Evolution and Natural Selection

What you'll learn

This revision guide covers evolution and natural selection as specified in the WJEC GCSE Biology curriculum. You'll understand how species change over time, the mechanisms driving these changes, and the evidence supporting evolutionary theory. This topic connects to inheritance, variation, and real-world examples like antibiotic resistance in bacteria.

Key terms and definitions

Evolution — the gradual change in the inherited characteristics of a population over time through the process of natural selection

Natural selection — the process by which organisms better adapted to their environment are more likely to survive, reproduce and pass on their advantageous characteristics to their offspring

Adaptation — a characteristic that helps an organism survive in its environment, developed through the process of natural selection

Mutation — a random change in the DNA sequence of an organism that can result in new alleles and genetic variation

Selective breeding (artificial selection) — the process by which humans breed organisms with desirable characteristics to produce offspring with those characteristics

Antibiotic resistance — the ability of bacteria to survive exposure to antibiotics that would normally kill them or slow their growth

Fossil — the preserved remains or traces of organisms that lived millions of years ago, found in rocks

Extinction — when all members of a species die out and the species no longer exists

Core concepts

Darwin's theory of evolution by natural selection

Charles Darwin proposed the theory of evolution by natural selection in 1859 after observations during his voyage on HMS Beagle. His theory explains how species change over time.

The key principles of natural selection are:

• Organisms produce more offspring than can survive (overproduction)
• There is genetic variation within populations due to mutations and sexual reproduction
• Competition exists for limited resources (food, water, shelter, mates)
• Organisms with characteristics best suited to their environment are more likely to survive (survival of the fittest)
• These organisms reproduce and pass advantageous alleles to their offspring
• Over many generations, the advantageous characteristics become more common in the population

Darwin's theory was groundbreaking but faced initial resistance because:

• It contradicted religious beliefs about creation
• There was insufficient evidence at the time
• The mechanism of inheritance (genes) was not yet understood
• It challenged the idea that species were fixed and unchanging

Evidence for evolution

Multiple lines of evidence support the theory of evolution:

Fossil evidence

Fossils provide a record of organisms from the past. The fossil record shows:

• Simple organisms appear in older rocks; more complex organisms in younger rocks
• Organisms have changed over time
• Some species have become extinct
• Transitional fossils show intermediate forms between groups (e.g., Archaeopteryx shows features of both reptiles and birds)

Fossils form when:

• Hard parts of organisms (bones, shells, teeth) are buried in sediment
• Minerals gradually replace the organic material
• The sediment hardens into rock over millions of years

The fossil record is incomplete because:

• Many organisms decomposed before fossilisation
• Soft-bodied organisms rarely form fossils
• Some fossils have been destroyed by geological activity
• Many fossils remain undiscovered

Anatomical evidence

Comparative anatomy reveals similarities between species:

• The pentadactyl limb (five-fingered limb) is found in mammals, reptiles, amphibians and birds with variations for different functions
• These homologous structures suggest common ancestry
• Similar structures adapted for different purposes (e.g., human arm, bat wing, whale flipper) indicate evolution from a common ancestor

Molecular evidence

DNA and protein analysis shows:

• All organisms use the same genetic code (DNA/RNA)
• Species that are closely related have more similar DNA sequences
• Humans and chimpanzees share approximately 98% of their DNA
• This molecular similarity supports the idea of common ancestry

Adaptation to the environment

Organisms are adapted to survive in their specific environments. Adaptations can be:

Structural adaptations — physical features:

• Polar bears have white fur for camouflage in snow
• Cacti have spines instead of leaves to reduce water loss
• Arctic foxes have small ears to reduce heat loss

Behavioural adaptations — actions that help survival:

• Migration in birds to find food and breeding grounds
• Hibernation in mammals to survive cold winters with limited food
• Nocturnal hunting to avoid daytime heat in desert animals

Functional (physiological) adaptations — internal processes:

• Camels can tolerate large changes in body temperature
• Desert plants have deep roots to access water
• Some bacteria produce toxins to kill competitors

Organisms well-adapted to their environment are more likely to survive, reproduce and pass on their genes. Over many generations, natural selection increases the frequency of advantageous adaptations in populations.

Evolution of antibiotic resistance in bacteria

Antibiotic resistance is a contemporary example of evolution by natural selection occurring rapidly:

The process:

1. Bacteria reproduce rapidly (some divide every 20 minutes)
2. Random mutations occur in bacterial DNA
3. Some mutations make bacteria resistant to antibiotics
4. When antibiotics are used, non-resistant bacteria are killed
5. Resistant bacteria survive and reproduce
6. Resistant bacteria pass resistance genes to offspring
7. The population becomes dominated by resistant bacteria

Factors accelerating antibiotic resistance:

• Overuse of antibiotics in medicine and agriculture
• Patients not completing antibiotic courses (allows some bacteria to survive)
• Use of antibiotics for viral infections (where they are ineffective)
• Poor infection control in hospitals
• Horizontal gene transfer between bacteria

Consequences:

• Diseases becoming harder to treat (e.g., MRSA, tuberculosis)
• Longer hospital stays
• Increased healthcare costs
• Higher mortality rates from previously treatable infections

Reducing antibiotic resistance:

• Only prescribe antibiotics when necessary
• Complete the full course of antibiotics
• Develop new antibiotics
• Improve hygiene to prevent infection spread
• Use narrow-spectrum antibiotics where possible
• Reduce antibiotic use in farming

Selective breeding (artificial selection)

Humans have bred plants and animals for thousands of years to produce organisms with desirable characteristics.

The selective breeding process:

1. Identify organisms with desired characteristics
2. Breed these organisms together
3. Select offspring showing the best characteristics
4. Breed these offspring
5. Repeat over many generations

Examples of selective breeding:

• Cattle bred for high milk yield or high meat production
• Dogs bred for specific traits (size, temperament, working ability)
• Crops bred for disease resistance, higher yield, or better taste
• Racehorses bred for speed and stamina

Advantages:

• Produces organisms with predictable characteristics
• Increases food production
• Develops varieties suited to specific environments
• Creates organisms that meet human needs

Disadvantages:

• Reduces genetic variation in populations
• Increased risk of genetic disorders (inbreeding)
• Organisms may be less able to adapt to environmental changes
• Some characteristics may have unforeseen consequences (e.g., breathing problems in flat-faced dog breeds)

Extinction

Extinction occurs when a species completely dies out. Causes include:

Natural causes:

• Climate change making environments unsuitable
• New predators entering the ecosystem
• New diseases
• Competition with other species for resources
• Catastrophic events (volcanic eruptions, asteroid impacts)

Human-caused extinction:

• Habitat destruction (deforestation, urbanisation)
• Hunting and overfishing
• Introduction of invasive species
• Pollution
• Climate change due to greenhouse gas emissions

Examples:

• Dodo — hunted to extinction by humans and introduced species
• Dinosaurs — likely caused by asteroid impact 65 million years ago
• Many species currently endangered due to human activity

Species may avoid extinction through:

• Conservation programmes (captive breeding, habitat protection)
• Legal protection
• Reintroduction to the wild
• Seed banks and gene banks preserving genetic material

Worked examples

Example 1: Natural selection in peppered moths

Question: The peppered moth exists in two forms: light-coloured and dark-coloured. Before the Industrial Revolution, most peppered moths were light-coloured. During the Industrial Revolution, soot darkened tree bark in industrial areas, and the dark-coloured form became more common.

(a) Explain how natural selection led to the increase in dark-coloured moths. [4 marks]

(b) Predict what happened to moth populations after pollution controls reduced soot in the late 20th century. [2 marks]

Mark scheme answers:

(a)

• Initially, most moths were light-coloured, with some dark moths due to mutation [1]
• Light moths were camouflaged on light tree bark; dark moths were easily seen by predators and eaten [1]
• When soot darkened trees, dark moths were now camouflaged and light moths were easily seen [1]
• Dark moths survived, reproduced and passed on alleles for dark colour; over generations, dark moths became more common [1]

(b)

• Light-coloured moths would have an advantage on lighter bark [1]
• The proportion of light-coloured moths would increase in the population [1]

Example 2: Antibiotic resistance

Question: A patient has a bacterial infection. The doctor prescribes a 7-day course of antibiotics but the patient stops taking them after 4 days when symptoms improve.

(a) Explain why stopping the antibiotic course early increases the risk of antibiotic resistance developing. [3 marks]

(b) Describe two other ways to reduce the development of antibiotic resistance. [2 marks]

Mark scheme answers:

(a)

• Not all bacteria will be killed after only 4 days [1]
• Bacteria that survive may carry resistance genes/mutations [1]
• These resistant bacteria reproduce, passing resistance to offspring, creating a resistant population [1]

(b)

• Only prescribe antibiotics for bacterial infections (not viral) [1]
• Reduce use of antibiotics in farming/agriculture [1]
• Improve hygiene/infection control to prevent spread [1]
• Develop new antibiotics [1] [Any 2 points, 1 mark each]

Example 3: Selective breeding

Question: A farmer wants to produce cows that give more milk.

(a) Describe how the farmer could use selective breeding to achieve this. [4 marks]

(b) Suggest one disadvantage of selective breeding in cattle. [1 mark]

Mark scheme answers:

(a)

• Select/identify cows that produce the most milk [1]
• Breed these cows together [1]
• Select offspring that produce the most milk [1]
• Repeat over many generations [1]

(b)

• Reduces genetic variation/diversity in the population [1]
• Increases risk of genetic diseases/disorders [1]
• Makes population less able to adapt to change/disease [1] [Any 1 point]

Common mistakes and how to avoid them

• Confusing adaptation with acclimatisation. Adaptation occurs over many generations through natural selection and involves genetic change. Acclimatisation is a short-term response by an individual organism (e.g., getting a tan). Always link adaptation to inheritance and evolution.

• Saying organisms "need" or "try" to evolve. Evolution has no direction or purpose. Mutations occur randomly, not in response to need. Write that "organisms with advantageous characteristics are more likely to survive," not "organisms adapt to survive."

• Thinking all members of a species are identical. Natural selection requires variation. Always mention that genetic variation exists within populations before explaining how selection acts on this variation.

• Forgetting timescale in evolution. Evolution usually takes many generations (except in rapidly reproducing organisms like bacteria). Always reference "over many generations" when discussing natural selection in multicellular organisms.

• Not explaining the full natural selection process. Include: variation exists, competition occurs, best-adapted survive, reproduce, pass on genes, characteristic becomes more common. Missing steps lose marks.

• Confusing selective breeding with genetic engineering. Selective breeding involves choosing which organisms reproduce together (no direct DNA manipulation). Genetic engineering involves directly modifying DNA. Use the correct term.

Exam technique for "Evolution and Natural Selection"

• Command words matter. "Describe" requires stating what happens; "explain" requires reasons/mechanisms using "because," "therefore," or "this means." For 4-mark questions on natural selection, cover variation → competition → survival → reproduction → inheritance → frequency change.

• Link to the context. Questions often provide scenarios (industrial melanism, antibiotic resistance, breeding programmes). Apply natural selection principles to the specific example rather than writing generic answers.

• Use scientific terminology precisely. Write "alleles" not "genes for characteristics," "advantageous characteristics" not "good features," "reproduce" not "have babies." This demonstrates biological understanding.

• Structure extended answers logically. For evolution questions, follow the chronological process: initial variation → environmental pressure → differential survival → reproduction → inheritance → population change. This ensures you don't miss steps.

Quick revision summary

Evolution is the change in inherited characteristics of populations over time. Natural selection drives evolution: genetic variation exists, organisms compete for resources, those best adapted survive and reproduce, passing advantageous alleles to offspring. Over generations, beneficial characteristics become more common. Evidence includes fossils, anatomical similarities, and DNA comparisons. Antibiotic resistance in bacteria demonstrates rapid evolution. Selective breeding allows humans to develop organisms with desired traits but reduces genetic variation. Extinction occurs when species die out due to environmental change, competition, or human activity.