IB Biology · Theme D Continuity and change · Ecosystems
D4.1 Natural selection
Populations vary, overproduce, and compete; the better adapted survive and reproduce, so their alleles spread. Mutation supplies new alleles, sexual reproduction new combinations, and only heritable traits can be selected. At HL, evolution is a change in allele frequencies; Hardy–Weinberg shows when nothing changes.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
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Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D4.1.1 Natural selection: the mechanism, and the paradigm shift it caused
Individuals vary in heritable traits; more offspring are produced than can survive.
Those better adapted to the selection pressures reproduce more, so their alleles spread.
It has run continuously for billions of years and produced Earth's biodiversity.
Darwin's mechanism replaced Lamarckism, the idea that acquired traits are inherited: a paradigm shift.
In Darwin's time it was already accepted that species evolved; what was missing was a convincing mechanism.
Students often think natural selection finished in the past. In fact it acts in every population today.
Students often think Darwin first proposed that species change. In fact the shift was his mechanism, not the idea of evolution.
D4.1.2 Where the variation comes from
Mutation changes DNA base sequence and is the only source of new alleles.
It occurs at random, not in response to need.
Sexual reproduction gives new combinations of existing alleles through meiosis and random fertilisation.
Crossing over, random orientation of bivalents and random gamete fusion do the shuffling.
Students often think organisms mutate to meet a need. In fact mutations are random; the environment selects among them.
Students often think sexual reproduction creates new alleles. In fact it only recombines the alleles already present.
D4.1.3 Too many offspring, too few resources
Populations overproduce offspring: more are born than the environment can support.
Carrying capacity is the maximum population a habitat can sustain, set by limiting resources.
Limiting resources include food, water, light, space, nesting sites and shelter.
The resulting competition means only some survive, so adaptations matter.
Students often think overproduction is irrelevant to selection. In fact without competition, differences in adaptation would barely affect survival.
Students often think only food limits a population. In fact water, space, light, breeding sites and mates can all limit it.
D4.1.4 Abiotic factors select too
An abiotic factor is non-living: temperature, water, light, salinity, soil pH.
Extreme temperature kills some individuals and not others, so it is a selection pressure.
Such factors are usually density-independent: they kill a similar proportion of crowded and sparse populations.
Students often think only predators and disease select. In fact a frost or drought selects for tolerant individuals just as surely.
Students often think density-independent factors kill a fixed number. In fact they kill a similar proportion whatever the density.
D4.1.5 Adaptation, fitness and competition within a species
An adaptation is a heritable trait that improves survival and reproduction.
Fitness is a genotype's survival value and reproductive potential: surviving offspring left, relative to others.
Intraspecific competition is between members of one species for the same resources and mates.
Individuals differ in adaptation, so some out-compete others; this is the basis of selection.
Students often think fitness means strength or size. In fact it is measured in surviving offspring.
Students often think selection is a contest between species. In fact it is mainly between individuals of the same species.
D4.1.6 Only heritable traits evolve
A heritable trait is encoded in the base sequence of genes and passes through gametes.
An acquired characteristic, such as trained muscle or a learned skill, is not in the genes.
So acquired characteristics are not inherited, and selection cannot change their frequency.
Students often think a tan or trained muscles pass to offspring. In fact nothing changes in the gametes' DNA.
Students often count learned behaviour as heritable. In fact it is acquired during life and not in the genes.
D4.1.7 Sexual selection: winning a mate
Sexual selection is a pressure arising from success in attracting or competing for mates.
Physical or behavioural traits can signal overall fitness, so they raise mating success.
Male birds of paradise evolved elaborate plumage and display because females choose them.
The alleles spread even though the plumage is costly and conspicuous.
Students often think selection can only favour survival traits. In fact fitness includes reproduction; a costly display can spread.
Students often call female choice artificial selection. In fact no human is involved; it is a natural selection pressure.
D4.1.8 Endler's guppies: selection pressures under experimental control
John Endler varied predators and gravel size for Trinidad guppies in ponds and stream transplants.
With the pike cichlid Crenicichla present, males evolved fewer, smaller, duller spots.
Without predators, female choice produced more, larger, brighter spots within a few generations.
Predation and sexual selection pulled the same trait in opposite directions.
Students often think a new mutation must appear once selection starts. In fact selection acted on variation already present.
Students often think every pressure favours the same male. In fact predators and females favoured opposite types; the population balanced.
D4.1.9 The gene pool HL
A gene pool is all the genes and their alleles present in a population.
Natural selection, mutation, migration and genetic drift change its allele frequencies over generations.
Students often think a gene pool is one individual's genes or the whole species' DNA. In fact it belongs to a population.
D4.1.10 Isolated populations drift apart in allele frequency HL
2028 guide: scope reduced — Specific allele frequencies of the example not required. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
Allele frequency is the fraction of all copies of a gene that are one allele.
Each diploid individual holds two copies, so allele frequency is not the fraction of carriers.
Geographically isolated populations have separate gene pools, so selection, drift and founder effects diverge them.
Databases show ABO allele frequencies differ markedly between human populations.
Students often equate allele frequency with the proportion of carriers. In fact heterozygotes contribute only one copy of two.
Students often think different frequencies mean different species. In fact one species can hold populations with different frequencies.
D4.1.11 Neo-Darwinism: selection as changing allele frequencies HL
Neo-Darwinism is Darwin's natural selection integrated with genetics.
Evolution is a change in allele frequencies in the gene pool.
Mutation supplies alleles; selection on heritable traits makes fitter alleles more frequent.
A harmful recessive allele disappears slowly, because heterozygotes hide it from selection.
Students often think a harmful allele is removed in a generation or two. In fact carriers shield it, and it fades ever more slowly as it becomes rare.
Students often think neo-Darwinism replaced a wrong Darwin. In fact it added genetics to a theory that stood.
D4.1.12 Directional, disruptive and stabilising selection HL
Directional favours one extreme, so the mean shifts, as with antibiotic resistance.
Disruptive favours both extremes over the middle, so variation grows or the population splits.
Stabilising favours the middle, so the mean stays but variation shrinks.
All three types change allele frequencies, so all three are evolution.
Students often think stabilising selection is not evolution. In fact alleles for extreme phenotypes fall in frequency.
Students often think disruptive means a disrupted environment. In fact it means both extremes are favoured over intermediates.
D4.1.13 Using the Hardy–Weinberg equation HL
With allele frequencies p and q, p + q = 1.
In equilibrium, genotype frequencies are p² + 2pq + q² = 1.
Usually q² is known from the recessive phenotype; take its square root, then p = 1 − q.
The dominant phenotype is p² + 2pq, since heterozygotes show it too.
Students often set q equal to the recessive phenotype frequency. In fact that frequency is q²; take the square root.
Students often write the heterozygote as pq. In fact it is 2pq, because it forms in two ways.
D4.1.14 When Hardy–Weinberg holds, and what a misfit means HL
Genetic equilibrium: allele and genotype frequencies stay constant across generations.
Conditions: large population, random mating, no mutation, no migration, no selection.
Genotypes that do not fit the equation show some condition is broken.
The broken condition could be non-random mating or migration, not necessarily selection.
Students often think a dominant allele must spread because it is dominant. In fact dominance affects phenotype, not frequency.
Students often think a misfit proves selection. In fact it shows only that some condition is not met.
D4.1.15 Artificial selection HL
Artificial selection is humans choosing individuals with desirable traits to breed from.
It is used on crop plants and domesticated animals; chosen alleles rise in frequency.
Breeders select from existing variation; the alleles arose by mutation.
Antibiotic resistance is natural selection: nobody chose the resistant bacteria to breed.
Students often call antibiotic resistance artificial selection because humans caused it. In fact the antibiotic acted as a natural selection pressure.
Students often think breeders create new alleles. In fact they only change the frequency of alleles already there.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 The replacement of Lamarckism by Darwin's theory of natural selection is described as a paradigm shift. What does the term 'paradigm shift' mean?
Answer and reasoning
A fundamental change in the framework of ideas within which scientists explain their observations — A paradigm is the accepted framework of ideas within which observations are interpreted. Darwin's mechanism replaced Lamarck's inheritance of acquired characteristics as the framework for explaining evolutionary change, so the change is a paradigm shift.
The first proposal by any scientist that species change over time rather than being fixed — A student who thinks Darwin discovered evolution itself picks this. In Darwin's time it was already widely understood that species evolved; what was unclear was the mechanism, and it was the mechanism that shifted.
The acceptance that characteristics acquired during life are passed on to offspring — A student who believes acquired characteristics are inherited assumes that this is the accepted modern explanation and so names it as the new framework. It is in fact Lamarckism, the framework that was replaced; acquired characteristics are not encoded in genes and so are not heritable.
A discovery that proves a scientific theory correct beyond any further doubt — A student who thinks science proves theories once and for all picks this. Scientific theories are not proved beyond doubt; Darwin's theory was later extended by genetics into neo-Darwinism, showing the framework can still develop.
2 Which statement correctly describes the roles of mutation and sexual reproduction in generating the variation on which natural selection acts?
Answer and reasoning
Sexual reproduction produces new alleles by combining together the DNA of two different parents — A student who thinks mixing parental DNA makes new alleles picks this. Every allele in an offspring came from a parent; sexual reproduction creates new combinations, not new alleles.
Mutation produces precisely the particular alleles that the environment requires for survival — A student who thinks mutations are directed by need picks this. Mutations are random changes in base sequence and occur whether or not they are useful; selection acts afterwards.
Mutation produces new alleles, and sexual reproduction produces new combinations of alleles — Mutation is the only source of new alleles. Meiosis and random fertilization then shuffle the alleles that already exist into new combinations in the offspring, so both processes contribute variation but in different ways.
A trait can spread only once a new mutation for it arises, as populations lack variation — A student who underestimates how much heritable variation an ordinary population already contains picks this. Mutation has been generating alleles continuously, so populations already carry the variation on which selection acts.
3 An unusually cold winter kills 70% of a population of lizards. The proportion killed is similar in crowded and in sparse populations. Which statement is correct?
Answer and reasoning
Low temperature is a density-independent abiotic selection pressure: lizards better able to tolerate cold are likelier to survive and breed — Extreme temperature is an abiotic factor whose effect does not depend on population density. Provided cold tolerance is partly heritable, the survivors are not a random sample, so allele frequencies in the population change.
Temperature cannot be a selection pressure, because the deaths were not the result of competition between the lizards — A student who equates selection with contests between organisms picks this. Any factor, biotic or abiotic, that affects survival according to heritable traits is a selection pressure.
The cold acted as a density-dependent factor, because it killed such a large number of lizards in the population — A student who reads 'density-dependent' as 'kills a lot' picks this. Density dependence is about whether the proportion affected changes with crowding; here it did not, so the factor is density-independent.
The surviving lizards became more cold-tolerant during the winter and will pass this tolerance on to their offspring — A student who thinks individuals adapt and pass the change on picks this. Tolerance acquired during life is not encoded in the base sequence of genes; the survivors were those that already carried alleles for greater cold tolerance.
4 Which statement describes the basis of natural selection due to intraspecific competition?
Answer and reasoning
Different species compete with one another, and the species that has the best adaptations survives — A student who treats the species as the unit of competition picks this. Natural selection arises from differences between individuals within a species, which use exactly the same resources.
Individuals improve their adaptations during life, and the improved ones out-compete the rest — A student who thinks of adaptation as something an individual does picks this. Adaptations are heritable traits individuals are born with; improvements acquired during life are not inherited.
Individuals of one species differ in adaptation, so some survive and reproduce more than others — Members of a species compete for the same resources and mates. Because they differ in heritable adaptations, they differ in survival and reproduction, and this difference is what natural selection acts on.
The strongest individuals win fights for resources, and strength is what biologists mean by fitness — A student who uses the everyday meaning of fitness picks this. Fitness is the survival value and reproductive potential of a genotype; strength contributes only where it increases surviving offspring.
5 Male birds of paradise have elaborate, brightly coloured plumage and perform complex courtship displays, while females are dull. The plumage is costly to grow and makes males easier for predators to see. Which explains how this plumage evolved?
Answer and reasoning
Females mate preferentially with males whose plumage signals overall fitness, so alleles for elaborate plumage rise in frequency — This is sexual selection. Differences in physical and behavioural traits that act as signs of overall fitness affect success in attracting a mate, so males with the most elaborate plumage leave more offspring despite the survival cost.
The plumage cannot have evolved by selection, because a trait that lowers survival is selected against rather than favoured — A student who thinks selection is only about survival picks this. Fitness includes reproduction; if the gain in mating success outweighs the survival cost, the alleles for the plumage spread.
The plumage evolved by artificial selection, because each female deliberately chooses which male's traits are passed on — A student who equates 'choice' with artificial selection picks this. Artificial selection is deliberate choice of traits by humans; mate choice within a species is a selection pressure arising in the animal's own environment, with no human involved.
Males that display more often grow brighter plumage during their lives and pass the brighter plumage on to their sons — A student who thinks acquired characteristics are inherited picks this. Plumage developed through use is not encoded in genes; the alleles for elaborate plumage were inherited and selected.
6 Endler transferred guppies from a stream containing the dangerous predator Crenicichla to a stream that had no guppies and only the weak predator Rivulus, which rarely eats adult guppies. Two years later, the males in the new stream had on average more coloured spots, and larger spots, than the population they came from. Which is the best interpretation?
Answer and reasoning
The result shows that predators and females favour the same trait, so the two pressures added together to increase spots — A student who expects all selection pressures to point the same way picks this. Crenicichla had been favouring fewer, smaller spots; the spots increased only once that opposing pressure was largely removed.
New mutations for extra spots must have arisen during the two years, because the trait could not have spread without them — A student who thinks each bout of selection needs a fresh mutation picks this. The source population already varied in spot number and size; only allele frequencies changed.
With strong predation removed, female preference for spotted males was no longer opposed by selection for camouflage — In the original stream, natural selection by Crenicichla favoured dull, small-spotted males while females favoured bright ones. Moving the fish to a stream with only a weak predator greatly reduced one of the two opposing pressures, so alleles for more and larger spots increased in frequency.
Individual males developed more spots once they were safer from predators and passed the new spots to their sons — A student who thinks acquired characteristics are inherited picks this. Spot pattern is genetically determined; the change over generations was a change in allele frequencies, not in individual fish.
7 A student searches an allele frequency database for the ABO blood group allele Iᴮ. The database gives a frequency of about 0.25 for a population in Central Asia and about 0.01 for an Indigenous population in South America. Which conclusion is best supported? HL
Answer and reasoning
The two populations cannot belong to the same species, because members of one species share the same allele frequencies — A student who expects a species to be genetically uniform picks this. Populations of one species commonly differ in allele frequencies; the human ABO alleles are a well-documented example.
The two populations have separate gene pools that have diverged in allele frequency while remaining one species — Geographically isolated populations of the same species can have different allele frequencies, because natural selection, genetic drift and founder effects act separately on each gene pool. Both populations are Homo sapiens; only the frequency of Iᴮ differs.
About 25% of the people in the Central Asian population have blood group B, since the allele frequency is 0.25 — A student who takes the allele frequency to be the proportion of people carrying Iᴮ picks this: 25% carry it and, since Iᴮ is dominant to i, they reason that 25% show group B. A frequency of 0.25 is the fraction of allele copies that are Iᴮ; the proportion of people with group B depends on the genotype frequencies and cannot be read off directly.
The Iᴮ allele will spread through the South American population because it is dominant to the allele i — A student who thinks dominant alleles spread because they are dominant picks this. Dominance affects the phenotype of heterozygotes, not allele frequency; under Hardy–Weinberg conditions a rare dominant allele stays rare.
A theory that replaced natural selection after it was shown to be wrong — A student who reads 'neo' as a replacement picks this. Neo-Darwinism keeps natural selection as the mechanism and adds the genetic basis of variation and inheritance.
Darwin's original proposal that species are not fixed but change over time — A student who thinks Darwin's contribution was the idea of evolution itself picks this. Evolution was already widely accepted in Darwin's time; neo-Darwinism refers to the later synthesis with genetics.
The idea that characteristics acquired in life can be passed on to the offspring — A student who believes acquired characteristics are inherited assumes the up-to-date theory must include that idea. Inheritance of acquired characteristics is Lamarckism, which Darwin's theory replaced; neo-Darwinism explains inheritance through alleles.
The integration of genetics with Darwin's theory of natural selection — Darwin developed natural selection without knowledge of genes. Biologists subsequently combined Mendelian inheritance, mutation and allele frequencies with natural selection; this integrated theory is known as neo-Darwinism.
9 In a large population, 16% of individuals show the phenotype of the homozygous recessive genotype for a gene with two alleles. Assuming Hardy–Weinberg equilibrium, what is the frequency of the homozygous dominant genotype? HL
Answer and reasoning
0.84 — A student who takes p² to be the frequency of the dominant phenotype picks 1 − 0.16 = 0.84. That is p² + 2pq, which includes the 0.48 of heterozygotes; the homozygous dominant genotype alone is p² = 0.36.
0.71 — A student who sets q equal to the recessive phenotype frequency (q = 0.16, p = 0.84) gets p² = 0.7056. The recessive phenotype frequency is q², so q = √0.16 = 0.4.
0.36 — q² = 0.16, so q = 0.4 and p = 1 − 0.4 = 0.6. The homozygous dominant frequency is p² = 0.6² = 0.36. (Heterozygotes are 2pq = 0.48, and 0.36 + 0.48 + 0.16 = 1.)
0.60 — A student who does not distinguish allele frequency from genotype frequency stops at p = 0.6. That is the frequency of the dominant allele; the frequency of the homozygous dominant genotype is p² = 0.36.
10 In a population of 1000 plants, the frequencies of alleles R and r are 0.6 and 0.4. The Hardy–Weinberg equation predicts 360 RR, 480 Rr and 160 rr plants. A survey finds 460 RR, 280 Rr and 260 rr. What can be concluded? HL
Answer and reasoning
At least one Hardy–Weinberg condition is not being met, such as random mating or equal survival of the genotypes — The allele frequencies calculated from the survey are still 0.6 and 0.4 (1200 R copies out of 2000), but the genotype frequencies do not fit p², 2pq, q²: there are too few heterozygotes. This indicates that a condition is broken, for example non-random mating (like mating with like) or differing survival, and further evidence is needed to decide which.
The shortage of heterozygous plants proves that natural selection is acting against the Rr genotype — A student who reaches for natural selection as the only explanation picks this. A deficit of heterozygotes is also produced by non-random mating; the data cannot distinguish between the possible broken conditions.
The population is in genetic equilibrium, because the two allele frequencies still add up to exactly 1 — A student who treats p + q = 1 as a test of equilibrium picks this. p + q = 1 is true of any two-allele population by definition; equilibrium is tested by whether the genotype frequencies fit p² + 2pq + q².
The dominant allele R is replacing r, because dominant alleles increase in frequency over the generations — A student who thinks dominance drives allele frequency picks this. The frequency of R in the survey is still 0.6; it is the genotype frequencies, not the allele frequencies, that depart from the prediction.
Read the ones marked not yet in Learn, then Verify.
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13 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 Which statement about natural selection as the mechanism of evolutionary change is correct?
Answer and reasoning
It produced the species that exist today and has now stopped acting upon any of them — A student who files natural selection under history picks this. Selection pressures still operate on every population; the evolution of antibiotic resistance is a present-day example.
It has operated continuously over billions of years and is still operating today — Natural selection operates wherever there is heritable variation, overproduction and a selection pressure. It has acted continuously over billions of years to produce the biodiversity of life on Earth and continues in every population now.
It causes the mutations that individuals need to survive new conditions — A student who thinks organisms produce the alleles they need picks this. Mutation is random and precedes selection; natural selection changes the frequencies of alleles that already exist.
It acts to improve each species as a whole rather than on individual organisms — A student who takes the species as the unit of selection picks this. Natural selection acts on differences between individuals in survival and reproduction; alleles spread because they raise the fitness of the individuals carrying them.
2 A pair of blue tits can raise about ten chicks each year, yet the number of blue tits in a woodland stays roughly constant from year to year. How does this promote natural selection?
Answer and reasoning
Overproduction plays no part, because chicks with the better alleles would survive equally well without competition — A student who reduces natural selection to 'the best adapted survive' picks this. Without overproduction and competition, small differences in adaptation would make little difference to survival.
The chicks compete for food only, because nesting sites, territory and other resources are not limiting in a woodland — A student who names food as the only resource picks this. Carrying capacity can be set by any limiting resource, including nesting holes, territory, water and shelter, and blue tits compete for all of these.
The chicks compete mainly with other bird species, and competition between blue tits themselves is unimportant — A student who sees selection as a contest between species picks this. The strongest competition is intraspecific, because blue tits need exactly the same food, nest sites and mates as each other.
Far more chicks are produced than the limited resources can support, so they compete and only some survive — Overproduction of offspring combined with limited food, nesting sites and territories means the woodland is at its carrying capacity. Chicks compete, and those whose heritable traits give them an advantage are more likely to survive and breed.
3 In a population of red deer, stag X is the largest and strongest male but leaves two offspring that survive to breed. Stag Y is smaller and weaker but leaves six offspring that survive to breed. Which stag has the higher fitness, and why?
Answer and reasoning
Stag X, because fitness means the physical strength and bodily condition of the individual animal — A student who uses the everyday meaning of fitness picks this. In natural selection, strength matters only if it leads to more surviving offspring, and here it did not.
Stag Y, because fitness is measured by the number of surviving offspring a genotype contributes — Fitness is the survival value and reproductive potential of a genotype: its contribution to the next generation. Stag Y's genotype is represented by three times as many offspring, so it has the higher fitness whatever its size.
Stag X, because natural selection favours traits that aid survival rather than reproduction — A student who thinks selection is only about survival picks this. Fitness depends on both survival and reproduction; an allele that increases offspring number spreads even in a smaller, weaker animal.
Neither, because fitness is a property of the deer species as a whole and not of individuals — A student who takes the species as the unit of selection picks this. Fitness is defined for genotypes and compared between individuals of the same species; that comparison is the basis of natural selection.
4 A woman develops large, strong arm muscles through years of rowing training. Will her children inherit larger arm muscles as a result of her training?
Answer and reasoning
Yes, because characteristics that an individual acquires during life are passed on to its offspring — A student who holds the Lamarckian view picks this. Acquired characteristics are not heritable, which is why Darwin's mechanism replaced Lamarckism.
Yes, if she teaches her children to row, because anything passed from parent to child is heritable — A student who uses the everyday meaning of 'inherit' picks this. A skill learned from a parent is acquired, not heritable; heritable traits are those determined by genes.
Yes, because the training causes mutations in the genes for muscle growth that her children will receive — A student who thinks mutations arise in response to need picks this. Mutations are random changes in base sequence, and exercise does not direct mutations into the gametes.
No, because muscle growth caused by training is not encoded in the base sequence of her genes — Enlarged muscles are an acquired characteristic caused by an environmental factor. Only traits encoded in the base sequence of DNA are passed on through gametes, so her training cannot be inherited; her children inherit only her alleles.
5 In elephant seals, males fight for control of a stretch of beach, and the largest males mate with most of the females there. Which statement about the evolution of large body size in male elephant seals is correct?
Answer and reasoning
Large males have the higher fitness simply because they are stronger, whether or not they succeed in mating — A student who uses the everyday meaning of fitness picks this. Fitness is reproductive potential; large size raises fitness here only because it leads to more offspring.
Large size is favoured by sexual selection because it increases a male's success in competing for mates — Sexual selection is a selection pressure arising from differences in mating success. Large males win fights, mate with more females and leave more offspring, so alleles for large size have increased in frequency in males.
Fighting for mates is not a selection pressure, since only survival differences drive evolution — A student who thinks selection is only about survival picks this. Differences in reproductive success change allele frequencies just as differences in survival do.
Large size evolved because it benefits the species by ensuring only the best genes are passed on — A student who thinks selection works for the good of the species picks this. Alleles for large size spread because they raise the fitness of the individual males carrying them, not for any benefit to the species.
6 In greenhouse ponds, Endler kept guppy populations on coarse or fine gravel with either the dangerous predator Crenicichla, the weak predator Rivulus, or no predator. At the start, males had a mean of about 10 coloured spots each. After about ten generations the mean was about 13 spots per male in ponds with no predator and in ponds with Rivulus, but about 9.5 spots per male in ponds with Crenicichla. In Crenicichla ponds the spots were also close in size to the gravel particles, whereas in no-predator ponds they were not. Which conclusion is best supported?
Answer and reasoning
The alleles for small spots arose by mutation in each pond only after the predators were added to it — A student who thinks selection needs a new mutation each time picks this. Ten generations is far too short for new alleles to appear and spread in every pond; existing variation was selected.
Because spot size came to match gravel size, the females must also have preferred males whose spots matched the gravel — A student who assumes predators and females favour the same trait picks this. Matching the gravel was favoured only where predators were present; without predators, spot size did not match the gravel, so female preference cannot have been for gravel-matched spots.
Males in the Crenicichla ponds grew smaller, gravel-matched spots during their lives and passed them on to their sons — A student who thinks acquired characteristics are inherited picks this. Spot pattern is genetically determined; the change over ten generations was a change in allele frequencies caused by differential survival and mating, not a change within individual fish.
The direction of selection on spot pattern depended on the predation pressure that was applied — The experiment controlled the selection pressures. Strong predation by Crenicichla selected for camouflage (fewer, gravel-matched spots) and spot number fell; with no predator, or only the weak predator Rivulus, sexual selection alone acted and spot number rose. The same starting population evolved in opposite directions.
7 A pond contains a population of 500 water snails. Which statement best describes the gene pool of this population? HL
Answer and reasoning
All the genes, and all the different alleles of those genes, carried by the 500 snails in the pond — A gene pool consists of all the genes and their different alleles present in a population. It belongs to the interbreeding group, here the 500 snails, and natural selection changes the frequencies of alleles within it.
The complete set of genes in one water snail, which is the same in every member of the species — A student who confuses gene pool with genome picks this. A genome is the genetic material of one individual; the gene pool is the total of all alleles in the whole population, and individuals differ in which alleles they carry.
The set of allele frequencies shared by every population of this snail species wherever it lives — A student who expects every population of a species to have the same allele frequencies picks this. Each separate population has its own gene pool, and populations of one species in different ponds can have different allele frequencies.
Only the alleles natural selection has favoured, because alleles reducing fitness are removed — A student who thinks selection eliminates unfavourable alleles at once picks this. The gene pool includes every allele present, including rare harmful recessive alleles hidden in heterozygotes.
8 A recessive allele causes a condition that is lethal before reproductive age in homozygotes. Heterozygotes are unaffected. Which statement describes how natural selection changes the frequency of this allele in the gene pool? HL
Answer and reasoning
It is removed from the gene pool within a generation, because every individual carrying it dies — A student who pictures selection acting directly on alleles picks this. Heterozygotes carry the allele but are unaffected, so they survive and pass it on.
It falls because the allele mutates back into the normal allele in individuals who need it in order to survive — A student who thinks mutation is directed by need picks this. Mutation is random and rare; the change in frequency is due to differential survival of genotypes, not to back-mutation.
It falls slowly over many generations, because the allele is hidden from selection in heterozygotes — Natural selection acts on heritable differences between individuals, that is on phenotypes. Only homozygotes are selected against, and as the allele becomes rarer a smaller fraction of its copies are in homozygotes, so its frequency declines more and more slowly.
It falls rapidly because the dominant normal allele out-competes the recessive allele in each generation — A student who thinks dominance itself drives frequency change picks this. Dominance describes the heterozygote phenotype; here it is the reason the recessive allele is protected from selection, not removed.
9 In a study of human births in the mid-twentieth century, babies of intermediate birth mass had the highest survival, while both very light and very heavy babies had higher mortality. Which type of selection does this show, and why? HL
Answer and reasoning
Stabilizing selection, because individuals with intermediate values of the trait are favoured over both extremes — Selection against both extremes with the intermediate phenotype favoured is stabilizing selection. The mean birth mass is unchanged but variation is reduced, and alleles contributing to extreme birth mass fall in frequency.
Disruptive selection, because the extremes of the range are disrupted by being selected against — A student who applies the everyday meaning of 'disruptive' picks this. Disruptive selection favours both extremes over the intermediate, the opposite of this pattern.
Directional selection, because whenever one phenotype is favoured over others the selection is directional — A student who uses directional as the default label picks this. Directional selection favours one extreme and shifts the mean; favouring the intermediate is stabilizing selection.
No selection, because the mean birth mass of the population does not change from generation to generation — A student who judges selection by the mean alone picks this. Differential survival among phenotypes is selection even when the mean is constant; the variance and the allele frequencies change.
10 Which statement about directional, disruptive and stabilizing selection is correct? HL
Answer and reasoning
Stabilizing selection leaves allele frequencies unchanged, since the mean phenotype does not shift — A student who reads a constant mean as a constant gene pool picks this. Stabilizing selection removes alleles for extreme phenotypes, which is a change in allele frequency.
All three types of selection result in a change in the allele frequencies of the gene pool — In each type some phenotypes leave more offspring than others, so the alleles associated with the favoured phenotypes increase in frequency. This is true of stabilizing selection as well, where alleles for the extremes are removed.
Disruptive selection is selection that follows a sudden disruption of the environment — A student who applies the everyday meaning of 'disruptive' picks this. Disruptive selection is defined by favouring both extremes; a sudden change favouring one extreme causes directional selection.
Only directional selection results in some phenotypes leaving more offspring than others — A student who treats directional selection as the only real kind picks this. All three types involve differential reproductive success; they differ in which part of the range is favoured.
11 A recessive condition affects 1 in 100 individuals in a large population that is in Hardy–Weinberg equilibrium. What proportion of the population are heterozygous carriers of the allele? HL
Answer and reasoning
0.09 — A student who calculates the heterozygote frequency as p × q gets 0.9 × 0.1 = 0.09. The heterozygote can be formed in two ways, so the term is 2pq = 0.18.
0.02 — A student who takes q = 0.01 (the phenotype frequency) and p = 0.99 gets 2pq = 0.0198. The phenotype frequency is q², so q = √0.01 = 0.1.
0.10 — A student who treats the allele frequency as the proportion of carriers stops at q = 0.1. The allele frequency is 0.1, but the proportion of individuals heterozygous for it is 2pq = 0.18.
0.18 — q² = 0.01, so q = 0.1 and p = 0.9. The heterozygote frequency is 2pq = 2 × 0.9 × 0.1 = 0.18, so 18% of the population are carriers, eighteen times the number actually affected.
12 After an antibiotic is used repeatedly in a hospital, the proportion of bacteria resistant to it rises sharply. Which statement correctly classifies this change? HL
Answer and reasoning
Artificial selection, because the change in the bacterial population was caused by a human action — A student who defines artificial selection as 'caused by humans' picks this. The guide is explicit that unintended consequences of human actions, such as antibiotic resistance, are due to natural selection.
Natural selection, because the antibiotic acted as a selection pressure and nobody chose which bacteria bred — Artificial selection requires deliberate human choice of traits for breeding. Here the increase in resistance is an unintended consequence of a human action: the antibiotic killed susceptible bacteria and the resistant ones survived and reproduced, which is natural selection.
Natural selection, because the bacteria mutated to become resistant in response to their exposure to the antibiotic — A student who thinks mutations are directed by need picks this. Resistance alleles arose by random mutation before exposure; the antibiotic selected the bacteria that already carried them.
Artificial selection, because the resistance allele was produced by the human use of the antibiotic — A student who thinks humans create the alleles they select picks this. Neither natural nor artificial selection creates alleles; both change the frequencies of alleles that arose by mutation.
13 How is artificial selection carried out in a crop plant such as wheat? HL
Answer and reasoning
By creating brand-new alleles for the desired traits and inserting them into the plants — A student who confuses breeding with genetic modification picks this. Artificial selection uses variation already present; breeders choose which plants reproduce and do not make new alleles.
By exposing the crop to a pesticide so that only the pesticide-resistant plants evolve — A student who classes every human-caused change as artificial selection picks this. Resistance evolving in response to a chemical is an unintended consequence, due to natural selection.
By choosing the plants with the most desirable traits and breeding only from them — Artificial selection is the deliberate choice of traits: in crop plants and domesticated animals, humans select the individuals with desirable traits, such as high yield, to breed from, so the alleles for those traits increase in frequency over generations.
By growing the plants in rich soil so that the better growth passes to seeds — A student who thinks acquired characteristics are inherited picks this. Growth gained from good soil is not encoded in the plants' genes and is not passed on; only inherited traits can be selected.
That was your twenty minutes. Real practice on D4.1 is past-paper questions marked against the mark scheme.
What the exam asks of D4.1
Paper 1A asks you to identify the type of selection from a graph, or to spot which statement about heritability or fitness is correct. Paper 1B gives Endler's guppy data or allele-frequency tables and asks you to interpret them. Paper 2 uses *outline* and *explain* for natural selection: variation, overproduction, competition, differential survival, then allele frequency change, in that order. HL questions use *calculate* for Hardy–Weinberg and *distinguish* for the three types of selection; show p, q and the equation every time.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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