the theory of evolution by natural selection after his famous voyage on HMS Beagle. His key observations and conclusions were:

graph TD
    A["Observation 1:<br/>Individuals within a species<br/>show variation"] --> E["Conclusion 1:<br/>Individuals with advantageous<br/>characteristics are more<br/>likely to survive"]
    B["Observation 2:<br/>Organisms produce more<br/>offspring than can survive"] --> E
    C["Observation 3:<br/>There is competition<br/>for limited resources"] --> E
    E --> F["Conclusion 2:<br/>Survivors reproduce and pass<br/>on their alleles to offspring"]
    F --> G["Conclusion 3:<br/>Over many generations,<br/>advantageous characteristics<br/>become more common"]
    G --> H["Evolution occurs"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#c8e6c9,stroke:#2e7d32
    style C fill:#fff9c4,stroke:#f9a825
    style E fill:#e1bee7,stroke:#6a1b9a
    style F fill:#ffccbc,stroke:#d84315
    style G fill:#ffccbc,stroke:#d84315
    style H fill:#ef9a9a,stroke:#c62828

Step-by-Step Process

1. Variation — individuals within a species show variation in their characteristics (caused by genetic differences)
2. Competition — organisms compete for limited resources (food, water, territory, mates)
3. Survival of the fittest — individuals with characteristics better suited to the environment are more likely to survive and avoid predation or disease
4. Reproduction — survivors breed and pass on their advantageous alleles to their offspring
5. Inheritance — over many generations, the frequency of the advantageous allele increases in the population
6. Evolution — the population gradually changes as advantageous characteristics become more common

Exam Tip: AQA (8464) often asks you to describe natural selection for a given scenario (e.g. antibiotic resistance in bacteria, or peppered moths). Use the step-by-step structure above and apply it to the specific example given.

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Example: Antibiotic Resistance in Bacteria

This is one of the most commonly examined examples of natural selection:

1. In a bacterial population, random mutations produce variation — some bacteria have a mutation that makes them resistant to an antibiotic
2. When the antibiotic is used, non-resistant bacteria are killed
3. The resistant bacteria survive because they are not affected by the antibiotic
4. The resistant bacteria reproduce rapidly (bacteria reproduce by binary fission)
5. The allele for resistance is passed on to all offspring
6. Over time, the entire population becomes antibiotic-resistant — this is a "superbug" such as MRSA

Why This Matters for Medicine

Problem	Consequence
Overuse of antibiotics	Increases the rate at which resistant strains develop
Incomplete courses of antibiotics	Allows partially resistant bacteria to survive and reproduce
MRSA (methicillin-resistant Staphylococcus aureus)	Cannot be treated with many common antibiotics
Reduced treatment options	Infections become harder and more expensive to treat

Exam Tip: AQA commonly asks about antibiotic resistance. Remember: the antibiotic does NOT cause the mutation. The mutation occurs randomly and before the antibiotic is applied. The antibiotic acts as a selection pressure that favours resistant bacteria.

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Example: Peppered Moths

Before the Industrial Revolution, most peppered moths were pale coloured, which camouflaged them against pale lichen-covered tree bark. Dark moths were rare.

During the Industrial Revolution:

1. Pollution killed lichen on trees and soot darkened tree bark
2. Pale moths were now easily seen by predators on dark bark
3. Dark moths were better camouflaged and survived more often
4. Dark moths reproduced and passed on the dark allele
5. The proportion of dark moths increased in polluted areas

After pollution was reduced:

• Lichen grew back, bark became lighter
• Pale moths were once again better camouflaged
• The proportion of pale moths increased again

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Evidence for Evolution

Type of Evidence	What It Shows
Fossils	Show how organisms have changed over millions of years; reveal intermediate forms between species
Antibiotic resistance	Shows natural selection occurring in real time within bacterial populations
DNA and genetic analysis	Closely related species have similar DNA sequences; the more similar the DNA, the more closely related
Comparative anatomy	Similar bone structures in different species (homologous structures) suggest common ancestry
Observed evolution	Peppered moths, Darwin's finches and other examples show evolution happening within observable timescales

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Other Theories of Evolution

Lamarck's Theory (Rejected)

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

This theory was rejected because:

• Characteristics acquired during an organism's lifetime do not change its DNA
• Only changes to DNA (in gametes) can be inherited
• There is no mechanism by which a stretched neck could alter the alleles in sperm or eggs

Wallace's Contribution

Alfred Russel Wallace independently developed a theory of evolution by natural selection at the same time as Darwin. Wallace:

• Collected evidence from the Malay Archipelago
• Wrote to Darwin describing his ideas
• Prompted Darwin to publish his theory in On the Origin of Species (1859)
• Is now recognised as a co-discoverer of natural selection

Exam Tip: AQA (8464) may ask you to compare Darwin's and Lamarck's theories. The key difference is that Darwin said variation is inherited and already exists before selection acts, while Lamarck thought organisms could acquire new characteristics and pass them on.

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Speciation

Speciation is the formation of new species through evolution. It occurs when populations of the same species become so different that they can no longer interbreed to produce fertile offspring.

How Speciation Occurs