Evolution by Natural Selection

This lesson covers evolution and the theory of natural selection as proposed by Charles Darwin. For AQA GCSE Biology, you need to understand how natural selection works, the evidence for evolution, and how evolution leads to the development of new species. You also need to know about antibiotic-resistant bacteria as a modern example of natural selection in action.

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What Is Evolution?

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

Evolution occurs through the process of natural selection, first described by Charles Darwin after his voyage on HMS Beagle to the Galapagos Islands in the 1830s.

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Charles Darwin and Natural Selection

Charles Darwin (1809-1882) proposed the theory of evolution by natural selection in his book On the Origin of Species (1859).

Darwin's key observations:

1. Individuals within a population show variation in their characteristics
2. There is a struggle for existence — organisms produce more offspring than the environment can support, leading to competition for resources (food, territory, mates)
3. Individuals with characteristics that are best suited to the environment are more likely to survive — this is called "survival of the fittest"
4. These individuals are more likely to reproduce and pass on their advantageous alleles to their offspring
5. Over many generations, the frequency of the advantageous alleles increases in the population
6. Over long periods of time, this can lead to the evolution of new species

graph TD
    A[Variation exists in population] --> B[Competition for resources]
    B --> C[Individuals with advantageous traits survive]
    C --> D[These individuals reproduce more]
    D --> E[Advantageous alleles passed to offspring]
    E --> F[Over generations the allele frequency increases]
    F --> G[Population evolves]
    G --> H[May lead to new species]

Exam Tip: When explaining natural selection, always include these five key steps: (1) variation, (2) competition/environmental change, (3) survival of the fittest, (4) reproduction, (5) inheritance of advantageous alleles. Missing any step will cost you marks.

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Why Darwin's Theory Was Controversial

Darwin's theory was controversial when first published for several reasons:

Reason	Explanation
Conflicted with religious beliefs	Many people believed God created all species individually and that species did not change over time
Insufficient evidence at the time	Darwin could not explain the mechanism of inheritance (genetics was not understood until the 20th century)
Challenge to human uniqueness	The idea that humans and apes share a common ancestor was deeply offensive to many
Lack of understanding of genetics	Without knowledge of DNA, genes, and mutations, the mechanism of variation was unclear
Rival theories	Other scientists, such as Jean-Baptiste Lamarck, proposed alternative theories (e.g. inheritance of acquired characteristics)

Lamarck vs Darwin

Lamarck proposed that organisms could pass on characteristics they acquired during their lifetime. For example, he suggested that giraffes developed long necks by stretching to reach tall trees, and this stretching was inherited by their offspring.

Feature	Darwin's theory	Lamarck's theory
Mechanism	Natural selection of existing variation	Inheritance of acquired characteristics
Source of variation	Random genetic variation already present	Organisms change during their lifetime through use/disuse
Example	Giraffes with naturally longer necks survived and reproduced more	Giraffes stretched their necks, and this was passed on
Status today	Accepted — supported by overwhelming evidence	Rejected — acquired characteristics are not inherited (changes to body cells do not affect gametes)

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Evidence for Evolution

Modern science provides a wealth of evidence supporting Darwin's theory:

1. Fossil Record

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
• Transitional forms (e.g. Archaeopteryx, which has features of both dinosaurs and birds)
• The order in which species appeared and disappeared
• Gradual changes in body structures over millions of years

However, the fossil record is incomplete because:

• Many organisms did not form fossils (soft-bodied organisms decompose without trace)
• Fossils may be destroyed by geological activity (erosion, tectonic movements)
• Many fossils have simply not been discovered yet

2. Antibiotic-Resistant Bacteria

The evolution of antibiotic-resistant bacteria such as MRSA (methicillin-resistant Staphylococcus aureus) is a modern, observable example of natural selection:

1. A population of bacteria shows natural variation — some bacteria may carry mutations that make them resistant to an antibiotic
2. When an antibiotic is used, it kills the non-resistant bacteria
3. The resistant bacteria survive and reproduce
4. The genes for resistance are passed to the next generation
5. Over time, the proportion of resistant bacteria in the population increases
6. The antibiotic becomes ineffective against the population

graph TD
    A[Bacterial population with natural variation] --> B[Antibiotic applied]
    B --> C[Non-resistant bacteria killed]
    B --> D[Resistant bacteria survive]
    D --> E[Resistant bacteria reproduce]
    E --> F[Resistance genes passed to offspring]
    F --> G[Population becomes mostly resistant]
    G --> H[Antibiotic no longer effective]

How to Reduce Antibiotic Resistance

Action	How it helps
Only prescribe antibiotics when necessary	Reduces the selection pressure for resistant bacteria
Patients must complete the full course	Ensures all bacteria are killed, not just the non-resistant ones
Do not use antibiotics for viral infections	Antibiotics have no effect on viruses; using them unnecessarily promotes resistance
Strict hygiene in hospitals	Prevents the spread of resistant bacteria between patients
Develop new antibiotics	Provides alternative treatments when existing antibiotics fail
Reduce antibiotic use in agriculture	Overuse in farming also promotes resistance

Exam Tip: MRSA and antibiotic resistance is one of the most heavily examined topics in GCSE Biology. Make sure you can explain how resistance develops using the steps of natural selection, and state measures to reduce it. This can appear as a 6-mark question.

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3. DNA and Genetic Evidence

Comparing the DNA sequences and protein structures of different species provides powerful evidence for evolution:

• Species that share a recent common ancestor have more similar DNA
• Humans and chimpanzees share approximately 98.8% of their DNA
• All life on Earth shares the same genetic code (the same base pairs, the same amino acids), suggesting a common origin

4. Anatomical Evidence (Comparative Anatomy)

Homologous structures — body structures in different species that have a similar underlying structure but may have different functions (e.g. the pentadactyl limb in humans, whales, bats, and dogs) — suggest descent from a common ancestor.

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Speciation

Speciation is the process by which a new species evolves from an existing one. It typically occurs through geographical isolation: