Natural selection and evolution

What you'll learn

Natural selection and evolution form a cornerstone of CIE IGCSE Biology, explaining how species change over time and adapt to their environments. This topic appears regularly in Paper 2 and Paper 4, particularly in extended response questions worth 5-6 marks. Understanding the mechanisms of natural selection, the evidence for evolution, and how variation leads to adaptation will prepare you for both data interpretation and essay-style questions.

Key terms and definitions

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

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

Variation — differences between individuals of the same species, caused by genetic factors, environmental factors, or both

Adaptation — an inherited characteristic that makes an organism well-suited to survival in its environment

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

Mutation — a random change in the DNA sequence that can introduce new alleles into a population

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

Competition — the interaction between organisms striving for the same limited resources in an environment

Core concepts

The mechanism of natural selection

Charles Darwin developed the theory of evolution by natural selection after observing variation in species during his voyage on HMS Beagle, particularly finches on the Galápagos Islands. Natural selection operates through the following stages:

1. Genetic variation exists within a population due to mutations and sexual reproduction
2. Selection pressure occurs when environmental factors (predation, disease, competition for resources) affect survival
3. Differential survival — individuals with advantageous characteristics are more likely to survive
4. Reproduction — survivors pass on their advantageous alleles to offspring
5. Increased frequency — over many generations, the advantageous alleles become more common in the population

The process requires reproduction to occur. Organisms do not change during their lifetime to become better adapted; rather, the population changes as certain characteristics become more frequent.

Sources of variation

Variation provides the raw material for natural selection. Without differences between individuals, no selection can occur.

Genetic variation arises from:

• Mutations in DNA creating new alleles
• Random fertilisation during sexual reproduction
• Independent assortment of chromosomes during meiosis
• Crossing over between homologous chromosomes

Environmental variation results from:

• Diet and nutrition
• Climate and weather conditions
• Exposure to sunlight
• Physical activity levels

Most characteristics show continuous variation influenced by multiple genes and environmental factors (height, mass, skin colour). Some show discontinuous variation controlled by a single gene with distinct categories (ABO blood groups, ability to roll tongue).

Evidence for evolution

CIE IGCSE Biology expects knowledge of several key evidence types:

Fossil record

• Fossils are preserved remains of organisms from millions of years ago
• The fossil record shows simpler organisms in older rocks and more complex organisms in younger rocks
• Intermediate forms (transitional fossils) show characteristics of two different groups
• Example: Archaeopteryx has features of both reptiles (teeth, bony tail) and birds (feathers, wings)

Comparative anatomy

• Pentadactyl limb — the five-digit limb structure found in mammals, birds, amphibians and reptiles
• Different species have modified this basic structure for different functions (human hand for manipulation, whale flipper for swimming, bat wing for flying)
• This suggests descent from a common ancestor with the basic pentadactyl structure

Molecular biology

• DNA and protein sequences can be compared between species
• More similar sequences indicate more recent common ancestry
• All organisms use the same genetic code, suggesting common origin

Darwin's finches as a case study

The Galápagos finches demonstrate natural selection in action. Darwin observed 13 species of finch on the islands, each with different beak shapes adapted to different food sources:

• Large, strong beaks for cracking large seeds
• Small, pointed beaks for picking small seeds
• Long, thin beaks for probing cacti for insects
• Sharp beaks for gripping and tearing flesh (in the vampire finch)

The mechanism explaining this diversity:

1. Ancestral finches arrived on the islands with variation in beak size and shape
2. Different islands had different available food sources
3. Finches with beaks best suited to the available food survived and reproduced more successfully
4. Over many generations, distinct species evolved adapted to specific niches

Antibiotic resistance in bacteria

Antibiotic resistance demonstrates evolution occurring rapidly and is a common exam example.

How resistance develops:

1. Within a bacterial population, random mutations may produce alleles for antibiotic resistance
2. When antibiotics are used, non-resistant bacteria are killed
3. Resistant bacteria survive and reproduce rapidly (binary fission can occur every 20 minutes)
4. The gene for resistance is passed to offspring
5. The population becomes predominantly resistant

Factors accelerating resistance:

• Overuse of antibiotics for minor infections
• Not completing the full course of antibiotics (leaves resistant survivors)
• Use of antibiotics in farming to promote animal growth
• Horizontal gene transfer between bacteria

Consequences and prevention:

• Infections becoming harder to treat (MRSA in hospitals)
• Need for new antibiotics, but development is slow and expensive
• Use antibiotics only when necessary
• Complete the full prescribed course
• Prevent infection through hygiene and vaccination

Selective breeding versus natural selection

Selective breeding (artificial selection) mirrors natural selection but humans choose which individuals reproduce.

Process of selective breeding:

1. Identify organisms with desirable characteristics
2. Breed them together
3. Select offspring showing the desired characteristics most strongly
4. Repeat over many generations

Examples in plants:

• Disease-resistant wheat varieties
• Large-fruited tomatoes
• Pest-resistant crop varieties
• Flowers with particular colours or scents

Examples in animals:

• Cattle with high milk yield or lean meat
• Dogs bred for specific behaviours or physical features
• Chickens producing more eggs
• Race horses for speed

Disadvantages of selective breeding:

• Reduces genetic diversity in the population
• Increases risk of inherited genetic defects (hip dysplasia in dogs)
• Inbreeding depression reduces overall fitness
• Population more vulnerable to new diseases or environmental changes

Adaptation types

Adaptations can be structural (physical features), behavioural (actions), or physiological (internal processes).

Desert animals (camel):

• Structural: large feet to spread weight on sand, long eyelashes to keep out sand
• Physiological: concentrated urine to conserve water, can tolerate body temperature changes
• Behavioural: nocturnal activity to avoid daytime heat

Arctic animals (polar bear):

• Structural: thick fur for insulation, white colour for camouflage, large paws to spread weight on ice
• Physiological: thick layer of blubber (fat) for insulation and energy storage
• Behavioural: hibernation during harshest winter months

Xerophytes (desert plants):

• Thick waxy cuticle to reduce water loss
• Reduced or absent leaves (CAM plants store water in stems)
• Extensive root systems to access underground water
• Stomata in pits or grooves to reduce transpiration

Worked examples

Example 1: Peppered moths and industrial melanism

Question: The peppered moth exists in two forms: light-coloured and dark-coloured. Before the Industrial Revolution, most moths were light-coloured. After industrialisation, the dark form became more common in polluted areas.

(a) Explain how natural selection led to an increase in dark-coloured moths in polluted areas. [5 marks]

(b) Predict what happened to moth populations after pollution was reduced in the 1960s. [2 marks]

Mark scheme answer:

(a)

• Light and dark moths both existed due to genetic variation / mutation [1]
• Industrial pollution blackened tree bark with soot [1]
• Light moths were more visible to predators (birds) on dark bark / dark moths were camouflaged [1]
• Dark moths had greater survival / more likely to reproduce [1]
• Dark moths passed on the allele for dark colour to offspring / allele frequency increased in population [1]

(b)

• Light-coloured moths became more common again [1]
• Because they were better camouflaged on clean, light-coloured bark / dark moths were more visible to predators [1]

Example 2: Antibiotic prescription data

Question: A doctor prescribed antibiotics to 100 patients with bacterial infections. The antibiotic killed 95% of the bacteria in most patients, but 8 patients showed no improvement.

(a) Suggest why the antibiotic was ineffective in some patients. [2 marks]

(b) Explain why doctors advise patients to complete the full course of antibiotics even if symptoms improve. [3 marks]

Mark scheme answer:

(a)

• The bacteria were resistant to that antibiotic [1]
• Due to a mutation / had the gene for resistance [1]

(b)

• Stopping early may leave some bacteria alive / not all bacteria killed [1]
• The survivors are more likely to be resistant / have some resistance [1]
• These will reproduce and the infection may return, harder to treat [1]

Example 3: Selective breeding in wheat

Question: A farmer wants to produce a new wheat variety that is both disease-resistant and produces high grain yield. Explain how the farmer could achieve this through selective breeding. [4 marks]

Mark scheme answer:

• Identify / select parent plants with disease resistance and high yield [1]
• Cross / breed these plants together [1]
• Select offspring that show both characteristics [1]
• Repeat the process over several / many generations [1]

Common mistakes and how to avoid them

• Mistake: Stating that individual organisms change or adapt during their lifetime to suit the environment (e.g., "the giraffe stretched its neck to reach leaves"). Correction: Individuals are born with characteristics; those with advantageous features survive better and reproduce more. The population changes over generations, not individuals during their lives.

• Mistake: Confusing natural selection with selective breeding. Correction: Natural selection involves environmental pressures determining survival; selective breeding involves humans deliberately choosing which organisms reproduce based on desired characteristics.

• Mistake: Writing that organisms "need" or "want" to develop a characteristic. Correction: Evolution has no direction or purpose. Random mutations provide variation; selection pressures determine which variants survive.

• Mistake: Thinking all mutations lead to evolution. Correction: Most mutations are neutral or harmful. Only mutations that provide a selective advantage in a particular environment increase in frequency through natural selection.

• Mistake: Stating that bacteria "become immune" to antibiotics or that antibiotics "make bacteria resistant". Correction: Resistant bacteria already exist due to random mutation. Antibiotics kill non-resistant bacteria, leaving resistant ones to reproduce.

• Mistake: In extended response questions, describing only one or two stages of natural selection. Correction: Full marks require mentioning variation, selection pressure, differential survival, reproduction, and change in allele frequency across generations.

Exam technique for Natural selection and evolution

• Command word awareness: "Explain" questions (common for this topic) require reasons or mechanisms, not just descriptions. For 5-6 mark questions on natural selection, structure your answer chronologically through the mechanism: variation → selection pressure → survival → reproduction → frequency change.

• Use of examples: Questions may provide unfamiliar contexts (insects developing pesticide resistance, rabbits and myxomatosis). Apply the standard natural selection mechanism to the new context rather than trying to recall specific facts about that example.

• Data interpretation: Paper 4 questions often include graphs showing population changes over time. Read axes carefully and link trends to selection pressures explicitly. Phrases like "correlation does not prove causation" are useful when data is limited.

• Comparative questions: When comparing natural and artificial selection, use a table format to ensure you make matched points. Identify similarities (both involve selection of advantageous characteristics, both change allele frequency) and differences (selection pressure source, timescale, genetic diversity impact).

Quick revision summary

Natural selection drives evolution when organisms with advantageous characteristics survive selection pressures, reproduce, and pass on beneficial alleles. Variation from mutations and sexual reproduction provides raw material for selection. Evidence includes fossils, comparative anatomy (pentadactyl limb), and molecular biology. Antibiotic resistance in bacteria demonstrates rapid evolution. Selective breeding applies similar principles with human choice of characteristics. Adaptations can be structural, behavioural, or physiological. Success in exam questions requires explaining the full mechanism across generations, not changes within individuals.