Classification and Biodiversity

IB Biology SL· Theme C: Interactions and Interdependencies, Unit 3· 25 min read

1. Modern Classification and the Three-Domain System★★☆☆☆⏱ 6 min

Classification has evolved from early Linnaean systems based on observable physical traits to modern systems based on evolutionary relatedness and genetic evidence. This shift led to the widespread adoption of the three-domain system, which replaced the older five-kingdom classification framework.

📘 Definition

Three-Domain System

The highest modern taxonomic classification that divides all life into three groups based on ribosomal RNA sequences and genetic differences.

Example:

The three domains are Bacteria, Archaea, and Eukarya.

📐 Worked Example

An unicellular organism has no nucleus and a cell wall made of peptidoglycan. Identify which domain it belongs to, justifying your answer.

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1. Recall the defining features of each domain:
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- Bacteria: Unicellular prokaryotes with cell walls containing peptidoglycan
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- Archaea: Unicellular prokaryotes with cell walls that lack peptidoglycan
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- Eukarya: All organisms with eukaryotic cells that have a nucleus and membrane-bound organelles
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1. Match the given traits to the domain definitions: the organism is prokaryotic (no nucleus) and has peptidoglycan in its cell wall.
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1. Conclusion: The organism belongs to the domain Bacteria.

Exam tip:

Always remember the difference in cell wall composition between Bacteria and Archaea — this is a common multiple choice question.

2. Cladistics and Cladogram Interpretation★★★☆☆⏱ 8 min

Cladistics is the modern approach to classification that groups organisms into clades based on shared derived characteristics inherited from a common ancestor, rather than just shared physical traits. Cladograms are branching diagrams used to visualize these hypothesized evolutionary relationships.

📘 Definition

Cladogram

A diagram where each branching point (node) represents a common ancestor, and branches represent descendant lineages.

📐 Worked Example

In a cladogram, the root splits into taxon A, and a separate clade. That clade splits into taxon B, and a sub-clade containing taxon C and taxon D. Which pair of taxa is most closely related?

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1. The closeness of relationship between taxa depends on how recently they shared a common ancestor.
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1. Nodes (branch points) mark common ancestors. More recent nodes mean more closely related lineages.
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1. C and D share their most recent common ancestor at the youngest node in the cladogram.
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1. B shares an older common ancestor with C and D, and A shares the oldest common ancestor with all other taxa.
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Conclusion: C and D are the most closely related pair.

3. Measuring Biodiversity with Simpson's Diversity Index★★★☆☆⏱ 7 min

Biodiversity describes the variety of life in a given area. At the species level, it is measured by two components: richness (number of different species present) and evenness (how evenly individuals are distributed across species). Simpson's Diversity Index accounts for both components.

📘 Definition

Simpson's Diversity Index

D = 1 - \frac{\sum n(n-1)}{N(N-1)}

A quantitative measure where $D$ ranges from 0 (low diversity, only one species) to 1 (very high diversity).

Example:

A forest with $D = 0.85$ is far more diverse than a monoculture crop field with $D = 0.02$ .

📐 Worked Example

A 100 m² quadrat contains 50 individuals of species A, 30 of species B, and 20 of species C. Calculate Simpson's Diversity Index for this community.

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1. Calculate total number of individuals $N$ :
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$N = 50 + 30 + 20 = 100$
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1. Calculate $n (n - 1)$ for each species:
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$A: 50 \times 49 = 2450,\quad B: 30 \times 29 = 870,\quad C: 20 \times 19 = 380$
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1. Sum all $n (n - 1)$ values:
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$\sum n(n-1) = 2450 + 870 + 380 = 3700$
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1. Calculate $N (N - 1)$ :
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$N(N-1) = 100 \times 99 = 9900$
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1. Substitute into the formula to find $D$ :
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$D = 1 - \frac{3700}{9900} = 1 - 0.374 = 0.626$

Exam tip:

Always show all working steps for calculation questions. You can earn partial marks for correct method even if your final answer is wrong.

4. Biodiversity and Conservation Significance★★☆☆☆⏱ 4 min

Accurate classification and measurement of biodiversity is the foundation of effective conservation. Conservation groups can only prioritize and protect endangered ecosystems and species if they correctly identify what organisms are present and how diverse the community is.

Ecosystems with higher native biodiversity are generally more resilient to disturbances like climate change, invasive species, and natural disasters. This makes maintaining high biodiversity a key goal for global conservation efforts.

✓ Quick check

Test your understanding:

What advantage does Simpson's Diversity Index have over just counting species richness?
- It is faster to calculate
- It accounts for evenness of species abundance
- It does not require counting individual organisms
- It always gives higher values
Reveal answer
1 —
Correct! Species richness only tells you how many different species are present. Simpson's index adds information about how evenly individuals are distributed between species, giving a more complete picture of biodiversity.

5. Common Pitfalls

Wrong move:

Claiming the traditional Linnaean five-kingdom system is the standard modern classification

Why:

Modern classification is based on genetic evidence, and the three-domain system is the current standard

Correct move:

Reference the three-domain system when asked about modern classification of life

Wrong move:

Judging relatedness from the spacing of tips on a cladogram

Why:

Tip spacing is usually arbitrary; only branching order (nodes) indicates relatedness

Correct move:

Trace back from tips to find the most recent common node to judge relatedness

Wrong move:

Interpreting a low Simpson's D value as high biodiversity

Why:

Simpson's D ranges from 0 to 1, where higher values equal higher biodiversity

Correct move:

Remember: higher $D$ = more diverse, lower $D$ = less diverse

Wrong move:

Confusing Archaea with Eukarya because both lack peptidoglycan cell walls

Why:

Archaea are prokaryotes with no nucleus, while Eukarya are all eukaryotes with a nucleus

Correct move:

Always check for the presence of a nucleus first when classifying domains

6. Quick Reference Cheatsheet

Concept	Key Information	Exam Tip
Three Domains	Bacteria: prokaryotes, peptidoglycan cell wall; Archaea: prokaryotes, no peptidoglycan; Eukarya: eukaryotes with nucleus	Check for nucleus first, then cell wall composition
Cladograms	Clade = common ancestor + all descendants; branching = common ancestor	Only branching order matters, not tip spacing
Simpson's D	$D = 1 - \frac{\sum n ( n - 1 )}{N ( N - 1 )}$ ; 0 = low diversity, 1 = high diversity	Always show full working for calculation marks
Biodiversity Components	Richness = number of species; Evenness = distribution of individuals	Simpson's D accounts for both, richness does not

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 1
Identify domain from organism characteristics
2023 · 2
Calculate Simpson's diversity index
2022 · 1
Interpret cladogram relatedness

Going deeper

Official SyllabusIB Biology SL 2025 SyllabusFull official syllabus reference for this topic

What's Next

Classification and biodiversity form the foundation of all ecological study, which is the core of Theme C: Interactions and Interdependencies in IB Biology SL. Now that you understand how we classify organisms and measure biodiversity, you can explore how different species interact with each other and their physical environment in ecological communities. Changes to biodiversity from human activity and climate change directly impact ecosystem stability, so the concepts you learned here connect directly to conservation management and global sustainability efforts. Mastering classification also supports your understanding of evolutionary relationships across all biology topics, from plant physiology to animal behavior.