B38: Evolution
The theory of evolution by natural selection
Key Definitions
Evolution: The change in the inherited characteristics of a population over many generations through the process of natural selection. This can result in the development of new species.
Natural selection: The process by which organisms with characteristics best suited to their environment are more likely to survive and reproduce, passing on their advantageous alleles to the next generation.
Extinction: When there are no remaining individuals of a species alive. Once a species is extinct, it cannot be brought back.
Darwin’s Theory of Natural Selection
1. Variation exists within a population (caused by genetic differences)
2. Organisms produce more offspring than the environment can support
3. There is competition for limited resources (survival of the fittest)
4. Individuals best adapted to the environment are more likely to survive
5. Survivors reproduce and pass on their advantageous alleles
6. Over many generations, the frequency of advantageous alleles increases
2. Organisms produce more offspring than the environment can support
3. There is competition for limited resources (survival of the fittest)
4. Individuals best adapted to the environment are more likely to survive
5. Survivors reproduce and pass on their advantageous alleles
6. Over many generations, the frequency of advantageous alleles increases
The key idea of natural selection is that organisms with the best adaptations to their environment are more likely to survive and reproduce, passing on their advantageous alleles. Over time, this causes the population to become better adapted.
Example 1: Peppered Moths
Before the Industrial Revolution, most peppered moths were pale (light-coloured), camouflaged against light tree bark. A small number were dark (melanic). When factories polluted the air, tree bark became dark with soot. Dark moths were now better camouflaged — birds ate more pale moths. The dark moths survived, reproduced, and passed on the allele for dark colour. Over time, the dark form became much more common in polluted areas.
Example 2: Antibiotic-Resistant Bacteria
When bacteria are exposed to antibiotics, most die. However, due to mutations, some bacteria have alleles that make them resistant. These resistant bacteria survive the antibiotic treatment and reproduce. The resistant population increases, and the antibiotic becomes less effective. This is direct evidence of natural selection happening now — and it is a serious medical problem (e.g. MRSA).
Wallace and Independent Discovery
Alfred Russel Wallace independently developed a theory of natural selection at the same time as Darwin. Wallace’s observations in the Malay Archipelago led him to the same conclusions. Darwin and Wallace jointly published their ideas in 1858, but Darwin’s 1859 book “On the Origin of Species” gave a more detailed explanation, which is why the theory is more associated with Darwin.
Example 3: Wallace’s Warning Colouration
Wallace observed that some animals have bright, conspicuous colours that serve as warning colouration (aposematism). Predators learn to associate bright colours with danger (e.g. poison), so brightly coloured prey survive and reproduce. This supported the theory of natural selection — the bright colour was an advantageous trait selected for because it reduced predation.
Evidence for Evolution
Fossil Record
Fossils are the preserved remains or traces of organisms that lived millions of years ago. The fossil record shows that organisms have changed over time — simple organisms appeared first, and more complex organisms appeared later. This provides evidence for evolution.
Example 4: Fossil Evidence — Horse Evolution
The fossil record shows that ancient horses were small (about the size of a fox) with multiple toes. Over millions of years, fossils show horses becoming larger and developing a single hoof. This gradual change in the fossil record is evidence for evolution by natural selection — horses adapted to changing grassland environments.
Example 5: MRSA — Evolution in Action
MRSA (Methicillin-resistant Staphylococcus aureus) is a bacterium that has evolved resistance to multiple antibiotics, including methicillin. This resistance arose through mutations and was selected for by the overuse of antibiotics. MRSA infections are very difficult to treat and are a major problem in hospitals. This is direct, observable evidence of evolution occurring in real time.
Extinction
Causes of Extinction
| Cause | Explanation | Example |
|---|---|---|
| Environmental change | Rapid climate change can destroy habitats faster than species can adapt | Many species threatened by global warming |
| New predators | Introduction of a new predator can wipe out prey populations | Dodo — hunted by humans and introduced animals |
| New diseases | A disease can spread rapidly and kill all individuals | Chytrid fungus killing amphibians worldwide |
| Competition | A more successful competitor outcompetes the species for resources | Introduced grey squirrels outcompeting red squirrels in the UK |
| Catastrophic events | Mass extinction events (e.g. asteroid impacts, volcanic eruptions) | Dinosaurs — likely killed by asteroid impact 66 million years ago |
Comparison: Evidence for Evolution
| Evidence Type | What It Shows | Strength |
|---|---|---|
| Fossil record | Organisms have changed over time; simple life came before complex life; transitional forms exist | Direct physical evidence from the past; shows gradual changes over millions of years |
| Antibiotic-resistant bacteria | Natural selection can be observed happening in real time | Can be observed within years; directly shows the mechanism of evolution |
| DNA/molecular evidence | Species with more similar DNA are more closely related | Quantitative and precise; confirms relationships suggested by fossils |
Practice Questions
Q1: Foundation Describe Darwin’s theory of natural selection in four key steps.
Q2: Foundation Explain how bacteria evolve resistance to antibiotics through natural selection.
Q3: Foundation Give two types of evidence for evolution and describe how each supports the theory.
Q4: Higher Explain three causes of extinction, giving a different example for each.
Q5: Higher Explain the role of Wallace in the development of the theory of evolution by natural selection.
Answers
- 1. Variation exists within a population due to genetic differences. 2. Organisms produce more offspring than the environment can support, leading to competition for limited resources. 3. Individuals with characteristics best suited to the environment are more likely to survive and reproduce (survival of the fittest). 4. Survivors pass on their advantageous alleles to their offspring, and over many generations, these alleles become more common in the population.
- Random mutations in bacterial DNA can produce alleles for antibiotic resistance. When antibiotics are used, non-resistant bacteria are killed, but resistant bacteria survive. These surviving bacteria reproduce and pass on the resistance allele. Over time, the population becomes dominated by resistant bacteria, making the antibiotic less effective. The antibiotic does not cause the mutation — it selects for resistant bacteria that already exist.
- 1. Fossil record: Fossils show that organisms have changed over time — older rocks contain simpler organisms, and more recent rocks contain more complex organisms. Transitional fossils show intermediate forms between groups (e.g. Archaeopteryx between reptiles and birds). 2. Antibiotic-resistant bacteria: The rapid evolution of antibiotic resistance is directly observable evidence that natural selection occurs. Resistant bacteria have evolved within decades, showing evolution happening in real time.
- 1. New predators: The dodo was hunted to extinction by humans and the introduced animals (pigs, rats) that ate their eggs. 2. Environmental change: Many species are currently threatened by global warming, which is changing habitats faster than species can adapt or migrate. 3. Competition: Grey squirrels introduced to the UK outcompete native red squirrels for food and habitat, leading to a decline in red squirrel populations.
- Alfred Russel Wallace independently developed the theory of natural selection while working in the Malay Archipelago. He sent his ideas to Darwin in 1858, and they jointly presented their findings to the scientific community. Wallace’s observations of warning colouration in animals supported natural selection. Although Wallace reached the same conclusions, Darwin published a more comprehensive explanation in “On the Origin of Species” (1859), which is why the theory is more commonly associated with Darwin.
Exam Tips
- When describing natural selection, always mention: variation → competition → survival of the fittest → reproduction → passing on alleles. Don’t say organisms “choose” to adapt — adaptation occurs through selection, not choice
- Antibiotic resistance is NOT caused by antibiotics — antibiotics do NOT cause mutations. Mutations happen randomly, and antibiotics SELECT for resistant bacteria by killing non-resistant ones
- For extinction questions, make sure you can explain at least three different causes with examples
- When asked about evidence for evolution, the two best answers are fossil record and antibiotic-resistant bacteria — these are the most commonly expected answers
- Don’t forget Wallace — some questions specifically ask about his contribution to the theory
🔢 Maths Skills
Mathematical Skills
Interpreting fossil record dating: fossils are dated using radioactive dating techniques. If a fossil is found in a rock layer that is 65 million years old, and a similar but more primitive fossil is found in a layer 80 million years old, you can infer that the species evolved between 80 and 65 million years ago.
Calculating bacterial population growth: if a resistant bacterium divides every 20 minutes, after 6 hours (18 divisions) one bacterium produces 2¹&sup8; = 262,144 bacteria. This shows how quickly antibiotic resistance can spread.
⚠️ Common Misconceptions
Watch Out!
Students often think evolution means organisms improve or get better. Wrong: Evolution means organisms improve over time Correct: Evolution produces organisms better adapted to their environment — not objectively “better” (e.g. cave fish lost their eyes, which is not an “improvement” in most contexts)
Students often think Darwin's theory was accepted immediately. Wrong: Darwin's theory was accepted straight away Correct: It was highly controversial because it conflicted with religious views and there was insufficient evidence at the time (genes and DNA were unknown)
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the theory of natural selection and why it was initially controversial.
Natural selection explains evolution as follows: variation exists within populations due to genetic differences; organisms produce more offspring than the environment can support, leading to competition; individuals with characteristics best suited to the environment survive and reproduce; they pass on their advantageous alleles to offspring; over many generations, the frequency of advantageous alleles increases. The theory was controversial because: it contradicted the prevailing religious belief that God created all species in their current form; it implied humans evolved from ape-like ancestors, which many found deeply troubling; the mechanism of inheritance was not understood (Darwin did not know about genes or DNA); and there were gaps in the fossil record that critics used as evidence against the theory.
Mark scheme: Up to 3 marks for explaining natural selection with key steps, up to 3 marks for reasons it was controversial
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A hospital reports that MRSA infections increased from 2% to 18% of all Staphylococcus aureus infections over 10 years, while antibiotic prescriptions also rose by 40%. Evaluate the evidence that antibiotic use is causing the increase in MRSA. What additional data would strengthen this conclusion?
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Biology exams?
Get the best revision books and guides to boost your grades.