Evolution, Biodiversity & Conservation

Complete Study Guide

Topics Covered

1. Water (A1.1)
2. Diversity of Organisms (A3.1)
3. Classification and Cladistics (A3.2) HL
4. Evolution and Speciation (A4.1)
5. Conservation of Biodiversity (A4.2)
6. Mixed Practice — Exam Style
7. May 2026 Prediction Questions

Aligned to IB Biology HL 2025 syllabus — Theme A

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Jump to section: Water · Diversity · Classification & Cladistics · Evolution & Speciation · Conservation · Mixed Practice · Predictions

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Section 1: Water (A1.1)

Water is arguably the most important molecule in biology. Its unique properties — all derived from hydrogen bonding — make it essential for life. This section focuses specifically on the Theme A syllabus requirements; the Biochemistry guide also introduces water’s role as a solvent in the context of biological molecules.

1.1 Hydrogen Bonding

Water ($\text{H}_2\text{O}$) is a polar molecule. The oxygen atom is more electronegative than hydrogen, creating partial charges: $\delta^-$ on oxygen and $\delta^+$ on each hydrogen.

These partial charges allow water molecules to form hydrogen bonds — weak electrostatic attractions between the $\delta^+$ hydrogen of one molecule and the $\delta^-$ oxygen of a neighbouring molecule. Each water molecule can form up to four hydrogen bonds simultaneously.

1.2 Thermal Properties in Detail

Specific heat capacity is the energy required to raise the temperature of $1 \text{ g}$ of a substance by $1 \text{ °C}$. Water’s value ($4.18 \text{ J g}^{-1}\text{K}^{-1}$) is exceptionally high because energy is used to break hydrogen bonds rather than increase kinetic energy.

Biological consequence: Large bodies of water (oceans, lakes) resist temperature fluctuations, providing stable habitats. Organisms with high water content (humans are ~60% water) resist rapid body temperature changes.

Latent heat of vaporisation is the energy required to convert $1 \text{ g}$ of liquid water to gas at its boiling point. Water’s high latent heat means that evaporating a small amount of water carries away a large amount of heat energy.

Biological consequence: Evaporative cooling is highly efficient — sweating in mammals, panting in dogs, and transpiration in plants all exploit this property.

1.3 Cohesion, Adhesion, and Transport

Cohesion (water-to-water attraction) creates surface tension — a “skin” at the water surface strong enough to support small organisms such as pond skaters.

Inside plants, cohesion is critical to the transpiration-adhesion-cohesion-tension mechanism:

1. Water evaporates from leaf mesophyll cells through stomata (transpiration).
2. This creates a tension (negative pressure) in the xylem.
3. Cohesion between water molecules transmits the tension down the xylem as a continuous column.
4. Adhesion between water molecules and the hydrophilic xylem walls prevents the column from pulling away.
5. Water is drawn up from the roots to replace what was lost.

1.4 Solvent Properties

Because water is polar, it dissolves:

• Ionic compounds (e.g. $\text{NaCl}$) — water molecules surround individual ions, with $\delta^-$ oxygen facing cations and $\delta^+$ hydrogen facing anions
• Polar molecules (e.g. glucose, amino acids) — hydrogen bonds form between water and polar groups on the solute

Hydrophobic (non-polar) substances such as lipids do not dissolve in water — they lack partial charges and cannot form hydrogen bonds with water.

1.5 Density Anomaly of Ice

Most substances are denser in the solid state. Water is anomalous: at $4\text{ °C}$, liquid water reaches maximum density. Below $4\text{ °C}$, hydrogen bonds lock molecules into an open lattice structure, making ice less dense than liquid water.

Biological significance: Ice floats, forming an insulating layer on the surface of lakes and oceans. Liquid water remains below, allowing aquatic organisms to survive through winter.

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Section 2: Diversity of Organisms (A3.1)

2.1 Domains and Kingdoms

Living organisms are classified into three domains based on fundamental cell structure and molecular evidence:

Within Domain Eukarya, organisms are further classified into kingdoms:

2.2 Binomial Nomenclature

All species are given a two-part Latin name following the system developed by Carl Linnaeus:

• The first name is the genus (capitalised)
• The second name is the species (lowercase)
• The full name is always italicised (or underlined when handwritten)

Example: Homo sapiens, Escherichia coli, Panthera leo

2.3 The Hierarchy of Taxa

Classification follows a nested hierarchy from broadest to most specific:

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

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Section 3: Classification and Cladistics (A3.2) HL

3.1 Phylogenetics and Cladograms

A cladogram is a branching diagram showing the evolutionary relationships between organisms based on shared derived characteristics. Each branch point (node) represents a common ancestor.

Key features of cladograms:

• Clades — a group consisting of an ancestor and all its descendants (a monophyletic group)
• Nodes — branch points representing speciation events
• Root — the common ancestor of all organisms in the cladogram
• Outgroup — a species included for comparison that is more distantly related

3.2 Homologous vs Analogous Structures

Homologous structures are anatomical features in different species that share a common evolutionary origin but may serve different functions. They provide evidence for divergent evolution.

• Example: The pentadactyl limb — the forelimbs of humans, whales, bats, and dogs all contain the same basic bone pattern (humerus, radius, ulna, carpals, metacarpals, phalanges) despite being used for grasping, swimming, flying, and running.

Analogous structures are features that serve similar functions but have different evolutionary origins. They result from convergent evolution — similar environmental pressures produce similar adaptations independently.

• Example: Wings of insects and wings of birds — both used for flight but structurally unrelated.

3.3 Molecular Evidence for Classification

Modern classification relies heavily on molecular evidence — comparisons of DNA base sequences, amino acid sequences, and ribosomal RNA.

Why molecular evidence is preferred:

• More objective than morphological comparisons
• Can be quantified precisely (percentage similarity)
• Less affected by convergent evolution
• Can be used even when organisms look very different externally

Key examples:

• rRNA sequences established the three-domain system (Archaea are molecularly closer to Eukarya than to Bacteria)
• Cytochrome c amino acid sequences — more similar between closely related species (human and chimpanzee differ by 0 amino acids; human and yeast differ by ~40)
• DNA hybridisation — heating double-stranded DNA from two species to measure how similar their sequences are

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Section 4: Evolution and Speciation (A4.1)

The meiosis-genetics guide introduces speciation through gene pools (D2.3). This section goes deeper into the evidence for evolution and the mechanisms of natural selection and speciation.

4.1 Evidence for Evolution

4.2 Natural Selection

Natural selection is the mechanism of evolution proposed by Darwin. It acts on phenotypic variation within a population.

The four conditions for natural selection:

1. Variation — individuals in a population show variation in their traits (caused by mutation and sexual reproduction)
2. Overproduction — more offspring are produced than can survive
3. Competition — individuals compete for limited resources (food, territory, mates)
4. Differential survival and reproduction — individuals with traits better suited to the environment are more likely to survive and reproduce, passing their alleles to the next generation

Over many generations, the frequency of advantageous alleles increases in the population — this is evolution by natural selection.

4.3 Gene Pools and Allele Frequencies

A gene pool is the total of all alleles for all genes in a population. Allele frequency is the proportion of a specific allele among all alleles for that gene.

Evolution can be defined as a change in allele frequencies in a population over time. Factors that change allele frequencies include:

• Natural selection — differential survival and reproduction
• Genetic drift — random changes in allele frequency, especially in small populations
• Gene flow — migration of alleles between populations
• Mutation — introduces new alleles

4.4 Types of Speciation

Speciation is the formation of new species through the evolution of reproductive isolation. Once populations can no longer interbreed to produce fertile offspring, they are considered separate species.

Allopatric speciation — step by step:

1. A geographic barrier (river, mountain range, ocean) splits a population
2. Gene flow between sub-populations ceases
3. Different selection pressures, mutations, and drift act on each group
4. Allele frequencies diverge over time
5. Reproductive isolation develops — if populations meet again, they can no longer interbreed

Sympatric speciation — polyploidy example:

1. An error in meiosis or mitosis doubles the chromosome number
2. The polyploid individual cannot produce fertile offspring with diploid members of the original species
3. If the polyploid can self-fertilise or mate with other polyploids, a new species arises in a single generation

4.5 Adaptive Radiation

Adaptive radiation is the rapid diversification of a single ancestral lineage into many new species, each adapted to a different ecological niche.

Conditions that promote adaptive radiation:

• Colonisation of a new, relatively empty habitat (e.g. volcanic islands)
• Mass extinction events that create vacant niches
• Evolution of a key innovation that opens new ecological opportunities

Classic example — Darwin’s finches:

• A single ancestral finch species colonised the Galápagos Islands
• Different islands had different food sources (seeds, insects, cacti)
• Natural selection favoured different beak shapes on each island
• Over time, 13+ species evolved with beaks adapted to specific diets

4.6 Punctuated Equilibrium vs Gradualism

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Section 5: Conservation of Biodiversity (A4.2)

5.1 Biodiversity Metrics

Biodiversity has three levels:

• Genetic diversity — variety of alleles within a species
• Species diversity — number and relative abundance of species in a community
• Ecosystem diversity — variety of habitats and ecosystems in a region

Species diversity has two components:

• Species richness — the total number of different species present
• Species evenness — how equally individuals are distributed among species

5.2 Threats to Biodiversity

5.3 Habitat Fragmentation and Edge Effects

Habitat fragmentation occurs when a large, continuous habitat is divided into smaller, isolated patches by human activities (roads, agriculture, urban development).

Consequences:

• Smaller populations in each fragment → increased genetic drift and inbreeding
• Reduced gene flow between fragments → reduced genetic diversity
• Species requiring large ranges (top predators) cannot be sustained
• Edge effects become proportionally greater in smaller fragments

Edge effects are changes in environmental conditions (temperature, humidity, light, wind) at the boundary between a habitat and the surrounding modified landscape. Species adapted to the interior of a habitat may not survive at the edge.

5.4 In Situ vs Ex Situ Conservation

5.5 CITES

CITES (Convention on International Trade in Endangered Species) is an international agreement that regulates the trade of wildlife and wildlife products.

• Appendix I: Species threatened with extinction — trade is banned except in exceptional circumstances (e.g. African elephant ivory, great apes)
• Appendix II: Species not yet threatened but trade must be controlled to prevent decline (e.g. many corals, orchids)
• Appendix III: Species protected in at least one country that has asked others for help controlling trade

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Mixed Practice — Exam Style

10 MCQs covering all sections (IB Paper 1 style)

1. [Water] Which property of water is most directly responsible for the cooling effect of sweating?

   A. High specific heat capacity

   B. High latent heat of vaporisation

   C. Cohesion between water molecules

   D. Solvent properties of water

2. [Water] Ice floats on liquid water because:

   A. Ice molecules have less kinetic energy than liquid water molecules

   B. Hydrogen bonds in ice form an open lattice structure, making ice less dense

   C. Water molecules in ice are smaller than in liquid water

   D. Ice contains fewer hydrogen bonds than liquid water

3. [Diversity] Which feature distinguishes Archaea from Bacteria?

   A. Archaea have a membrane-bound nucleus

   B. Archaea have 80S ribosomes

   C. Archaea lack peptidoglycan in their cell walls

   D. Archaea are all heterotrophic

4. [Classification] Two species share a homologous structure. This indicates:

   A. They evolved in similar environments

   B. They share a common ancestor

   C. They have identical DNA sequences

   D. They occupy the same ecological niche

5. [Classification] HL In a cladogram, a clade is defined as:

   A. Any two species that are positioned next to each other

   B. An ancestor and all of its descendants

   C. A group of species that look similar

   D. The outgroup and its closest relative

6. [Evolution] Which of the following is NOT required for natural selection to occur?

   A. Variation in traits within a population

   B. Overproduction of offspring

   C. Inheritance of acquired characteristics

   D. Differential survival and reproduction

7. [Evolution] Adaptive radiation is best described as:

   A. The extinction of multiple species at the same time

   B. The gradual change of one species into another

   C. The rapid diversification of one lineage into many species occupying different niches

   D. The convergent evolution of unrelated species

8. [Conservation] Simpson’s Diversity Index is $0.85$ for Habitat X and $0.35$ for Habitat Y. Which conclusion is correct?

   A. Habitat X has fewer species than Habitat Y

   B. Habitat X has greater species diversity than Habitat Y

   C. Habitat Y has greater species evenness

   D. Both habitats have the same species richness

9. [Conservation] Which is an example of ex situ conservation?

   A. Establishing a marine protected area

   B. Creating a wildlife corridor between forest fragments

   C. Maintaining a captive breeding programme in a zoo

   D. Banning hunting within a national park

10. [Conservation] Habitat fragmentation leads to reduced biodiversity primarily because:

    A. Fragments receive more rainfall than continuous habitats

    B. Smaller, isolated populations experience increased genetic drift and inbreeding

    C. Edge effects cause fragments to become warmer, benefiting all species equally

    D. Fragmentation increases gene flow between populations

Show Answers

1. B — High latent heat of vaporisation. Evaporation of sweat requires significant energy to break hydrogen bonds between water molecules, drawing heat away from the skin. High specific heat capacity (A) buffers temperature changes but does not directly explain the cooling effect of evaporation.

2. B — In ice, hydrogen bonds form a regular open lattice with greater spacing between molecules than in liquid water, making ice less dense. Option D is the opposite — ice has more ordered hydrogen bonds than liquid water.

3. C — Archaea lack peptidoglycan in their cell walls, while Bacteria have peptidoglycan. Both are prokaryotic (no nucleus), and both have 70S ribosomes. Archaea also have distinctive ether-linked membrane lipids.

4. B — Homologous structures (same basic anatomy, potentially different function) indicate descent from a common ancestor (divergent evolution). Option A describes analogous structures (convergent evolution).

5. B — A clade is a monophyletic group consisting of an ancestor and ALL of its descendants. Species adjacent on a cladogram are not necessarily in the same clade, and similarity in appearance (C) does not define a clade.

6. C — Inheritance of acquired characteristics is a Lamarckian concept and is NOT part of Darwinian natural selection. Natural selection requires heritable genetic variation, overproduction, and differential survival.

7. C — Adaptive radiation is the rapid diversification of a single ancestral lineage into many species, each adapted to a different niche. It is distinct from convergent evolution (D), which involves unrelated species becoming similar.

8. B — A higher Simpson’s Diversity Index indicates greater species diversity (more species and/or more even distribution). $D = 0.85$ is much higher than $D = 0.35$.

9. C — Captive breeding in a zoo is ex situ (off-site) conservation. Options A, B, and D are all in situ (on-site) strategies that protect species within their natural environment.

10. B — Fragmentation creates smaller, isolated populations that are more susceptible to genetic drift and inbreeding, reducing genetic diversity and adaptive potential. Reduced gene flow (not increased, as in D) compounds the problem.

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May 2026 Prediction Questions

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IB Biology HL — Evolution, Biodiversity & Conservation — Complete Study Guide — 2025 Syllabus — Good luck!