Evolution and Biodiversity (SL) — IB Biology SL SL Study Guide

For: IB Biology SL candidates sitting IB Biology SL.

Covers: All core SL content for Evolution and Biodiversity, including evidence for evolutionary change, natural selection and adaptation, Linnaean classification of life, and cladistics with phylogenetic tree interpretation.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Biology SL style for educational use. They are not reproductions of past IBO papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official IBO mark schemes for grading conventions.

1. What Is Evolution and Biodiversity (SL)?

Evolution refers to the cumulative change in the heritable characteristics of a population over successive generations, driven by natural selection and other mechanisms, while biodiversity describes the total variety of living organisms on Earth across all levels of biological organization, from genetic variation within species to entire ecosystems. This is Topic 5 in the IB Biology SL syllabus, worth ~10% of your final exam marks, and is tested in both Paper 1 multiple-choice questions and Paper 2 short and extended response questions. Examiners frequently ask for real-world examples of evolutionary processes, so memorizing the case studies included in this guide will directly help you gain marks.

2. Evidence for evolution

Evolution is a widely accepted scientific theory because it is supported by multiple independent lines of empirical evidence, outlined below:

1. Fossil record: Fossils are preserved remains or traces of past organisms, dated via radiometric testing to reveal a chronological sequence of species appearance. For example, the 55-million-year fossil record of horses shows gradual, consistent changes: from the small, forest-dwelling Hyracotherium (4 toes on front feet, low-crowned teeth for browsing leaves) to modern Equus (1 toe per foot, high-crowned teeth for grazing grass), matching the global shift from forest to grassland habitats.
2. Selective breeding (artificial selection): Humans have intentionally selected for desirable heritable traits in domesticated species for thousands of years, leading to dramatic phenotypic change. All modern dog breeds, from Chihuahuas to Great Danes, are descended from the gray wolf (Canis lupus), demonstrating that populations can change rapidly in response to selection pressure.
3. Homologous structures: Anatomical features with the same basic structural plan, derived from a common ancestor, even if they have different functions in modern species. The pentadactyl (5-digit) limb is a classic example: it has the same core bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges) in human hands (grasping), bat wings (flying), whale flippers (swimming), and horse legs (running).
4. Molecular evidence: All living organisms use the same genetic code, and conserved proteins (e.g., cytochrome c, used in cellular respiration) have highly similar amino acid sequences in closely related species. Humans and chimpanzees have only 1 difference in the 104 amino acids of cytochrome c, while humans and rhesus monkeys have 12 differences, confirming a closer evolutionary relationship between humans and chimps.

Worked exam example (2 marks): Question: Outline one piece of evidence from selective breeding that supports evolution. Full-mark answer: (1) All domestic dog breeds are derived from the gray wolf, with thousands of years of human selection for traits like size, coat type, and behavior producing hundreds of distinct breeds. (2) This demonstrates that heritable traits in a population can change significantly in response to selection pressure, which is the core mechanism of evolution.

3. Natural selection and adaptation

Natural selection, first proposed by Charles Darwin and Alfred Russel Wallace in 1858, is the primary mechanism of evolution. It follows four non-negotiable postulates:

1. Overproduction of offspring: All species produce more offspring than can survive to reproductive age, leading to competition for limited resources (food, shelter, mates).
2. Heritable variation within populations: Random mutations and sexual reproduction (crossing over in meiosis, random fertilization) produce genetic and phenotypic variation between individuals in a population.
3. Differential survival and reproduction: Individuals with adaptations (heritable traits that increase fitness, or survival/reproductive success) in their local environment are more likely to survive and produce more offspring than individuals with less favorable traits.
4. Inheritance of favorable traits: Adaptations are passed to offspring, so the frequency of favorable alleles increases in the population over successive generations, leading to evolutionary change.

A common exam case study is antibiotic resistance in bacteria: random mutations produce rare bacterial individuals with alleles that allow them to break down or avoid the effects of an antibiotic. When the antibiotic is applied, non-resistant bacteria are killed, leaving resistant individuals to reproduce rapidly without competition. The resistance allele is passed to offspring, so the frequency of resistance in the population increases over generations, leading to widespread antibiotic-resistant strains.

Worked exam example (3 marks): Question: Explain how natural selection leads to the high frequency of dark-colored peppered moths in polluted industrial areas of 19th-century England. Full-mark answer: (1) Pre-existing random mutation produced two heritable color variants in peppered moth populations: light speckled and dark melanic. (2) In polluted areas, soot killed light-colored lichen on tree trunks and blackened bark, so dark moths were more camouflaged from bird predators, giving them higher survival and reproduction rates than light moths. (3) The dark color allele was passed to offspring, so the frequency of dark moths increased from <2% to >90% of the population in 50 years.

4. Classification of biodiversity

Taxonomy is the science of naming, describing, and classifying living organisms, using the hierarchical Linnaean system, with seven core taxa ordered from most general (largest group) to most specific (smallest group):

Mnemonic: King Philip Came Over For Good Soup = Kingdom → Phylum → Class → Order → Family → Genus → Species

A species is defined as a group of organisms that can interbreed to produce fertile, viable offspring, and are reproductively isolated from other such groups. All species are named using binomial nomenclature: a two-part name Genus species, where the genus name is capitalized, the species name is lowercase, and the full name is italicized (or underlined if handwritten). For example, humans are Homo sapiens, gray wolves are Canis lupus.

In 1990, the three-domain system was introduced based on ribosomal RNA sequence data, replacing the older five-kingdom system as the highest level of classification:

1. Archaea: Prokaryotic organisms with no peptidoglycan in their cell walls, often found in extreme environments (hot springs, deep-sea hydrothermal vents).
2. Bacteria: Prokaryotic organisms with peptidoglycan in their cell walls, including common pathogens like E. coli and Streptococcus.
3. Eukarya: Eukaryotic organisms with a nucleus and membrane-bound organelles, including the kingdoms Animalia, Plantae, Fungi, and Protista.

Worked exam example (2 marks): Question: List the three domains of life, and identify one feature that distinguishes Archaea from Bacteria. Full-mark answer: (1) The three domains are Archaea, Bacteria, Eukarya. (2) Archaea have no peptidoglycan in their cell walls, while Bacteria do.

5. Cladistics and phylogenetic trees

Cladistics is a modern method of classification that groups organisms based on shared derived characteristics (synapomorphies) inherited from a common ancestor, rather than superficial physical similarity. A clade is a monophyletic group, meaning it includes a single ancestral species and all of its descendants, with no excluded members. A synapomorphy is a trait unique to a specific clade: for example, hair and mammary glands are synapomorphies of the mammal clade, as no non-mammal species has these traits.

A phylogenetic tree (or cladogram) is a diagram that shows evolutionary relationships between groups of organisms:

• Branching points (nodes) represent a common ancestor that split into two or more descendant lineages.
• The more closely related two species are, the more recently they shared a common ancestor, so they are placed closer together on the tree.
• Molecular data (DNA or amino acid sequences) is the most reliable evidence for building cladograms, as neutral mutations accumulate at a relatively constant rate (the molecular clock hypothesis), allowing scientists to estimate the timing of lineage divergence.

For example, if you compare the amino acid sequence of hemoglobin for four species: human and chimp have 0 differences, human and gorilla have 1 difference, human and rhesus monkey have 8 differences, the cladogram will have rhesus monkey branching off first, then gorilla, then human and chimp sharing the most recent common ancestor.

Worked exam example (3 marks): Question: Outline two pieces of information that can be deduced from a cladogram. Full-mark answer: (1) The relative evolutionary relatedness of species: species that share a more recent node (common ancestor) are more closely related. (2) The order in which new traits evolved: derived traits that appear on a branch before a node are shared by all species that descend from that node.

6. Common Pitfalls (and how to avoid them)

Examiners report these are the most frequent mark-losing mistakes on this topic:

1. Wrong move: Claiming selection pressure causes mutations (e.g., "bacteria mutate to become resistant to antibiotics"). Why students do it: Confusing cause and effect, assuming the environment induces adaptive mutations. Correct move: Always state that mutations are random and pre-existing; the selection pressure only favors survival of individuals with the favorable mutation, it does not cause the mutation.
2. Wrong move: Listing the Linnaean hierarchy in reverse order (species first, kingdom last). Why students do it: Misremembering the mnemonic, or misreading the question's request for order from most to least specific. Correct move: Use the "King Philip" mnemonic, and double check the question's required order before writing your answer.
3. Wrong move: Citing analogous structures (e.g., bird wings and insect wings) as evidence for common ancestry. Why students do it: Confusing homologous (same structure, common ancestor, different function) and analogous (same function, different structure, convergent evolution, no common ancestor) structures. Correct move: Only use homologous structures as evidence for common ancestry; analogous structures are evidence for convergent evolution, not shared descent.
4. Wrong move: Stating that individual organisms evolve. Why students do it: Confusing individual acclimatization with population-level evolutionary change. Correct move: Evolution is defined as change in the heritable characteristics of a population over generations; individuals cannot evolve, as their genetic makeup does not change during their lifetime.
5. Wrong move: Writing binomial names with both words capitalized, or not italicizing them. Why students do it: Forgetting binomial nomenclature rules. Correct move: Capitalize only the genus name, use lowercase for the species name, and italicize the full name (underline if handwritten) to get full marks for naming questions.

7. Practice Questions (IB Biology SL Style)

Question 1 (Paper 1 Multiple Choice, 1 mark)

Which of the following provides the strongest evidence for a close evolutionary relationship between two species? A. Similar habitat B. Identical amino acid sequence of a conserved protein C. Similar body shape D. Similar diet

Worked solution: Correct answer = B. Molecular sequence data directly reflects genetic similarity inherited from a common ancestor, making it the most reliable evidence for relatedness. A, C, and D are incorrect because similar habitat, body shape, or diet can result from convergent evolution, not shared ancestry. 1 mark for B, 0 for all other options.

Question 2 (Paper 2 Short Answer, 3 marks)

(a) Define the term evolution (1 mark). (b) Outline one piece of fossil evidence that supports evolution (2 marks).

Worked solution: (a) Full mark answer: Evolution is the cumulative change in the heritable characteristics of a population over successive generations. 1 mark for this exact definition; 0 marks if you mention individuals instead of populations, or non-heritable changes. (b) Full mark answer: (1) The horse fossil record shows a 55-million-year sequence of gradual changes, including increasing body size, reduced toe number from 4 to 1, and higher-crowned teeth adapted for grazing grass. (2) This sequence matches the global shift from forest to grassland habitats, showing that populations adapted to changing selection pressures over time, which is consistent with evolutionary theory. 1 mark per point, maximum 2 marks.

Question 3 (Paper 2 Extended Response, 4 marks)

Explain how natural selection leads to the development of adaptive traits in a population, using a named example.

Worked solution: Any valid example (antibiotic resistance, peppered moths, giraffe neck length) is acceptable, as long as it links to all four postulates of natural selection. Full mark answer for antibiotic resistance: (1) Random mutations in bacterial populations produce rare individuals with alleles that confer resistance to a specific antibiotic. (2) Bacteria produce far more offspring than can survive, leading to competition for resources. (3) When the antibiotic is applied, non-resistant bacteria are killed, leaving resistant individuals to survive and reproduce at higher rates. (4) The resistance allele is passed to offspring, so the frequency of resistance in the population increases over generations, making the antibiotic ineffective. 1 mark per point, maximum 4 marks.

8. Quick Reference Cheatsheet

9. What's Next

This topic forms the foundational context for multiple other IB Biology SL topics: Topic 4 (Ecology), where evolutionary adaptations determine species niches and community interactions; Topic 6 (Human Physiology), where you will learn about the evolution of antibiotic resistance in human pathogens; and Option C (Ecology and Conservation), where biodiversity measurement and conservation efforts rely on evolutionary classification frameworks. Understanding evolution will also help you contextualize every biological trait you learn about across the syllabus, as all features of living organisms are products of evolutionary processes.

If you struggle with any part of this topic, from interpreting phylogenetic trees to memorizing classification rules, you can ask Ollie, our AI tutor, for personalized explanations, additional practice questions, or step-by-step walkthroughs of exam problems. You can also find more study resources and official past paper practice for IB Biology SL on the homepage.