IB Biology · Theme A Unity and diversity · Ecosystems
A4.1 Evolution and speciation
Evolution is change in a population's heritable characteristics; acquired traits do not count. Sequences, selective breeding and homologous structures are the evidence, and convergence explains lookalikes. New species arise only by splitting, once reproductive isolation lets populations diverge.
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
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).
Learn
A4.1.1 Evolution is heritable change in a population, and why it is still called a theory
Evolution is change in the heritable characteristics of a population over generations.
The population evolves, not the individual: allele frequencies shift.
Acquired characteristics such as trained muscles are not genetic, so not inherited.
That rules out Lamarckism, the idea that acquired traits are passed on.
A theory is a well-supported explanation; evolution is unlikely to be falsified, but no theory can be proved by matching every fact.
Students often think an individual evolves by adapting in its lifetime. In fact only heritable change across a population counts.
Students often think "theory" means guess. In fact it names a pragmatic truth that explains a broad range of observations.
A4.1.2 Sequence similarity is evidence of common ancestry
Compare base sequences of DNA or RNA, or amino acid sequences of proteins.
Differences accumulate after two lineages split from a common ancestor.
Fewer differences means a more recent common ancestor.
Across many species the pattern matches a branching tree of descent.
Students often think the simpler living species is the ancestor. In fact both descend from an ancestral population that no longer exists.
Students often think similar environments give similar sequences. In fact convergence copies features, not base sequences.
A4.1.3 Selective breeding shows how fast heritable change can happen
Humans choose which animals or plants breed, based on heritable traits.
Over generations this changes the population: evolution by artificial selection.
The breeder selects from variation already present; it does not create it.
Dog breeds versus the grey wolf, or crops from wild cabbage, show how fast change happens.
Students often think human-driven change is not evolution. In fact heritable characteristics changed; the cause of selection is irrelevant.
Students often think very different breeds are separate species. In fact all dog breeds remain one species.
A4.1.4 Homologous structures share an origin, not a function
Homologous structures are similar because they were inherited from a common ancestor.
They may now do quite different jobs.
The pentadactyl limb: one upper bone, two lower bones, wrist or ankle bones, five digits.
Human arm, bat wing, whale flipper and horse leg all follow this pattern.
Students often define homologous by shared function. In fact it is shared origin; function may differ.
Students often think five digits is simply the best design. In fact each species inherited and modified the same ancestral pattern.
A4.1.5 Analogous structures come from convergent evolution
Analogous structures share a function but have different evolutionary origins.
Similar selection pressures favoured similar solutions in unrelated lineages.
Insect and bird wings; the streamlined bodies of sharks and dolphins.
This is convergent evolution; the species stay as unrelated as before.
Students often think converging species grow more closely related. In fact they only look more alike; relatedness is unchanged.
Students often think dolphins are close to sharks. In fact dolphins are mammals; their shark-like form is convergence.
A4.1.6 New species only ever come from splitting old ones
Speciation is a pre-existing species splitting into two or more.
It is the only way new species have appeared, so it increases the species count.
Extinction decreases it; the two act independently.
Gradual change within one lineage, without splitting, is not speciation.
Students often think a slowly changing species becomes a new one. In fact without a split it is evolution, not speciation.
Students often think a big mutation in one individual makes a new species. In fact that individual can still breed with its parents' species.
A4.1.7 Isolation starts speciation; differential selection completes it
Reproductive isolation stops alleles passing between two gene pools.
Geographical isolation by a river, mountains or sea is one way to achieve it.
Differential selection then favours different traits in each population until they cannot interbreed.
The Congo River split bonobos from common chimpanzees; neither swims, and conditions differ across it.
Bonobos, for example, have no gorillas competing for food.
Students often think separation alone makes new species. In fact divergence under different selection must follow.
Students often think the environment causes useful mutations. In fact mutations are random; the environment selects among them.
A4.1.8 Allopatric and sympatric speciation HL
Allopatric: populations diverge in different places, separated by a geographical barrier.
Sympatric: populations diverge in the same area, without a physical barrier.
Behavioural isolation: different courtship signals stop mating.
Temporal isolation: breeding or flowering at different times.
In both cases isolation ends gene flow and the gene pools diverge.
Students often think sympatric speciation hides a small barrier. In fact behaviour, timing or polyploidy isolates, not geography.
Students often think temporal isolation means different eras. In fact it means different breeding times in living populations.
A4.1.9 Adaptive radiation fills vacant niches HL
Adaptive radiation: one ancestral species diversifies rapidly into many.
It happens where vacant niches exist, such as new islands or after an extinction.
Each species specialises on a different resource or habitat.
So close relatives coexist without competing, raising biodiversity.
Students often think close relatives cannot share a place. In fact different niches remove the competition.
Students often confuse adaptive radiation with convergence. In fact radiation is one ancestor becoming many; convergence is many lineages becoming alike.
A4.1.10 Barriers and sterile hybrids keep gene pools apart HL
Courtship behaviour is species-specific, so it often prevents hybridisation in animals.
When hybrids form they are usually sterile, so alleles go no further.
A horse (64) and a donkey (62) make a mule with 63 chromosomes.
Its chromosomes cannot all pair in meiosis, so no viable gametes form.
Students often think a mule is weak or incomplete. In fact it is healthy with a full set of genes; only pairing in meiosis fails.
Students often think only physical barriers stop breeding. In fact ignoring another species' courtship is barrier enough.
A4.1.11 Plants can form new species in one generation HL
A sterile hybrid becomes fertile if its chromosome number doubles: polyploidy.
Every chromosome then has a homologous partner for meiosis.
The polyploid cannot breed with either parent species, so it is a new species at once.
This abrupt speciation has produced many species of knotweed, genus Persicaria.
Students often think extra chromosome sets are lethal. In fact polyploid plants are common and healthy.
Students often think the polyploid is still its parents' species. In fact crosses with parents give unbalanced, sterile offspring.
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 Which of these changes would a biologist regard as evolution?
Answer and reasoning
The frequency of an allele for dark wing colour rises in a moth population over many generations. — Evolution is change in the heritable characteristics of a population. A rise in the frequency of an allele across generations is exactly such a change, whatever the cause of the selection.
The muscles a weightlifter enlarges by training are inherited by the weightlifter's children. — A student who accepts the Lamarckian idea that acquired characteristics are inherited picks this. Enlarged muscles are acquired during life and are not genetic in origin, so they cannot be inherited and no evolution results.
An individual Arctic fox grows a thicker and whiter coat each year as winter approaches. — A student who thinks an individual evolves when it changes during its life picks this. The fox's seasonal coat is a response by one individual, not a change in the heritable characteristics of a population.
Young chimpanzees learn from adults how to crack nuts with stones, and the skill spreads. — A student who counts anything handed down from parents to offspring as heritable picks this. A learned skill spreads by imitation, not through alleles, so its spread is not evolution.
2 The amino acid sequence of the same protein was compared in four species. Compared with species W, species X differs at 2 positions, species Y at 15 positions and species Z at 31 positions. Which conclusion is supported by the data?
Answer and reasoning
Z is the most primitive of the four species, because its sequence is furthest from W's. — A student who reads sequence differences as a ranking from primitive to advanced picks this. Differences accumulate in both lineages after they split, so 31 differences means W and Z diverged longest ago, not that Z is primitive.
X lives in the same kind of habitat as W, because similar conditions give similar proteins. — A student who thinks the environment shapes sequences picks this. Sequence similarity is inherited from a common ancestor; similar habitats produce convergent features, not similar amino acid sequences.
W shares a more recent common ancestor with X than it does with either Y or Z. — Differences in amino acid sequence accumulate over time after two lineages split from a common ancestor. W and X differ least, so their common ancestor is the most recent; Z, with the most differences, diverged from W earliest.
X is the ancestor of W, because the two sequences are almost identical. — A student who pictures living species as ancestors of other living species picks this. Both W and X are present-day species descended from the same ancestral population; the near-identical sequence shows a recent split, not descent of one from the other.
3 Cabbage, kale, broccoli, cauliflower, kohlrabi and Brussels sprouts were all produced from wild cabbage (Brassica oleracea) by selective breeding over the past few thousand years. What does this show about evolution?
Answer and reasoning
The heritable characteristics of a population can change greatly in a short time when selection is strong. — Variation between crop varieties, and between them and the original wild species, shows how rapidly evolutionary change can occur. Selective breeding changed the heritable characteristics of these populations within a few thousand years.
Breeders were able to produce each new characteristic by causing the mutation that was needed for it. — A student who thinks breeders manufacture the features they want picks this. Breeders cannot cause a particular mutation; they select among the heritable variation already present, generation after generation.
It is not evidence of evolution, because the changes were brought about by humans rather than by nature. — A student who treats 'natural' as part of the definition of evolution picks this. Evolution is change in the heritable characteristics of a population; selective breeding produces such change, and its speed is what makes it evidence.
Each variety has now become a separate species, because the varieties differ so much in their appearance. — A student who defines species by appearance picks this. All the varieties remain members of Brassica oleracea; they are not reproductively isolated from each other, so no speciation has occurred.
4 The arm of a human, the wing of a bat and the flipper of a whale are used for grasping, flying and swimming, yet each has one upper bone, two lower bones, a group of wrist bones and five digits. Why is this evidence for evolution?
Answer and reasoning
Five digits is the best possible arrangement for any limb, so each of the species evolved the pattern separately. — A student who explains every structure by function picks this. A whale has no use for separate digits, and a paddle or a wing could be built in many ways; the shared pattern is inherited, not required by function.
The shared bone pattern was inherited from a common ancestor and has been modified for different functions. — The pentadactyl limb is a homologous structure. The same underlying pattern in limbs with different functions is explained by descent with modification from a common ancestor, not by the demands of grasping, flying or swimming.
The limbs are analogous structures, which show that different functions evolve from different origins. — A student who has swapped the two terms picks this. Analogous structures have the same function but different origins; these limbs share an origin and differ in function, so they are homologous.
Bats and whales are older species than humans, so they are our ancestors and passed the pattern on. — A student who thinks living species are the ancestors of other living species picks this. Humans, bats and whales are all descended from a common ancestor that had the pentadactyl limb; none of the living species is the ancestor of another.
5 Sharks, which are fish, and dolphins, which are mammals, both have streamlined bodies, a dorsal fin and paddle-shaped front fins. Which statement about these features is correct?
Answer and reasoning
They are homologous, showing that dolphins are more closely related to sharks than they are to other mammals. — A student who reads any resemblance as kinship picks this. Dolphins are mammals and share a far more recent common ancestor with other mammals; the resemblance to sharks arose independently.
They are homologous, because in both animals the features perform the same function in water. — A student who has swapped the definitions picks this. Same function with different origins is the definition of analogous structures; homologous structures share an origin regardless of function.
They are analogous: they evolved separately because similar selection favoured similar shapes in water. — Analogous structures have the same function but different evolutionary origins. Fish and mammals acquired streamlined bodies and fins independently, by convergent evolution under the similar selection pressures of fast movement through water.
They show that the two species are converging on each other and will eventually merge into a single species. — A student who takes 'converge' to mean the lineages come together picks this. Convergent evolution makes features similar, not species; sharks and dolphins remain as distantly related as they were.
A species changes gradually over millions of years until it looks quite different from its ancestors. — A student who thinks enough change turns a species into a new one picks this. Gradual evolutionary change within one lineage is evolution but is not speciation, because no splitting occurs and no additional species results.
A river splits a population into two halves, which are at once regarded as separate species. — A student who takes geographical separation to be the whole of speciation picks this. The river only stops gene flow; the halves become separate species only if differential selection then makes them diverge until they can no longer interbreed.
An individual is born with a mutation that makes it different from all others of its species. — A student who thinks a single mutant individual is a new species picks this. Speciation is the splitting of a population into groups that can no longer interbreed; the mutant is still a member of its parents' species and can breed with them.
A population splits into two groups that then become unable to interbreed with each other. — Speciation is the splitting of a pre-existing species into two or more species that can no longer interbreed. It is the only way in which new species have appeared.
7 Bonobos live south of the Congo River and common chimpanzees live north of it; neither species swims. How did the two species arise from one ancestral population?
Answer and reasoning
Apes on each side of the river developed the features they needed to survive there and passed them on to their offspring. — A student who accepts inheritance of acquired characteristics picks this. Features developed during an individual's life are not inherited; the populations diverged because selection favoured different alleles on the two sides.
The river prevented interbreeding, and different selection pressures on each side caused the populations to diverge. — The Congo River gave geographical, and therefore reproductive, isolation. Conditions on the two sides differ, for example in food supply and competitors, so differential selection made the isolated gene pools diverge until they became separate species.
The river caused mutations to occur that made the apes on each side of it different from one another. — A student who thinks the environment directs mutation picks this. Mutations arise at random; the river stopped gene flow, and differential selection on existing variation on each side caused the divergence.
Separation by the river was on its own enough to turn the two populations into two separate species. — A student who takes isolation to be the whole of speciation picks this. Isolation stopped gene flow, but the populations became separate species only because differential selection then made them diverge.
8 What do allopatric speciation and sympatric speciation have in common? HL
Answer and reasoning
Both require reproductive isolation of the populations, so that their gene pools can diverge. — In both, gene flow between the populations must stop; this reproductive isolation is geographic in allopatric speciation and behavioural or temporal in sympatric speciation. Once isolated, the gene pools diverge until the populations cannot interbreed.
Both require a physical barrier, though in sympatric speciation the barrier is very small. — A student who models all speciation on the allopatric example picks this. Sympatric speciation occurs without any physical barrier; isolation is achieved by differences in behaviour or timing.
Both require the populations to live in separate areas at some stage in the process. — A student who thinks populations sharing an area cannot stop interbreeding picks this. Sympatric speciation takes place entirely within one area.
Both are complete as soon as the populations stop interbreeding, before any genetic change. — A student who takes isolation to be the whole of speciation picks this. Reproductive isolation is the starting condition; the populations become separate species only after their gene pools have diverged.
9 A single species of finch colonized a remote group of islands that had no other land birds. Its descendants now form many species, each with a beak suited to a different food. Which statement explains this? HL
Answer and reasoning
This is convergent evolution, because unrelated birds reaching the islands evolved similar beaks. — A student who conflates the two terms picks this. The species all descend from one colonizing species and have become more different from each other, which is divergence in an adaptive radiation, the opposite of convergence.
Vacant niches let the descendants diversify, and specializing on different foods lets the species coexist. — With no other land birds present, food niches were vacant. Adaptive radiation filled them: each species became adapted to a different food, so closely related species coexist without competing, increasing the biodiversity of the islands.
The finches developed the beaks they needed for each food and passed those beaks on to their offspring. — A student who accepts inheritance of acquired characteristics picks this. Beak shape did not change by use; selection on each food source favoured birds whose heritable beak shape suited it.
The species must each live on a separate island, because related species cannot share an area without conflict. — A student who applies competitive exclusion to all related species picks this. Species adapted to different foods occupy different niches, so several can coexist on one island without competing.
10 A horse has 64 chromosomes and a donkey has 62. A mule, their hybrid, is healthy and vigorous but almost always sterile. Why is the mule sterile? HL
Answer and reasoning
It has only half of the horse's genes and half of the donkey's, so it lacks a complete set of instructions. — A student who thinks a hybrid is made from two incomplete halves picks this. The mule has a complete haploid set from each parent, which is why it develops into a healthy animal; the sets simply cannot pair in meiosis.
Horses and donkeys must be the same species, so the sterility is a result of inbreeding. — A student who thinks producing offspring makes two organisms one species picks this. Horses and donkeys are separate species; the mule's sterility is the mechanism that keeps their alleles from mixing.
Its 63 chromosomes cannot all pair up during meiosis, so it does not form viable gametes. — The mule inherits 32 chromosomes from the horse and 31 from the donkey. These are not homologous pairs, so pairing in meiosis fails and functional gametes are not produced, even though the animal itself is vigorous.
A hybrid is an unnatural mixture, so its body rejects the combination of horse and donkey genes. — A student who regards hybrids as defective picks this. The stem states that the mule is healthy and vigorous; its sterility is a failure of chromosome pairing in meiosis, not of its body.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
8 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A student says: 'Evolution by natural selection is only a theory, so it has not been proved and could easily be overturned.' Which response best evaluates this claim?
Answer and reasoning
The claim is right: a scientific theory is just an untested idea, and evolution will become a fact once enough evidence has been gathered. — A student who uses the everyday meaning of 'theory' picks this. In science a theory is a well-supported explanation; evolution stays a theory not because evidence is lacking but because science cannot formally prove any theory true.
The theory cannot be formally proved true, but it predicts and explains so many observations that it is unlikely ever to be falsified. — This is the nature-of-science position in the guide: no theory can be proved true by correspondence, so evolution remains a theory, but it is a pragmatic truth supported by a broad range of observations and is unlikely ever to be falsified.
The claim is wrong because the theory has been proved true by direct observation of every prediction that it makes. — A student who believes science proves its theories picks this. No finite set of observations can prove a theory true for all cases; the theory of evolution is accepted as a pragmatic truth, not a proved one.
The claim is right because the theory only describes past events, which cannot be tested by scientific experiment. — A student who thinks only directly observed events can be tested picks this. The theory makes predictions about present-day sequence data, homologous structures and selective breeding, and these predictions have been tested repeatedly.
2 For one gene, two species have identical amino acid sequences in the protein, yet the base sequences of the gene differ at six positions. Which statement explains this?
Answer and reasoning
They are not evidence of divergence, because they have had no effect on the protein. — A student who counts only changes that alter the phenotype picks this. The six base substitutions arose by mutation after the lineages split, so they are evidence of divergence whether or not they alter the protein.
The genes must actually be identical, so the six differences are sequencing errors. — A student who believes every base change alters the protein picks this. Several codons specify the same amino acid, so base substitutions can occur without changing the amino acid sequence; no error need be involved.
Similar environments keep the proteins alike while the DNA of each species drifts apart. — A student who thinks the environment shapes sequences picks this. The protein is identical because the genetic code is degenerate, so the six base substitutions do not change any amino acid; similar habitats produce convergent features, not conserved sequences.
Some base substitutions leave the amino acid that is coded for unchanged. — The genetic code is degenerate: several codons code for the same amino acid, so a base substitution can leave the amino acid sequence unchanged. This is why base sequences reveal more differences between species than protein sequences do.
3 Over a period of time, 40 speciation events and 25 extinctions were recorded in a group of organisms. Over the same period several other species in the group changed gradually without splitting. What happened to the number of species in the group?
Answer and reasoning
It rose by 15, because only the splitting of species adds to the total and only extinction removes from it. — Each speciation event adds one species by splitting one into two, and each extinction removes one: 40 - 25 = 15. Gradual change without splitting is not speciation and does not alter the count.
It rose by more than 15, because the species that changed gradually also became new species. — A student who counts gradual change as speciation picks this. A species that changes without splitting remains one species, so it adds nothing to the total; only the 40 splits and 25 extinctions count.
It fell by 25, because each speciation changes an existing species rather than adding one. — A student who thinks speciation replaces the old species with a new one picks this. Speciation is splitting, so one species becomes two and the total rises by one at each event.
It stayed the same, because in nature each speciation event is balanced by the extinction of a species. — A student who believes the number of species is held in balance picks this. Speciation and extinction are independent processes; here 40 additions and 25 losses give a net increase of 15.
4 A population of lizards was split when rising sea level cut off an island from the mainland. Many generations later, island and mainland lizards can no longer produce fertile offspring together. Which statement about this speciation is correct?
Answer and reasoning
Separation was enough by itself, and the two populations were different species as soon as they were cut off. — A student who equates isolation with speciation picks this. When the sea rose the two groups were still one species; they became separate species only after generations of divergence.
The populations only became different species once the lizards began to look different from one another. — A student who defines species by appearance picks this. The test is reproductive isolation: the populations are separate species because they can no longer produce fertile offspring, whatever they look like.
Separation stopped gene flow, so selection on each side could change the two gene pools independently. — Geographical isolation achieved reproductive isolation, ending gene flow between the populations. Differential selection on the island and the mainland then changed the two gene pools in different directions until the lizards could no longer interbreed.
Lizards on the island developed the features they needed and then passed them on to their young. — A student who accepts Lamarckian inheritance picks this. Features developed during life are acquired and not inherited; the island population changed because selection altered allele frequencies over generations.
5 Two populations of a fly species live in the same orchard but lay their eggs on different fruit species, which ripen at different times, so the flies rarely mate with each other. Which type of speciation could this lead to, and why? HL
Answer and reasoning
Allopatric speciation, because the two kinds of fruit tree act as a geographical barrier between the populations. — A student who looks for a physical barrier in every example picks this. The flies share the same orchard and can reach each other; the barrier is timing, not geography, so any speciation would be sympatric.
Neither, because temporal isolation refers to species that lived at different times in Earth's history. — A student who attaches 'temporal' to geological time picks this. Temporal isolation is a difference in the timing of reproduction between present-day populations, exactly as in these flies.
Neither, because populations that share the same habitat are bound to interbreed and cannot separate. — A student who thinks only physical separation can stop gene flow picks this. Reproductive isolation can be temporal: flies that mate at different times do not interbreed even though they live together.
Sympatric speciation, because temporal isolation is cutting gene flow with no geographical barrier. — The populations occupy the same area, so any speciation is sympatric. Mating at different times is temporal isolation, one of the forms of reproductive isolation that can allow gene pools sharing a habitat to diverge.
6 Two closely related bird species live in the same forest. The males of each species sing a different song, and females respond only to the song of their own species. What is the significance of this for the two species? HL
Answer and reasoning
It is not a barrier to hybridization, because only physical differences can stop two species breeding. — A student who thinks barriers must be physical picks this. Courtship behaviour often prevents hybridization in animals; a song that the other species does not respond to is an effective barrier, and no physical difference is needed.
It lets the two species coexist by preventing them from competing with each other for food. — A student who confuses reproductive barriers with niche separation picks this. Song has nothing to do with feeding; its significance is that it prevents interbreeding.
It shows that they are really one species, because they are so alike and live in the same forest. — A student who defines species by appearance picks this. Species are defined by reproductive isolation: the song difference keeps the two gene pools apart, so they are separate species however alike they look and wherever they live.
It prevents hybridization, so that alleles do not pass between the two gene pools. — Species-specific courtship is a barrier to hybridization. Because females respond only to their own species' song, the two species do not interbreed, and their alleles remain in separate gene pools even though they share a habitat.
7 Suppose one Persicaria species has 20 chromosomes in each diploid cell and another has 24. A hybrid forms between them, and in one hybrid plant the chromosome number doubles. Which statement is correct? HL
Answer and reasoning
The doubled plant has 44 chromosomes, can pair them all in meiosis, and is a new species isolated from both parents. — The hybrid receives 10 chromosomes from one parent and 12 from the other, 22 in all, which have no homologous partners. Doubling gives 44, so every chromosome has a partner and meiosis works. Crosses with either parent give unbalanced, sterile offspring, so the polyploid is a new species.
The doubled plant remains sterile, because its chromosomes still come from two different species. — A student who blames sterility on the mixture of species picks this. The hybrid was sterile because its 22 chromosomes had no partners; after doubling each has an identical partner, so pairing and meiosis succeed.
The doubled plant is still the same species as its parents, because it can breed with either of them. — A student who assumes interbreeding with the parents continues picks this. A cross with the 20-chromosome parent gives offspring with 22 + 10 = 32 chromosomes, which cannot pair properly, so the polyploid is reproductively isolated.
The doubled plant dies, because carrying extra sets of chromosomes is lethal, as it is in human cells. — A student who generalizes from human chromosome disorders picks this. Polyploidy is common and well tolerated in plants; a complete extra set is balanced, unlike a single extra chromosome.
8 A botanist claims that species R of Persicaria arose abruptly by hybridization between species P and species Q, followed by polyploidy. Which observation would best support this claim? HL
Answer and reasoning
R appears in the fossil record after a long series of intermediate forms between P and Q. — A student who expects every speciation to be gradual picks this. A polyploid species arises in a single generation, so a long series of intermediates would count against the claim rather than for it.
The diploid chromosome number of R equals the diploid numbers of species P and Q added together. — A hybrid of P and Q has one haploid set from each; doubling gives two sets from each, so the polyploid's diploid number is the sum of the two parents' diploid numbers. This is the signature of speciation by hybridization and polyploidy.
R produces fully fertile offspring when crossed with either species P or species Q. — A student who thinks the polyploid can still breed with its parents picks this. Fertile offspring with P would mean R is not reproductively isolated from P, which would count against R being a separate polyploid species.
R has fewer chromosomes than either P or Q, as expected for a hybrid with half of each parent's set. — A student who thinks a hybrid carries only half of each parent's chromosomes picks this. A hybrid has a complete haploid set from each parent, and after polyploidy has twice that, so R should have more chromosomes than either parent, not fewer.
That was your twenty minutes. Real practice on A4.1 is past-paper questions marked against the mark scheme.
What the exam asks of A4.1
Paper 1A asks you to classify an example as evolution or not, homologous or analogous, allopatric or sympatric. Paper 1B may give sequence-difference tables or breed data and ask what they show about ancestry or rate of change. Paper 2 uses *define* for evolution, *outline* for one line of evidence, and *explain* for how isolation and differential selection produce species, using the bonobo and chimpanzee case. At HL, expect *explain* on hybrid sterility and polyploidy and *distinguish* on speciation modes; give the mechanism and the named example.
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 ·
DP Science Cafe