IB DP Biology: Evolution, speciation and conservation of biodiversity (DP Biology)

This study guide follows the IB Biology guide, first assessment 2025, and covers sections A4.1 (Evolution and speciation) and A4.2 (Conservation of biodiversity) in Theme A, Unity and diversity. A4.1.1 to A4.1.7 and all of A4.2 are SL and HL; A4.1.8 to A4.1.11 are additional higher level, so they are marked HL only. It applies to the Diploma Programme Biology examinations from the first assessment in 2025.

Related pages: revision notes, practice questions, the IB Biology hub, the printable checklist and the Theme A overview. To find weak spots first, try the free 10-minute diagnostics.

What this unit covers

Section	Focus	Tier
A4.1.1	Evolution defined	SL and HL
A4.1.2	Sequence evidence	SL and HL
A4.1.3	Selective breeding	SL and HL
A4.1.4	Homologous structures	SL and HL
A4.1.5	Convergent evolution	SL and HL
A4.1.6	Speciation by splitting	SL and HL
A4.1.7	Isolation and differential selection	SL and HL
A4.1.8	Sympatric and allopatric speciation	HL only
A4.1.9	Adaptive radiation	HL only
A4.1.10	Hybridization barriers, sterile hybrids	HL only
A4.1.11	Abrupt speciation in plants	HL only
A4.2.1 to A4.2.2	Biodiversity and species numbers	SL and HL
A4.2.3 to A4.2.4	Extinction and ecosystem loss	SL and HL
A4.2.5 to A4.2.6	Evidence and causes of the crisis	SL and HL
A4.2.7 to A4.2.8	Conservation approaches and EDGE	SL and HL

A4.1.1 Evolution defined

Evolution is a genetic change in the features of a population over generations. Heritable means genetic, so muscles built by training are not evolution. Population means the group evolves, not the individual: allele frequencies change, an individual’s alleles do not.

This definition separates Darwinian evolution from Lamarckism, which proposed that acquired characteristics are inherited. Natural selection explains a broad range of observations and is unlikely ever to be falsified, but science cannot formally prove it true, so it is a pragmatic truth.

A4.1.2 Sequence data

Closely related species share a recent common ancestor, so they have had less time to accumulate differences. Base sequences of DNA or RNA, and amino acid sequences of proteins, can be compared directly. The more differences, the longer ago the lineages diverged.

Worked example. Here are 12 amino acids from the same protein in four species (hypothetical data).

Species	Sequence
P	GDVEKGKKIFVQ
Q	GDVEKGKKIFAQ
R	GDIEKGKKLFAQ
S	GNIAKGEKLWVQ

Counting differences position by position: P and Q differ at 1 position (8.3%), P and R at 3 (25.0%), P and S at 6 (50.0%), Q and R at 2 (16.7%), Q and S at 7 (58.3%), R and S at 5 (41.7%). P and Q are the most closely related. R is next. S is the most distant. P and Q branched apart most recently, S earliest.

Why DNA can differ without the protein differing. The genetic code is degenerate: several codons code for one amino acid. Compare ATG GCT AAG CTT GGA CCA with ATG GCA AAG CTC GGT CCA. Three bases differ (3 of 18), yet both code for Met-Ala-Lys-Leu-Gly-Pro. DNA can therefore show more differences than the protein.

A4.1.3 Selective breeding

Humans choose which animals and plants breed. Domesticated dogs show huge variation in size, coat and behaviour, yet all descend from wolves. Cabbage, broccoli, kale and kohlrabi are varieties of one wild species, Brassica oleracea, selected for different plant parts. Differences between varieties, and from the wild ancestor, arose rapidly. Selection can change populations quickly. Natural selection works the same way, with the environment selecting.

A4.1.4 Homologous structures

Homologous structures have the same underlying structure because they were inherited from a common ancestor, even if they now do different jobs. The key example is the pentadactyl limb of tetrapods. A human arm, bat wing, whale flipper and horse leg all have one upper bone (humerus), two lower bones (radius and ulna), wrist bones, and digits. The bones differ in proportion: long finger bones support a bat’s wing membrane, short stout ones form a flipper. A shared plan serving different functions is best explained by descent from a common ancestor, adapted by selection in each lineage.

A4.1.5 Convergent evolution

Analogous structures do the same job but arose separately in different lineages. They arise by convergent evolution: unrelated lineages face similar selection pressures and evolve similar solutions. Examples: the wings of a bat and an insect (a bat wing is a modified pentadactyl limb with skin stretched over bones, an insect wing is a cuticle outgrowth), and the streamlined bodies and fins of sharks and dolphins. Check internal structure: different origins mean analogous.

A4.1.6 Speciation by splitting

Speciation is the formation of new species. Every new species arises by the splitting of a pre-existing species, so speciation increases the total number of species, and extinction decreases it. Gradual change in one lineage over time, with no split, is not speciation.

Worked example. A region has 1,200 species. Over a period, 35 new species form by splitting and 60 become extinct. Net change = 35 − 60 = −25, so 1,175 species remain. Extinction exceeds speciation, so diversity falls.

A4.1.7 Reproductive isolation and differential selection

Populations of one species diverge when gene flow between them stops. This is reproductive isolation. A common route is geographical isolation, such as a river, mountain range or sea. The separated populations experience different conditions, so differential selection favours different alleles in each. Mutation also occurs independently. Over many generations allele frequencies differ so much that the populations no longer interbreed successfully: they are separate species.

Example. Bonobos live south of the Congo River and common chimpanzees live north and east of it. The river separated an ancestral population. Different conditions on each side led to differential selection and divergence.

A4.1.8 Sympatric and allopatric speciation (HL only)

Isolation that stops interbreeding may be geographic, behavioural or temporal. Allopatric speciation happens when populations are physically separated, so geographic isolation stops gene flow first. Sympatric speciation happens in the same area, so isolation must be behavioural (for example, different courtship signals) or temporal (breeding at different times of day or year). Both need isolation, variation and divergence under selection.

A4.1.9 Adaptive radiation (HL only)

Adaptive radiation is the rapid evolution of many related species from one ancestor, each adapted to a different niche. It occurs where niches are vacant, such as on newly formed islands or after a mass extinction. Each species uses a different resource, so close relatives coexist without competing, increasing biodiversity.

A4.1.10 Barriers to hybridization (HL only)

Barriers keep the gene pools of species apart. Courtship behaviour is specific to a species, so an animal usually ignores the displays of other species. If hybridization does occur, hybrids may be sterile. A mule (horse × donkey) is sterile: a horse has 64 chromosomes and a donkey 62, so a mule has 32 + 31 = 63. These cannot all pair up in meiosis, so viable gametes are not formed. Alleles cannot pass between species through the hybrid.

A4.1.11 Abrupt speciation in plants (HL only)

Plants can form new species in one generation. Two species hybridize and the hybrid is sterile, as its chromosomes lack partners. Then polyploidy: the number doubles, each chromosome gains a partner and meiosis works.

Worked example (hypothetical). Species X has 2n = 14 and species Y has 2n = 12. Gametes have 7 and 6 chromosomes, so the hybrid has 13 chromosomes, which cannot pair into bivalents, so it is sterile. If chromosome number doubles to 26, each chromosome has a homologue, gametes form, and the plant is fertile. A cross with X gives offspring with unpaired chromosomes, so the new species is isolated. The knotweed or smartweed genus Persicaria contains many species formed by hybridization and polyploidy.

A4.2.1 Levels of biodiversity

Biodiversity is the variety of living things, seen at several levels. There are three: ecosystem diversity (variety of habitats and communities), species diversity (the number of species and their relative abundance) and genetic diversity (variety of alleles within a species).

A4.2.2 Species numbers now and in the past

Millions of species have been named and described, and many more remain undiscovered. Fossil evidence suggests there are more species alive now than at any time in the past. Classification is pattern recognition, and the same observations can be classified differently: “splitters” recognize more species than “lumpers”.

A4.2.3 Anthropogenic extinction

Use at least three case studies.

• North Island giant moa (Dinornis novaezealandiae): a terrestrial megafaunal bird, hunted by the first human settlers and extinct within a few centuries of their arrival.
• Caribbean monk seal (Neomonachus tropicalis): a marine species killed for oil and meat, and affected by overfishing of its reefs. The last confirmed sighting was in 1952, and it was declared extinct in 2008.
• A species from your own area. Choose one familiar to you. The thylacine (Tasmanian tiger) is one option: hunted, with habitat loss, and the last captive animal died in 1936.

A4.2.4 Causes of ecosystem loss

Study only human causes. Mixed dipterocarp forest in Southeast Asia has been lost through logging of tall timber trees, clearance for plantations such as oil palm, and fire. Add one case from your region.

Worked example. A forest reserve covers 3,200 km² in the first survey and 2,480 km² nine years later. Loss = 720 km². Percentage loss = 720 ÷ 3,200 × 100 = 22.5%. Mean loss = 80 km² per year.

A4.2.5 Evidence of the crisis

Evidence comes from reliable surveys of many habitats worldwide and from reports of IPBES, the intergovernmental biodiversity and ecosystem services science panel. Its 2019 assessment reported that about one million species face extinction and that the current extinction rate is at least tens to hundreds of times the average of the last 10 million years. Surveys must be repeated to show change in species richness (number of species) and evenness (how similar the abundances are).

Worked example. Site A has four species with 25 individuals each. Site B has four species with 70, 10, 10 and 10. Both have richness 4 and 100 individuals, but A is even and B is dominated by one species. If richness falls from 48 to 39 species between surveys, the change is (39 − 48) ÷ 48 × 100 = −18.8%.

Nature of science: evidence is stronger if peer-reviewed and published so methods can be checked. Citizen scientists add coverage but bring concerns such as variable identification skill.

A4.2.6 Causes of the crisis

Overarching cause: human population growth. Specific: hunting and other over-exploitation; urbanization; deforestation and land clearance for agriculture, causing loss of natural habitat; pollution; and pests, diseases and invasive alien species carried around the world by global transport.

A4.2.7 Conservation approaches

No single approach is enough, and different species need different measures.

• In situ: protecting species in their natural habitat, including managed nature reserves, rewilding (restoring wild species and processes) and repair of degraded ecosystems.
• Ex situ: zoos and botanic gardens, with germ plasm kept in seed or tissue banks.

Large mammals with big home ranges need in situ protection. A tiny population may need ex situ breeding as insurance.

A4.2.8 EDGE of Existence

The EDGE of Existence programme prioritizes species that are evolutionarily distinct and globally endangered. An evolutionarily distinct species has few close relatives, so it represents a long unique branch of the tree of life. Losing it removes much evolutionary history at once. Prioritizing which species to save raises ethical, political, cultural and economic questions, so it is debated.

Common errors

• Writing that individuals evolve. Populations evolve.
• Calling bat and insect wings homologous. They are analogous.
• Treating gradual change in one lineage as speciation.
• Saying a mule is sterile without referring to unpaired chromosomes.

Official syllabus

International Baccalaureate Organization, Biology guide, first assessment 2025, published February 2023. Sections A4.1 and A4.2.