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

These notes condense sections A4.1 (Evolution and speciation) and A4.2 (Conservation of biodiversity) of the IB Biology guide, first assessment 2025, for final-weeks recall. A4.1.8 to A4.1.11 are additional higher level and marked HL only; everything else is SL and HL. For full explanations and worked examples, read the study guide, then test yourself with the practice questions. Course pages: IB Biology hub, printable checklist and free 10-minute diagnostics.

Key definitions

Term	Meaning
Evolution	A genetic (heritable) change in the features of a population over generations
Homologous structure	Same basic structure from a shared ancestor, possibly different function
Analogous structure	Same function, different evolutionary origin
Convergent evolution	Similar selection pressures produce similar features in unrelated lineages
Speciation	Formation of a new species by splitting of an existing species
Reproductive isolation	No gene flow between populations
Differential selection	Different alleles favoured in different environments
Biodiversity	Variety of life at ecosystem, species and genetic levels
In situ	Conservation in the natural habitat
Ex situ	Conservation outside the natural habitat

Evidence for evolution (A4.1.1 to A4.1.5)

• Evolution is a change in a population. Acquired traits that are not genetic do not count. Lamarckism is rejected.
• Sequences: more differences in DNA, RNA or protein sequences means a more distant common ancestor. Fewer differences means closer relatives.
• Selective breeding: dog breeds from wolves and the varieties of Brassica oleracea show that variation is heritable and that populations change quickly under selection.
• Homologous: pentadactyl limb with humerus, radius and ulna, carpals and digits in human, bat, whale and horse.
• Analogous: bat and insect wings. Same function, different origin, so convergent evolution.

Method: working out relatedness from sequences

1. Align the sequences.
2. Count the differences for each pair.
3. Convert to a percentage if the sequences are different lengths or if asked.
4. Fewest differences means the most recent common ancestor.
5. State the conclusion in terms of ancestry, not “similarity of appearance”.

Reminder. Sequences of 12 amino acids that differ at 3 positions differ by 3 ÷ 12 × 100 = 25.0%. DNA can differ at positions where the protein does not, because the genetic code is degenerate.

Must-know distinction: homologous or analogous?

Feature	Homologous	Analogous
Ancestry	Shared	Separate
Function	Often different	Same
Internal structure	Same plan	Different plan
Explained by	Common descent	Convergent evolution

Speciation (A4.1.6 to A4.1.7)

• Every new species arises from a pre-existing species splitting.
• Speciation increases the number of species. Extinction decreases it.
• Gradual change in a single lineage is not speciation.

Method: explaining speciation in steps

1. A population is split (for example by a river).
2. Gene flow stops: reproductive isolation.
3. Different conditions mean differential selection, and mutations arise independently.
4. Allele frequencies diverge.
5. The populations no longer breed successfully with each other, so they are separate species.

Specific example. Bonobos live south of the Congo River and common chimpanzees live north of it. The river separated the ancestral population, and differential selection led to divergence.

HL only: speciation in more detail (A4.1.8 to A4.1.11)

Allopatric and sympatric (HL only).

Feature	Allopatric	Sympatric
Location	Separate areas	Same area
First barrier	Geographical	Behavioural or temporal
Common requirement	Reproductive isolation, variation, divergence under selection	Same

Isolation may take three forms: geographic, behavioural or temporal.

Adaptive radiation (HL only). Many related species evolve from one ancestor, each adapted to a different niche. It follows the appearance of vacant niches. Close relatives coexist without competing, so biodiversity increases.

Barriers to hybridization (HL only).

• Courtship behaviour is species-specific, so it usually prevents mating between species.
• Hybrids may be sterile. A mule (horse × donkey) is sterile, because 32 + 31 = 63 chromosomes cannot all pair in meiosis.
• Both stop alleles from mixing between species.

Abrupt speciation in plants (HL only). Hybridization gives a sterile hybrid. Polyploidy doubles the chromosomes, so they pair, and the plant is fertile. A new species can appear in one generation. Persicaria (knotweed or smartweed) has many species formed this way.

Biodiversity and extinction (A4.2.1 to A4.2.4)

• Three levels: ecosystem diversity, species diversity, genetic diversity.
• Millions of species are described, and many more are undiscovered. Fossils suggest more species are alive now than at any time in the past.
• “Splitters” recognize more species than “lumpers”, because one set of observations can be grouped in more than one way.
• Giant moa (North Island): terrestrial megafauna, hunted by early settlers.
• Caribbean monk seal: marine species, killed for oil and meat, and affected by overfishing. Last confirmed sighting 1952, declared extinct 2008.
• A local species of your choice: learn the cause of its loss.
• Ecosystem loss: mixed dipterocarp forest in Southeast Asia lost through logging, plantation clearance and fire. Add one local example.

The crisis, its causes and responses (A4.2.5 to A4.2.8)

• Evidence: repeated surveys across habitats worldwide, and IPBES reports. Repeats are needed to show change in species richness and evenness.
• Evaluating evidence: peer-reviewed and published sources let methods be checked. Citizen science widens coverage but raises concerns about identification and consistency.
• Causes: human population growth overall. Specifically over-exploitation, urbanization, deforestation and land clearance for agriculture, pollution, and pests, diseases and invasive alien species carried by global transport.
• Conservation: no single approach suffices. In situ: managed nature reserves, rewilding, repairing degraded ecosystems. Ex situ: zoos, botanic gardens, banks of seed or tissue (germ plasm).
• EDGE of Existence: prioritizes species that are both evolutionarily distinct and globally endangered, because losing them loses a large amount of evolutionary history. Prioritization is a matter for debate.

Reminder. Richness is the number of species. Evenness is how similar their abundances are. A change from 48 to 39 species is (39 − 48) ÷ 48 × 100 = −18.8%.

Small worked reminders

Percentage loss of habitat. A reserve shrinks from 3,200 km² to 2,480 km². Loss = 720 km². 720 ÷ 3,200 × 100 = 22.5%. Always divide by the original value.

Richness and evenness. Site A has four species with 25 individuals each. Site B has four species with 70, 10, 10 and 10. Richness is 4 in both. Site A is even; Site B is dominated by one species. Report both measures when comparing sites.

Polyploid chromosome count (HL only). Species X has 2n = 14 and Y has 2n = 12. The hybrid has 7 + 6 = 13 chromosomes and is sterile. After doubling it has 26, which can pair.

Command-word frames

• Distinguish: state a difference for both items, not only one.
• Explain: give the cause and the effect, linked by “so” or “because”.
• Outline: give the main points briefly.
• Evaluate: give strengths and limitations, then a judgement.
• Suggest: apply known biology to an unfamiliar case.

Quick self-test

1. Define evolution.
2. Why is a bodybuilder’s muscle growth not evolution?
3. Two protein sequences of 20 amino acids differ at 5 positions. Calculate the percentage difference.
4. Name the bones in the upper and lower pentadactyl limb.
5. A shark fin and a dolphin flipper look alike but have different origins. What type of structures are they?
6. A region has 800 species. 12 form by speciation and 30 go extinct. How many remain?
7. (HL only) State one way sympatric speciation differs from allopatric speciation.
8. (HL only) A hybrid plant has 9 chromosomes and is sterile. What number would a fertile polyploid have?
9. Name three levels of biodiversity.
10. Distinguish in situ from ex situ conservation, with an example of each.

Answers

1. A heritable change in the features of a population over generations.
2. It is an acquired change that is not inherited in the genes.
3. 5 ÷ 20 × 100 = 25.0%.
4. Humerus (upper); radius and ulna (lower).
5. Analogous structures (convergent evolution).
6. 800 + 12 − 30 = 782.
7. Sympatric speciation occurs in the same area, so isolation is behavioural or temporal rather than geographical.
8. 18, as chromosome doubling gives each chromosome a partner.
9. Ecosystem diversity, species diversity, genetic diversity.
10. In situ is in the natural habitat (nature reserve); ex situ is outside it (seed bank or zoo).

Where marks are usually lost

• Writing “individuals evolve” instead of populations.
• Calling analogous structures homologous, or the reverse.
• Describing similar appearance rather than shared ancestry as the evidence.
• Calling gradual change speciation.
• Leaving out reproductive isolation or differential selection from a speciation explanation.
• Writing “the mule is sterile because it is a hybrid” without the unpaired chromosomes.
• Listing causes of the crisis without linking each to a loss of habitat, species or alleles.
• Not stating richness and evenness separately.
• Naming a conservation approach without saying why it suits the species.
• Dividing by the later value instead of the original when finding a percentage change.
• Treating the EDGE programme as a list of the most endangered species only: distinctness matters too.
• Using a case study with no named cause, such as writing only that a species “was hunted”.

Official syllabus

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