Evolution

Combined Science (Trilogy) AQA
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B19: Evolution

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Theory of evolution, natural selection and classification

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📋 Key Concepts

Evolution: The change in the inherited characteristics of a population over successive generations through the process of natural selection. This may result in the development of new species.

Key Terms

📝 Darwin's Theory of Natural Selection

Natural selection explains how evolution occurs. Organisms with characteristics best suited to their environment are more likely to survive and reproduce, passing on their advantageous genes to their offspring.
Steps of natural selection:
  1. Variation: Within a population, there is genetic variation (differences in characteristics due to different alleles)
  2. Competition: Organisms compete for limited resources (food, mates, territory)
  3. Survival of the fittest: Individuals with characteristics best suited to the environment are more likely to survive ("fittest" means best adapted, not strongest)
  4. Reproduction: The survivors are more likely to reproduce and pass on their advantageous alleles to their offspring
  5. Over generations: The frequency of the advantageous alleles increases in the population. Over many generations, this can lead to the development of new species.
Example 1

Explain how natural selection could lead to the evolution of antibiotic resistance in bacteria.

Solution:

1. Variation: random mutations produce some bacteria with genes for antibiotic resistance.

2. When antibiotics are used, most non-resistant bacteria are killed.

3. The resistant bacteria survive (they are best adapted to the environment containing the antibiotic).

4. The resistant bacteria reproduce, passing on the resistance alleles to their offspring.

5. Over time, the population becomes increasingly resistant to the antibiotic.

Example 2

Before the Industrial Revolution, most peppered moths were pale. After industrial pollution darkened tree bark, dark moths became more common. Explain this using natural selection.

Solution:

Variation existed in the moth population (pale and dark forms). Before industrialisation, pale moths were camouflaged on light tree bark and avoided predation. Dark moths were easily seen and eaten. After pollution darkened the bark, dark moths were now better camouflaged. They survived and reproduced more successfully, passing on the dark allele. Over generations, the frequency of dark moths increased in the population.

📝 Wallace and Independent Discovery

Alfred Russel Wallace independently developed the theory of evolution by natural selection. He sent his ideas to Darwin, which prompted Darwin to publish his own work (On the Origin of Species, 1859). Wallace is best known for his work on warning colouration in animals (e.g. bright colours in poisonous species that warn predators) and his observations of geographic distribution of species.
Why Darwin is more famous: Darwin had been developing his theory for many years with extensive evidence. His book provided a more comprehensive explanation with detailed supporting evidence from years of research (including his voyage on HMS Beagle).

📝 Evidence for Evolution

Fossils: The preserved remains or traces of organisms that lived millions of years ago. Fossils provide evidence for evolution because they show how organisms have changed over time — older rock layers contain simpler organisms, and more recent layers contain more complex organisms.
Limitations of the fossil record:
  • Many organisms decompose before they can fossilise
  • Fossils that did form may have been destroyed by geological activity
  • Not all fossils have been discovered yet
  • Soft-bodied organisms rarely fossilise
Antibiotic-resistant bacteria: This is direct evidence of evolution that can be observed happening now. Bacteria evolve resistance to antibiotics through natural selection within a few generations, showing evolution in action.
Example 3

Explain how the development of antibiotic-resistant bacteria provides evidence for evolution.

Solution:

When bacteria are exposed to an antibiotic, most die but any with a mutation for resistance survive. These resistant bacteria reproduce, passing on the resistance gene. Over a short time, the population becomes resistant. This is a direct, observable example of natural selection (evolution) occurring within a short timescale, supporting Darwin's theory.

📝 Extinction

Extinction: When no individuals of a species remain alive. Extinction can be caused by:
  • Environmental changes (e.g. climate change, habitat destruction)
  • New predators
  • New diseases
  • Competition from another species for the same resources
  • Catastrophic events (e.g. volcanic eruptions, asteroid impacts)
  • Changes to the food web (loss of a food source)
Example 4

The dodo became extinct in the 17th century. Explain the likely causes of its extinction.

Solution:

The dodo lived on Mauritius and had no natural predators, so it was flightless and unafraid of danger. When humans arrived, they hunted the dodos for food. Humans also introduced new predators (rats, cats, pigs) that ate dodo eggs. The dodos' habitat (forest) was destroyed for agriculture. Combined, these factors — new predators, hunting by humans, and habitat destruction — caused the dodo to become extinct.

📝 Classification

Classification: The system of organising living organisms into groups based on their characteristics and evolutionary relationships.
Linnaeus system classifies organisms into a hierarchy of groups (from largest to smallest):
Hierarchy of classification:
Kingdom → Phylum → Class → Order → Family → Genus → Species

Memory aid: King Philip Came Over For Good Soup
Binomial naming system: Every species has a two-part Latin name consisting of the genus name (capital letter) followed by the species name (lower case). Both parts are written in italics or underlined. Example: Homo sapiens (humans).
Rules of binomial naming:
  • Genus name has a capital letter, species name is lower case
  • Both names are in italics (or underlined if handwritten)
  • The binomial name is unique to each species — it avoids confusion from using common names in different languages
Example 5

Explain why the binomial naming system is important in science.

Solution:

Common names can be confusing — the same organism may have different names in different languages or regions, and different organisms may share the same common name. The binomial system gives each species a unique, internationally recognised two-part Latin name. This allows scientists worldwide to communicate precisely about the same species, avoiding confusion.

❓ Practice Questions

Q1: Describe the process of natural selection in five steps.

Q2: Explain how the development of antibiotic-resistant bacteria provides evidence for evolution.

Q3: State three causes of extinction and give an example for each.

Q4: Describe the Linnaean classification system, including the hierarchy of groups and the binomial naming system.

Q5: Explain why Darwin's theory of natural selection was not immediately accepted when it was first published.

✅ Answers

  1. 1. Variation exists in a population due to genetic differences. 2. Organisms compete for limited resources. 3. Individuals with characteristics best suited to the environment survive ("survival of the fittest"). 4. Survivors reproduce and pass on their advantageous alleles. 5. Over many generations, the frequency of advantageous alleles increases in the population, leading to evolution.
  2. When bacteria are exposed to antibiotics, random mutations mean some bacteria have resistance genes. These resistant bacteria survive while non-resistant ones die. The resistant bacteria reproduce, passing on resistance alleles. Over time, the population becomes resistant. This shows evolution by natural selection happening in real time, supporting Darwin's theory.
  3. Any three from: (1) New predators — e.g. rats introduced to islands preying on native birds; (2) Environmental change — e.g. climate change making habitat unsuitable; (3) New diseases — e.g. chytrid fungus killing amphibians; (4) Competition — e.g. introduced species outcompeting native species for food; (5) Catastrophic events — e.g. asteroid impact causing mass extinction.
  4. Linnaeus classified organisms into a hierarchy: Kingdom → Phylum → Class → Order → Family → Genus → Species (from largest to smallest). Organisms in the same group are more closely related. The binomial system gives each species a two-part Latin name: Genus (capital) + species (lower case), e.g. Homo sapiens. It is written in italics and provides a unique, internationally recognised name.
  5. Darwin's theory was not immediately accepted because: (1) it challenged the religious view that God created all species; (2) there was insufficient evidence at the time (the fossil record was incomplete); (3) the mechanism of inheritance was not yet understood (DNA and genes had not been discovered); (4) people found it hard to accept that humans evolved from other organisms.

🎯 Exam Tips

🔢 Maths Skills

Mathematical Skills

Interpreting fossil data: When analysing fossil data, calculate the age of rock layers and read from stratigraphic diagrams. Fossils in deeper rock layers are older. Use percentages to show changes: e.g. if the proportion of antibiotic-resistant bacteria in a hospital increases from 5% to 35% over 10 years, the increase is 30 percentage points. Calculate the rate of change: 30 ÷ 10 = 3 percentage points per year. When interpreting data on allele frequencies, calculate how the frequency of a resistance allele changes over generations.

⚠️ Common Misconceptions

Watch Out!

1. Evolution means organisms get better over time. Wrong: evolution is about species improving and becoming more advanced. Correct: evolution means becoming better adapted to the current environment, not objectively "better" — what is advantageous in one environment may be disadvantageous in another.

2. Darwin's theory was accepted immediately. Wrong: Darwin's theory of natural selection was quickly accepted by the scientific community. Correct: Darwin's theory was controversial when published in 1859 because it challenged religious beliefs about creation, the fossil record was incomplete, and the mechanism of inheritance (DNA/genes) was not yet understood.

✍️ 6-Mark Question

Extended Answer

6 marks: Explain natural selection and why it was initially controversial.

Natural selection explains how evolution occurs: variation exists within a population due to genetic differences (mutations). Organisms compete for limited resources such as food, mates and territory. Individuals with characteristics best suited to their environment are more likely to survive and reproduce — this is "survival of the fittest." The survivors pass on their advantageous alleles to their offspring, and over many generations the frequency of these alleles increases in the population, potentially leading to new species. Darwin's theory was initially controversial for several reasons: it challenged the prevailing religious view that God created all species separately; the fossil record was incomplete and did not provide sufficient evidence; the mechanism of inheritance was not understood (genes and DNA had not been discovered); and people found it disturbing that humans could have evolved from other organisms rather than being specially created.

Mark scheme: 1 mark — variation exists in populations; 1 mark — competition for resources; 1 mark — best-adapted survive and reproduce; 1 mark — advantageous alleles increase over generations; 1 mark — at least two reasons for controversy (religious challenge, incomplete evidence, unknown inheritance mechanism); 1 mark — explanation linked to historical context.

📊 AO3: Analyse & Evaluate

Analysis and Evaluation

Antibiotic X was introduced in 1990. In 1990, 2% of Staphylococcus aureus infections were resistant; by 2000 this was 18%; by 2010 it was 52%. In 2010, the hospital introduced strict antibiotic stewardship (only prescribing X when essential). By 2020, resistance had fallen to 31%. Describe the trend and explain it using natural selection. Evaluate whether the stewardship programme was effective and suggest why resistance did not return to 2%.

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