D4.1 Natural selection

Natural selection explains how heritable variation, selection pressures, differential survival, reproduction and allele-frequency changes drive evolutionary adaptation in populations in evolving populations.

Learning objectives

D4.1.1Natural selection as mechanism• Natural selection is the mechanism driving evolutionary change• It acts on heritable variation and can produce adaptation, speciation, and biodiversity D4.1.2Roles of mutation and sexual reproduction• Mutation creates new alleles, especially when germ-line mutations are inherited• Meiosis and random fertilization create new combinations of existing alleles D4.1.3Overproduction and competition• Overproduction of offspring leads to high mortality in limited environments• Competition for food, space, mates, and other resources promotes selection D4.1.4Abiotic factors as selection pressures• Abiotic factors can act as density-independent selection pressures• Temperature, drought, light, salinity, and pH can favour different variants D4.1.5Differences in adaptation, survival, reproduction• Individuals vary in adaptation, survival, and reproductive success• Fitness means passing alleles to offspring in a particular environment D4.1.6Traits must be heritable• Natural selection causes evolution only if traits are heritable• Acquired characteristics are not inherited through DNA base sequences D4.1.7Sexual selection• Sexual selection favours traits that increase mate choice or mating competition success• Displays, ornaments, and behaviours such as birds-of-paradise plumage can be selected D4.1.8Modelling selection• Selection can be modelled by experimentally controlling selection pressures• Endler's guppy experiments test predation pressure, colour pattern, and mating success D4.1.9(HL)—Gene pool concept• A gene pool contains all genes and alleles in an interbreeding population• Evolution can be measured as changes in allele frequencies over generations D4.1.10(HL)—Allele frequencies• Allele frequency is the proportion of a specific allele in the gene pool• Geographically isolated populations can diverge in allele frequencies D4.1.11(HL)—Changes in allele frequency• Natural selection increases alleles linked to higher survival or reproduction• Neo-Darwinism combines Mendelian genetics, mutation, recombination, and Darwinian selection D4.1.12(HL)—Types of selection• Directional selection favours one extreme phenotype• Stabilizing favours intermediate phenotypes; disruptive favours both extremes D4.1.13(HL)—Hardy-Weinberg equation• Hardy-Weinberg equations calculate allele and genotype frequencies in equilibrium• Use p + q = 1 and p² + 2pq + q² = 1 for two alleles D4.1.14(HL)—Hardy-Weinberg conditions• Equilibrium requires large population, random mating, no selection, mutation, migration, or drift• Deviations from expected frequencies indicate evolutionary forces acting D4.1.15(HL)—Artificial selection• Artificial selection deliberately chooses parents with desired heritable traits• Crop, livestock, and pet breeding show directed change; resistance can arise unintentionally

Natural selection is differential survival and reproduction caused by heritable differences among individuals. Across generations, alleles associated with greater reproductive contribution can become more frequent.

Individuals in a population vary.
Some variation is heritable.
The environment creates selection pressures.
Heritable variants cause differences in survival or mating success.
Individuals leave different numbers of surviving offspring.
Allele frequencies change across generations.

Individuals are selected; populations evolve. An individual does not genetically adapt because it needs to—the frequency of inherited variants changes in descendants.

Mutation location	What can inherit it?	Evolutionary consequence
germ-line cell or its precursor	gametes and potentially offspring	can add a new allele to the population gene pool
somatic cell	descendant cells within that individual	normally ends when the individual dies

Mutation is random with respect to what the organism needs. Selection is non-random in the sense that existing variants leave different numbers of offspring in a particular environment.

A new allele may be harmful, neutral or advantageous depending on phenotype and environment; mutation does not mean automatically beneficial or harmful.

Process	Source of new combination
crossing over	homologous chromosomes exchange DNA between non-sister chromatids
independent assortment	maternal and paternal homologs enter gametes in varied combinations
random fertilization	one of many possible sperm fuses with one of many possible eggs

Sexual reproduction usually reshuffles existing alleles; mutation is the ultimate source of a completely new allele.

Recombination does not guarantee a better offspring. It creates varied genotypes on which environmental selection pressures can act.

potential offspring production exceeds the number the environment can support → food, space, light, territories or mates become limiting → many individuals die or fail to reproduce → variants differ in representation among the offspring that breed

Competition type	Who competes?	Why it matters for selection
intraspecific	members of the same species	they share a niche and compete for very similar resources and mates
interspecific	members of different species	overlapping resource use can also change survival and reproduction

A roughly stable population size does not mean every offspring survives. High offspring production can be balanced by high pre-reproductive mortality.

Abiotic factor	Variant that could be favoured in one context
drought	lower water loss or deeper roots
salinity	ion exclusion or salt tolerance
temperature	enzymes and membranes that function at local extremes
light	leaf form or pigment investment suited to the light regime
pH	proteins and transport systems stable at local pH

A density-independent pressure can affect survival regardless of how crowded the population is, although its total impact may still differ among places and times.

The environment does not cause organisms to develop the needed inherited trait. It changes which already varying individuals contribute most to the next generation.

Individual outcome	Contribution to evolutionary fitness
survives but produces no surviving offspring	little or none
reproduces early and offspring later breed	high contribution
wins mates but offspring have low survival	mating success may not become high lifetime fitness

Fitness is relative to a particular environment. A phenotype advantageous during drought may be neutral or costly when water is abundant.

Better adapted does not mean perfect. Selection compares variants currently present and is constrained by trade-offs, ancestry and chance.

Feature	Acquired change	Heritable variant
origin	arises during one individual’s lifetime through use, injury or environment	encoded in inherited genetic or transmissible biological information
inheritance	usually not copied into gamete DNA	can be passed through gametes
population effect	ends with the individual	can change frequency across generations

Selection can act on a phenotype, but evolutionary change follows only when differences affecting reproductive success are heritable. Ordinary muscle gain or a scar is not inherited as a new DNA sequence.

Parallel giraffe sequences contrast Lamarckian individual neck lengthening with Darwinian inherited variation and differential reproduction across generations.

The population already contained pale and melanic heritable forms.
Soot darkened resting surfaces and reduced pale lichen cover in industrial areas.
Birds more often detected the less-camouflaged form.
Better-camouflaged moths survived and reproduced at a higher rate.
The allele associated with that form increased locally across generations.

When air pollution later declined and lichens returned, the direction of the camouflage advantage could reverse. Fitness belongs to a phenotype–environment relationship, not permanently to one colour.

Pale and melanic peppered moth forms are shown side by side, with pale associated with non-polluted habitats and melanic with industrially polluted habitats.

Route	Selection event	Example trait
mate choice	one sex preferentially mates with particular partners	bright plumage or complex courtship display
mating competition	individuals compete directly or indirectly for access	body size, weapons, territories or dominance behavior

heritable variation in display or competition trait → unequal mating success → unequal offspring contribution → associated alleles increase

Persistent differences in selected traits between sexes can produce sexual dimorphism, such as bright displaying males and more cryptic females.

Effect of an exaggerated ornament	Possible fitness consequence
more visible or energetically costly	lower survival through predation or resource cost
attractive display or honest condition signal	greater chance of mating
species-specific courtship signal	mating isolation from other populations

The trait can spread when its gain in reproductive success exceeds its survival cost. Fitness integrates the whole route to descendants, not survival alone.

Sexual selection names the pressure—competition for mates or mate choice—rather than competition for food or escape from predators.

Design element	Selection experiment question
independent variable	which pressure is deliberately changed?
dependent variable	which phenotype, survival or mating outcome is measured?
controls	which habitat, starting population and observation conditions stay comparable?
replication	are multiple populations exposed to each treatment?
time	are enough generations observed for frequency change?

Random assignment and controlled treatments make a causal claim stronger; repeated measurements reveal whether a pattern persists rather than reflecting one generation of chance.

A simulation or enclosure simplifies nature. Its value depends on whether the manipulated pressure and measured fitness components represent the biological system.

Treatment	Predator pressure	Prediction for male spots
K	none	female choice can favour conspicuous males
R	weak, Rivulus hartii	sexual selection remains influential
C	dangerous, Crenicichla alta	conspicuous males suffer stronger predation

Fine and coarse gravel were crossed with predator treatments. Under predation, spot size matching the substrate can improve camouflage; without strong predation, contrast can improve visibility to females.

Guppies were allowed to breed, predator treatments were imposed, and spot number and size were measured over later generations in replicated ponds.

Evidence	Interpretation
spots increased before strong predators were added	female choice favored conspicuous males
mean spot number fell with C. alta	dangerous predators selected against conspicuous males
weak-predator field transplants became more colorful over 15 generations	reduced predation allowed sexual selection to dominate
spot size tracked gravel under predation	background matching improved camouflage

Natural and sexual selection acted on the same heritable color traits in opposing directions. The observed phenotype reflected the balance of those pressures, not one universal best color.

Guppy treatments K, R and C show spot number rising with no or weak predation and falling under dangerous C. alta predation.

mutation creates alleles + sexual reproduction reshuffles them → offspring vary → limited resources and abiotic or biotic pressures produce unequal survival and mating → heritable variants reach offspring unequally → population trait and allele frequencies change

Claim to test	Evidence needed
adaptation occurred by selection	heritable variation plus unequal reproductive contribution across generations
sexual selection acted	trait differences predict mating success
environmental pressure caused change	controlled manipulation or strong comparative evidence links pressure to fitness
population evolved	heritable trait or allele frequency changed, not merely one individual

Avoid purpose-driven language. Populations do not create useful mutations on demand; selection filters inherited variation already produced by mutation and recombination.

A gene pool is the complete set of genes and their alleles in an interbreeding population. Individuals carry samples of that pool; gametes sample alleles again for the next generation.

Level	What is counted?
diploid individual	two allele copies at one autosomal locus
population	all allele copies carried by all breeding individuals
allele frequency	proportion of all locus copies that are a specified allele

When allele frequencies change from one generation to another, the gene pool is changing—this is population-level evolutionary change even before a new species appears.

For allele A in a diploid population:

$f(A)=\frac{2N_{AA}+N_{Aa}}{2N_{total}}$

Genotype	Individuals	A copies contributed
AA	30	60
Aa	50	50
aa	20	0
total	100	110 of 200 copies

f(A) = 110/200 = 0.55; therefore f(a) = 0.45. Heterozygotes contribute one copy of each allele, not two copies of the dominant allele.

A barrier reduces or stops gene flow between populations.
Mutation, selection and genetic drift act separately in each gene pool.
Different environments can favor different inherited variants.
Allele frequencies diverge across generations.
Accumulated genetic and reproductive differences may eventually contribute to speciation.

Force	Directional expectation
gene flow	tends to make populations more similar
different local selection	can make populations diverge predictably
drift in small populations	can make frequencies diverge by chance
mutation	introduces new alleles independently

Geographic allele-frequency databases can reveal divergence, but the pattern alone does not identify which force caused it; sampling and population definition must also be checked.

mutation supplies new alleles → meiosis and fertilization create genotypes → genotypes influence heritable phenotypes → selection acts through individual survival and reproduction → allele frequencies change in the population gene pool

Darwinian description	Genetic description
individuals vary	individuals carry different alleles and genotypes
some leave more offspring	alleles are transmitted unequally
population becomes adapted	advantageous inherited variants rise in frequency

Selection is not the only force that changes allele frequency. Mutation, migration and genetic drift also alter gene pools, though not necessarily because one phenotype has higher fitness.

Mode	Highest fitness	Distribution after selection
directional	one extreme	mean shifts toward that extreme
stabilizing	intermediate	variance narrows around the existing mean
disruptive	both extremes	intermediates decline; distribution can become bimodal

The graphs describe phenotype distributions, but evolutionary change ultimately requires associated heritable alleles or allele combinations to change frequency.

Before-and-after phenotype distributions show directional selection favoring one extreme, stabilizing selection favoring intermediates and disruptive selection favoring both extremes.

Resistance variation exists before or arises independently of treatment.
The antibiotic kills susceptible bacteria more effectively.
Resistant survivors reproduce and contribute a larger fraction of the next population.
Resistance alleles rise in frequency; plasmid transfer can spread them between cells.

The antibiotic does not teach individual bacteria to become resistant. It changes the survival difference between pre-existing or newly mutated variants.

A mixed bacterial population is exposed to antibiotic; susceptible cells die, resistant survivors reproduce, and resistance can also move by plasmid transfer.

Birth-weight region	Selective disadvantage can arise from
very low	immature organs, low reserves and greater heat loss
intermediate	lowest mortality under the measured historical conditions
very high	delivery complications and associated maternal or fetal risks

When both extremes leave fewer surviving offspring, intermediate phenotypes contribute more. The mean may remain similar while the distribution narrows.

Human birth weight is affected by genes and environment. Historical hospital data show selection on the phenotype, but do not make every difference genetic or define one universal optimum.

environment contains two contrasting opportunities → individuals near either phenotypic extreme exploit one successfully → intermediate phenotype fits neither well → both extremes contribute more offspring → a bimodal distribution can develop

Yearling male plumage	Territorial outcome proposed in the study
dull brown	may be tolerated as non-threatening and gain territory
bright blue	may be avoided as a strong rival and gain territory
intermediate	attacked more often and gains poorer territory

Disruptive selection can increase divergence, but two peaks do not by themselves prove speciation. Reproductive isolation and continued genetic divergence are additional requirements.

Symbol	Meaning for two alleles A and a
p	frequency of A
q	frequency of a
p²	expected frequency of AA
2pq	expected frequency of Aa
q²	expected frequency of aa

$p+q=1$

$p^2+2pq+q^2=1$

p and q are allele frequencies; p², 2pq and q² are genotype frequencies. The equations do not require p to represent the dominant allele.

Convert the recessive phenotype count to a frequency; under complete dominance this is q².
Calculate q = √q².
Calculate p = 1 − q.
Calculate expected genotype frequencies p², 2pq and q².
Check that the three genotype frequencies sum to 1.

This inference is valid only when the population and locus satisfy the Hardy–Weinberg model and the recessive phenotype identifies homozygous recessives accurately.

A Hardy–Weinberg workflow starts from observed q squared, takes a square root for q, calculates p, then predicts p squared, 2pq and q squared and checks their sum.

Quantity	Calculation	Result
q²	observed non-taster frequency	0.35
q	√0.35	0.592
p	1 − 0.592	0.408
p²	0.408²	0.166
2pq	2 × 0.408 × 0.592	0.483
check	0.166 + 0.483 + 0.350	0.999 ≈ 1

In 200 people, expected counts are approximately 33 homozygous dominant, 97 heterozygous and 70 homozygous recessive after multiplying each frequency by 200.

Keep extra digits until the final answer. Early rounding makes the genotype frequencies fail to sum closely to 1.

Condition	Change prevented in the model
very large population	random genetic drift is negligible
random mating	genotype pairing is not biased
no selection	genotypes contribute equally
no mutation	alleles do not convert into new states
no migration	no alleles enter or leave through gene flow

Under these conditions, random mating produces p²:2pq:q² genotype frequencies and allele frequencies remain stable across generations.

The model is an ideal baseline, not a claim that real populations satisfy every condition perfectly.

Estimate allele frequencies from the population sample.
Use p², 2pq and q² to calculate expected genotype frequencies and counts.
Compare expected with observed genotype counts using an appropriate statistical test.
If the departure exceeds sampling expectation, investigate which assumptions are violated.

Possible violation	Additional evidence to seek
selection	survival or reproduction differs by genotype
non-random mating	mate pairs differ from random expectation
migration	immigrants carry different allele frequencies
drift	small population and random change
mutation	documented new allele conversion

A deviation does not by itself prove natural selection. Several forces, population structure, genotyping error or sampling can produce the pattern.

Choose a target phenotype that has heritable variation.
Select parents showing the desired value.
Breed them and measure offspring.
Retain offspring with the strongest desired phenotype.
Repeat across generations so associated alleles become more frequent.

Rapid improvement can reduce genetic diversity and bring linked harmful alleles or correlated traits along with the selected phenotype.

A maize sequence compares a small wild-type cob with progressively larger cobs produced by repeated selection across generations.

Feature	Artificial selection	Natural selection
source of reproductive advantage	human breeder chooses parents	environmental survival and mating pressures
variation required	heritable variation	heritable variation
population result	chosen alleles change frequency	alleles linked to higher fitness change frequency
guaranteed organismal benefit	no; target serves human goal	no universal benefit; adaptation is environment-specific

Antibiotic resistance is usually natural selection caused by a human-created pressure, not artificial selection: humans do not deliberately choose resistant bacteria as breeding parents.

Both processes reveal the same inheritance principle—repeated unequal reproduction changes a population over generations.

count allele copies in the gene pool → establish p and q → use Hardy–Weinberg as a no-change expectation → compare observed genotype or allele frequencies across generations → use phenotype, fitness and population evidence to identify selection or another evolutionary force

Pattern or action	First model to examine
mean shifts toward one extreme	directional selection
variance narrows around intermediate values	stabilizing selection
both extremes rise and intermediates fall	disruptive selection
observed genotypes differ from p²:2pq:q²	one or more equilibrium assumptions may be violated
humans repeatedly choose parents	artificial selection

Keep description, calculation and cause separate. A changed distribution describes an outcome; allele counts quantify evolution; causal claims require evidence about inheritance, survival, mating, migration, drift or deliberate breeding.

8 marks

Explain how evolution may happen in response to environmental change with evidence from examples.

8 marks

Explain how sexual reproduction can eventually lead to evolution in offspring.

4 marks

Outline how overpopulation of a species in a given environment may lead to evolution.

2 marks

Explain how natural selection is influenced by changes in the environment.

1 mark

In a natural population, what is a feature of individuals that are better adapted?

1 mark

What is required for natural selection to occur?

I. Acquired characteristics
II. Advantageous characteristics
III. Genetic variation

6 marks

Explain what is meant by exaggerated traits and how they may develop in males of a species.

1 mark

John Endler experimented on populations of guppies (Poecilia reticulata) with different colouration. A male guppy fish is shown with large spots, which makes the fish more attractive to females, but more visible to predators.

The table shows the male colouration of guppy fish and number of predators in three different ponds.

	Predator X	Predator Y	Male guppy colouration
Pond 1	12	0	Large colourful spots
Pond 2	15	0	Medium colourful spots
Pond 3	5	17	None/very small drab spots

What can be concluded from the data?

3 marks

Discuss how the isolation of populations due to the fragmentation of forests could lead to changes in gene pools.

1 mark

Define allele frequency.

4 marks

If a wild population of cats contained both curled ears and normal ears, explain how the proportion of these two phenotypes could change in the population.

2 marks

Phenotypic variation allows natural selection within populations. Compare and contrast directional and disruptive selection.

3 marks

Discuss the use of the Hardy-Weinberg equation in population genetics studies.

2 marks

State two assumptions made when using the Hardy-Weinberg equation.
1.

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—D4.2 Stability and changeRead

D4.1 Natural selection

Learning objectives

Natural Selection Changes Populations Through Unequal Reproductive Success

Mutation Creates New Alleles; Germ-Line Location Makes Them Heritable

Sexual Reproduction Rearranges Existing Alleles into New Genotypes

Overproduction Makes Reproductive Success Unequal When Resources Are Limited

Abiotic Conditions Select Existing Variants Without Regard to Population Density

Fitness Means Alleles Reaching Future Generations, Not Strength or Longevity Alone

Only Heritable Differences Can Accumulate Across Generations

Peppered-Moth Frequencies Changed When Camouflage Changed Survival

Sexual Selection Rewards Traits That Increase Access to Mates

A Mating Advantage Can Persist Despite a Survival Cost

A Selection Experiment Must Manipulate Pressure and Measure Reproductive Consequences

Endler Separated Predation Strength from Gravel Background

Endler’s Guppies Reveal a Trade-off Between Predation and Mate Choice

Natural Selection Connects Variation, Environment and Reproduction Across Generations

A Gene Pool Contains Every Allele Available to an Interbreeding Population

Allele Frequency Counts Allele Copies, Not Individuals

Isolation Lets Population Gene Pools Diverge

Neo-Darwinism Connects Darwin’s Selection to Mendelian Alleles

Three Selection Modes Differ in Which Phenotypes Have Highest Fitness

Antibiotics Cause Directional Selection of Resistant Bacteria

Stabilizing Selection Removes Both Extremes Around an Intermediate Optimum

Disruptive Selection Favors Both Extremes over Intermediates

Hardy–Weinberg Separates Allele Frequencies from Genotype Frequencies

A Recessive Phenotype Reveals q², Not q

Worked Hardy–Weinberg Example: 35% Recessive Phenotype

Hardy–Weinberg Equilibrium Requires Forces of Change to Be Absent

Departure from Hardy–Weinberg Identifies a Question, Not a Single Cause

Artificial Selection Repeats Human Choice of Heritable Parents

Artificial and Natural Selection Differ in Selector, Not in Inheritance

Population Genetics Measures How Selection and Other Forces Change Gene Pools

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