Understand Natural Selection - IB Biology

SECTION 1

Historical Context & Motivation

For most of human history, people assumed that species were fixed and unchanging. Naturalists catalogued the diversity of life but had no mechanism to explain how that diversity arose. The idea that populations could change over generations was radical — it required both careful observation and a theoretical framework that could explain adaptation, the process by which organisms become better suited to their environments over time.

1798 • Malthus on Population

Thomas Malthus published An Essay on the Principle of Population, arguing that populations grow faster than their food supply. This idea of a 'struggle for existence' later inspired Darwin's thinking.

1809 • Lamarck's Inheritance of Acquired Traits

Jean-Baptiste Lamarck proposed that organisms could pass on traits they developed during their lifetime. Although incorrect, his work was one of the first formal theories of biological change over time.

1859 • Darwin Publishes On the Origin of Species

Charles Darwin presented natural selection as the mechanism for evolution, drawing on decades of observation during the HMS Beagle voyage and pigeon-breeding experiments.

1858 • Wallace's Independent Discovery

Alfred Russel Wallace independently conceived the theory of natural selection while studying wildlife in Southeast Asia. His letter to Darwin prompted the joint presentation of their ideas to the Linnean Society.

1930s–1940s • The Modern Synthesis

Scientists combined Darwinian natural selection with Mendelian genetics, creating a unified framework known as the Modern Synthesis. This showed that mutations provide the raw material on which natural selection acts.

The central question that Darwin and Wallace answered was deceptively simple: if organisms produce more offspring than can survive, and those offspring vary, then which individuals survive and reproduce? Their answer — that individuals with traits better suited to their environment are more likely to pass those traits to the next generation — remains one of the most powerful ideas in all of science.

SECTION 2

Core Principles of Natural Selection

Natural selection is not a random process. It requires specific conditions in a population. When all four conditions below are met simultaneously, evolution by natural selection will occur. These conditions are sometimes remembered by the acronym VISA — Variation, Inheritance, Selection pressure, and Adaptation (differential reproduction).

Variation

Individuals in a population differ in their traits — such as beak size, fur colour, or enzyme efficiency. This phenotypic variation is essential because natural selection can only act when there are differences to select among.

Inheritance

At least some of the variation must be heritable — that is, passed from parents to offspring through genes. Traits that are purely environmental (like a scar) cannot be selected for genetically.

Overproduction & Competition

Populations tend to produce more offspring than the environment can support. This leads to a struggle for existence in which not all individuals survive to reproduce.

Differential Survival & Reproduction

Individuals whose traits give them an advantage in a particular environment are more likely to survive and reproduce. This is often called fitness — the relative reproductive success of a genotype.

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KEY TAKEAWAY

Think of natural selection like a filter at a concert entrance. Everyone in the crowd (population) is a bit different — different heights, different ticket types. The door (environment) only lets certain people through. Those who fit the criteria get inside (survive and reproduce), and the next 'generation' of concertgoers looks more like those who passed through. The door doesn't create the differences — it simply sorts them.

SECTION 3

Visualising Natural Selection

The diagram below shows how natural selection shifts the distribution of a trait across generations. Imagine a population of beetles that vary in colour from light green to dark green. In an environment with dark foliage, darker beetles are harder for predators to spot, giving them a selective advantage. Over several generations, the average colour in the population shifts toward darker green.

In Generation 1, beetle colour is centred around a medium shade. After five generations of predation favouring darker beetles, the mean shifts to the right — this is directional selection. Notice how the curve's shape narrows slightly, reflecting reduced variation as selection removes lighter phenotypes.

This type of shift, where one extreme of a trait distribution is favoured, is called directional selection. It is just one of three modes — the others being stabilising selection (favouring the average) and disruptive selection (favouring both extremes). All three modes begin with the same four conditions: variation, heritability, overproduction, and differential reproduction.

SECTION 4

How Natural Selection Works at the Genetic Level

While Darwin described natural selection in terms of observable traits, the mechanism operates on allele frequencies — the relative proportions of different versions of a gene within a population. When one allele confers an advantage, its frequency increases over generations because its carriers leave more offspring.

ALLELE FREQUENCY

p + q = 1

For a gene with two alleles: p = frequency of the dominant allele, q = frequency of the recessive allele. If natural selection favours carriers of allele A, then p increases and q decreases over generations.

HARDY-WEINBERG GENOTYPE FREQUENCIES

p² + 2pq + q² = 1

This equation predicts genotype frequencies in a population where no evolution is occurring. If observed genotype frequencies deviate from Hardy-Weinberg predictions, one or more evolutionary forces — including natural selection — may be at work.

The Hardy-Weinberg principle serves as a null model — a baseline for comparison. It assumes no mutation, no migration, random mating, no genetic drift (infinite population size), and no natural selection. When real populations violate these assumptions, allele frequencies change, and that change is evolution. Natural selection is the only one of these forces that consistently produces adaptation.

SELECTION COEFFICIENT

w = 1 − s

w = fitness of a genotype (relative to the fittest genotype, which has w = 1). s = selection coefficient (the reduction in fitness). For example, if a genotype has s = 0.1, its carriers produce 10% fewer surviving offspring than the fittest genotype.

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IB Exam Tip

The IB Biology syllabus expects you to understand that natural selection changes allele frequencies in a population. You do not need to memorise complex Hardy-Weinberg calculations, but you should be able to explain why allele frequencies shift when selection is acting.

SECTION 5

Three Modes of Natural Selection

Natural selection can act on a trait distribution in three distinct ways, depending on which phenotypes are favoured. Understanding these modes helps explain why populations sometimes become more uniform, sometimes shift in one direction, and sometimes split into two distinct forms.

The dashed violet curves show the original trait distribution. Solid curves show the result after selection: stabilising narrows the curve, directional shifts it, and disruptive splits it into two peaks.

Mode of Selection	Phenotype(s) Favoured	Effect on Variation	Example
Stabilising	Intermediate / average	Decreases variation	Human birth weight
Directional	One extreme	Shifts mean; may reduce variation	Antibiotic-resistant bacteria
Disruptive	Both extremes	Increases variation; may lead to speciation	Beak size in seedcracker finches

Summary of the three modes of natural selection

SECTION 6

Worked Example: Peppered Moths

One of the most famous examples of natural selection in action is the case of the peppered moth (Biston betularia) during the Industrial Revolution in England. Let's walk through the logic step by step, connecting observations to the four conditions of natural selection.

Industrial Melanism in Peppered Moths

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Step 1 — Identify the Variation

Before industrialisation, the peppered moth population contained two forms: a light-coloured morph (peppered/speckled) and a rare dark-coloured (melanic) morph. This variation in wing colour existed because of different alleles at a single gene locus. The dark morph is determined by a dominant allele.

Variation present: light vs. dark wing colour, determined by heritable alleles.
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Step 2 — Identify the Selection Pressure

During the Industrial Revolution, soot from factories darkened tree bark. Light-coloured moths, which had been well-camouflaged against pale lichen-covered bark, became highly visible to bird predators. Dark moths, previously conspicuous, were now camouflaged against the darkened trees.

Selection pressure: predation by birds, which hunt by sight.
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Step 3 — Determine Differential Survival

In polluted areas, dark moths had higher survival rates because they were harder for birds to spot on dark bark. They survived longer, mated more often, and produced more offspring than the light morph. This is differential reproduction — the dark morph had higher fitness in the polluted environment.

Dark morph has higher fitness (w ≈ 1); light morph has reduced fitness (w < 1) in polluted areas.
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Step 4 — Predict the Change in Allele Frequency

Because the dark allele is dominant and its carriers reproduced more, the frequency of the dark allele (let's call it p) increased in polluted areas. By the mid-20th century, over 95% of peppered moths in industrial regions were dark. This represents a dramatic shift in allele frequency — evolution by natural selection.

p (dark allele) increased from rare (~0.01) to dominant (~0.90+) in industrial areas over approximately 50 generations.
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Step 5 — Evaluate the Reversal

After the Clean Air Acts of the 1950s reduced pollution, tree bark lightened again. The selection pressure reversed: light moths regained their camouflage advantage, and the frequency of the light morph increased. This confirmed that the change was driven by natural selection, not random chance, because the direction of change tracked the environment.

The reversal demonstrates that natural selection is environment-dependent and not directionally 'progressive'.

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KEY TAKEAWAY

The peppered moth example shows all four conditions of natural selection in action. It also demonstrates a crucial point: natural selection does not have a goal. The 'best' phenotype depends entirely on the current environment. Change the environment, and the direction of selection changes too.

SECTION 7

Evidence For and Limitations of Natural Selection

Natural selection is supported by an overwhelming body of evidence from multiple independent fields. However, it is important to understand that natural selection is not the only mechanism of evolution — other forces such as genetic drift, gene flow, and mutation also change allele frequencies in populations.

Line of Evidence	What It Shows	Example
Fossil record	Transitional forms show gradual change in traits over geological time	Whale ancestors with vestigial hind limbs
Comparative anatomy	Homologous structures suggest common ancestry; analogous structures show convergent evolution	Pentadactyl limb in mammals, birds, reptiles
Molecular biology	DNA sequence similarities correlate with evolutionary relatedness	Humans and chimpanzees share ~98.7% of DNA
Direct observation	Selection can be observed in real time in species with short generation times	Antibiotic resistance in bacteria; beak size changes in Darwin's finches
Biogeography	Distribution of species matches predictions of descent with modification	Unique marsupials in Australia; island endemism

Multiple independent lines of evidence support evolution by natural selection

Limitations & Common Misconceptions

✓ Natural selection ≠ all of evolution. Genetic drift can change allele frequencies by chance, especially in small populations. Gene flow introduces alleles from other populations. Mutation creates new alleles.
✓ Individuals do not evolve. A single organism cannot change its genotype during its lifetime. Natural selection acts on individuals, but evolution occurs in populations over generations.
✓ Natural selection does not produce 'perfect' organisms. It can only select among existing variation. Trade-offs, constraints, and changing environments mean that no organism is perfectly adapted.
✓ Survival is not enough. What matters is reproductive success. An organism that survives to 100 but never reproduces has a fitness of zero.

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PERSPECTIVE

Natural selection is the only evolutionary mechanism that produces adaptation — the match between an organism and its environment. Genetic drift, mutation, and gene flow can change allele frequencies, but they do not consistently 'improve' the fit between organisms and their environments the way natural selection does.

SECTION 8

Connection to Advanced Evolutionary Theory

Natural selection is the cornerstone of evolutionary biology, but modern evolutionary theory has expanded well beyond Darwin's original framework. Understanding how natural selection connects to concepts like speciation, sexual selection, and coevolution provides a richer picture of how life evolves.

Concept	How It Relates to Natural Selection	IB Syllabus Link
Sexual selection	A form of natural selection where traits are favoured because they increase mating success, even if they reduce survival (e.g., peacock tails)	D4.1 — Natural selection
Speciation	When populations become reproductively isolated and natural selection (or drift) causes them to diverge enough that they can no longer interbreed	D4.1 — Continuity and change
Coevolution	Two species exert reciprocal selection pressures on each other, driving evolutionary change in both (e.g., predator-prey arms races, pollinator-flower mutualisms)	D4.1 — Interactions
Artificial selection	Humans act as the selective agent, choosing which organisms breed. Darwin used this as evidence that selection can cause dramatic change	D4.1 — Evidence for evolution
Kin selection & altruism	Natural selection can favour behaviours that reduce an individual's fitness if they help closely related individuals reproduce (Hamilton's rule)	HL extension

How natural selection connects to broader evolutionary concepts

As you continue in IB Biology, you will encounter these ideas in greater depth. The key insight to carry forward is that natural selection is not just a historical idea — it is an active, ongoing process that shapes every population on Earth right now. From the evolution of drug-resistant viruses to the adaptation of crops to climate change, understanding natural selection gives you the tools to analyse some of the most pressing biological questions of our time.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A farmer notices that some of his chickens have thicker feathers than others. He moves his flock to a much colder climate. After several generations, most of the flock has thick feathers. Identify each of the four conditions of natural selection in this scenario.

Show answer

The four conditions are satisfied as follows. Variation: chickens differ in feather thickness. Inheritance: feather thickness is at least partially genetic and can be passed to offspring. Overproduction and competition: more chickens are born than can survive in the cold environment. Differential survival and reproduction: chickens with thicker feathers are better insulated, survive longer, and produce more offspring. Over generations, alleles for thick feathers increase in frequency.

Question 2: BASIC CALCULATION

In a population of 500 wildflowers, 320 are red (dominant phenotype, genotypes RR or Rr) and 180 are white (recessive phenotype, genotype rr). Assuming Hardy-Weinberg equilibrium, calculate the frequency of the recessive allele (q).

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The frequency of the rr genotype is q² = 180 ÷ 500 = 0.36. Taking the square root, q = √0.36 = 0.6. Therefore, the frequency of the recessive allele (r) is 0.6 or 60%. The frequency of the dominant allele is p = 1 − 0.6 = 0.4. Note that even though the dominant phenotype (red) is more common, the recessive allele is actually more frequent in this population. This is a common counterintuitive result and highlights why allele frequencies, not phenotype ratios, are the proper measure of genetic composition.

Question 3: INTERMEDIATE

A population of lizards lives on an island where the rocks are dark. Most lizards are dark-coloured and well-camouflaged. A volcanic eruption covers the island in light-coloured ash. Predict what type of natural selection will now act on the lizard population, and describe how the distribution of body colour will change over the next 20 generations. Justify your reasoning.

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This scenario describes directional selection favouring lighter body colour. Before the eruption, dark lizards had high fitness because their camouflage reduced predation on dark rocks. After the eruption, the substrate changed to light-coloured ash, so lighter lizards are now better camouflaged. Predators will disproportionately eat the dark lizards, reducing the frequency of alleles for dark colouration. Over 20 generations, the mean body colour will shift from dark to light, and the frequency of 'light colour' alleles will increase. The speed of this shift depends on the strength of predation pressure, the degree of heritability of colour, and how much genetic variation for lighter colour already exists in the population.

Question 4: APPLIED

Antibiotics are becoming less effective against many bacterial infections. Using your knowledge of natural selection, explain why doctors recommend that patients always finish their entire course of antibiotics, even if they feel better after a few days. What happens at the population level if the course is not completed?

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When a patient takes antibiotics, the drug kills the most susceptible bacteria first. If the patient stops taking the medication early, some bacteria with partial resistance — those that can survive low drug concentrations — may still be alive. These survivors reproduce, and because resistance is genetically determined, their offspring inherit resistance alleles. This is directional selection: the antibiotic acts as a selection pressure that eliminates susceptible genotypes and favours resistant ones. Completing the full course ensures the drug concentration stays high enough to kill even partially resistant bacteria, minimising the chance that resistant alleles increase in the bacterial population. Incomplete courses effectively 'train' the bacterial population to resist the drug, contributing to the global crisis of antibiotic resistance.

Question 5: CRITICAL THINKING

A student claims: 'Cheetahs evolved to run fast because they needed to catch prey.' Evaluate this statement using your understanding of natural selection. Identify the misconceptions it contains, and rewrite the statement so that it accurately describes how natural selection operates.

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The student's statement contains two major misconceptions. First, it implies purpose or intent in evolution — as if cheetahs 'decided' to evolve speed. Natural selection has no foresight or goal; it is a consequence of differential reproduction, not a deliberate process. Second, it suggests that need drives genetic change. In reality, variation must already exist before selection can act on it. A corrected statement would be: 'Among ancestral cheetah populations, individuals that happened to be faster (due to heritable genetic variation) were more successful at catching prey. These faster individuals survived longer and produced more offspring, passing on alleles associated with speed. Over many generations, the average running speed of the cheetah population increased through directional selection.' This version emphasises that variation comes first, selection acts on existing differences, and the outcome is a shift in population-level allele frequencies — not a change within any single individual.

SUMMARY

Lesson Summary

Summary

Natural selection is the process by which individuals with heritable traits better suited to their environment leave more offspring, causing allele frequencies to change over generations. It requires four conditions: variation in traits, heritability of those traits, overproduction of offspring leading to competition, and differential survival and reproduction. Darwin and Wallace independently proposed this mechanism in the mid-19th century, and it was later unified with Mendelian genetics in the Modern Synthesis.

Natural selection can operate in three modes: stabilising selection favours the average phenotype and reduces variation, directional selection shifts the population mean toward one extreme, and disruptive selection favours both extremes and may lead to speciation. Evidence from the fossil record, comparative anatomy, molecular biology, direct observation, and biogeography all support evolution by natural selection. While natural selection is the only mechanism that produces adaptation, it works alongside genetic drift, gene flow, and mutation to drive the full complexity of evolutionary change.