IB Biology · Theme A: Unity and diversity · SL and HL
A3.1 Diversity of organisms
A one-page summary of A3.1 Diversity of organisms, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What is a species?
What patterns are seen in the diversity of genomes within and between species?
What A3.1 covers
What makes a species?
- A3.1.1Variation is a defining feature of life
- A3.1.1…and some traits fall into categories
- A3.1.2Species as groups with shared traits
- A3.1.3One name, recognised worldwide
- A3.1.4The biological species concept
- A3.1.5Where the boundary blurs
- A3.1.5Related, but reproductively isolated
Chromosomes and genomes
- A3.1.6Chromosome numbers differ by species
- A3.1.7Reading a karyogram
- A3.1.8Gene, allele, genome
- A3.1.9The C-value paradox
- A3.1.10 · SkillComparing genome sizes
- A3.1.11Uses of whole genome sequencing
Species boundaries and modern tools
- A3.1.12 · HLWhen the BSC breaks down
- A3.1.12 · HLBacteria break the rule twice over
- A3.1.13 · HLChromosome number as a species trait
- A3.1.14 · HL · SkillBuilding a dichotomous key
- A3.1.15 · HLSpecies detection from environmental DNA
A3.1 Diversity of organisms: summary
Species concepts
- Morphological: grouped by appearance — look-alikes can still differ genetically.
- Biological (BSC): interbreed to produce fertile offspring — horse × donkey mule is sterile.
Fuzzy boundaries
- Ring species: every neighbour interbreeds except where the ring closes.
- Chronospecies: fossil lineage split into species is unavoidably arbitrary.
Chromosomes
- Number varies by species: human 46, chimp 48, always even.
- Human chr 2 = ancestral ape chromosomes 12 + 13, fused.
Genomes
- Gene / allele / genome / SNP — one product, one version, all DNA, one-base difference.
- C-value paradox: genome size and gene number are only loosely related.
HL · Species boundaries
- Asexual organisms: no interbreeding, so no BSC test.
- Bacteria: horizontal gene transfer blurs lineages further.
HL · Modern tools
- Chromosome number as extra evidence for a species split.
- Dichotomous keys and eDNA barcoding identify species by feature or by trace DNA.
Key terms
- Species
- Under the biological species concept, a group able to interbreed and produce fertile offspring.
- Binomial name
- A two-part scientific name (genus + species epithet) unique to one species.
- Karyotype / karyogram
- A cell's chromosome complement / the ordered image of its chromosomes.
- Gene / allele / genome
- A DNA sequence coding for a product / one version of a gene / an organism's entire DNA.
- SNP
- Single-nucleotide polymorphism — a one-base difference in DNA sequence between individuals.
- Non-coding DNA
- DNA that doesn't code for protein, including introns and repeated sequences.
- Horizontal gene transfer
- Movement of DNA directly between cells that aren't parent and offspring, common in bacteria.
- Dichotomous key
- An identification tool built from a series of two-choice steps based on observable features.
- Environmental DNA (eDNA)
- Genetic material shed into the environment and recoverable without capturing the organism.
- DNA barcoding
- Comparing a short standardised DNA region against a reference database to identify species.
Sample exam questions
Three of the 46 multiple-choice questions for A3.1. Try each one before opening the answer.
Question 1. Which statement about variation between organisms is correct?
- Identical twins show no variation at all.
- No two individuals are identical in all traits.
- Variation is found only between different species.
- Variation is found only in animals.
Show the answer
Answer: B. Variation between and within species is a defining feature of life.
Question 2. Human chromosome 2 has banding patterns that match two separate chimpanzee chromosomes. Which hypothesis does this support?
- Two ancestral chromosomes fused to form it.
- Chimpanzees evolved from humans.
- Humans have more genes than chimpanzees.
- Chromosome 2 split into two in chimpanzees.
Show the answer
Answer: A. Fusion of two ancestral chromosomes explains why humans have 46 chromosomes and chimpanzees 48.
Question 3. Use the key to identify a tree whose leaves are not divided into leaflets, are not lobed and have no spines.
- Oak
- Holly
- Beech
- Ash
Show the answer
Answer: C. Couplet 1b leads to 3; 3b leads to 5; 5b gives beech.
Linking questions
Questions that connect A3.1 to other parts of the course, the kind that come up in Paper 2.
- How are individual species grouped into larger categories such as genus, family and higher taxa?
- How does variation within a species provide the raw material for natural selection and the formation of new species? (see D1.3)
- What is the molecular origin of the mutations that create variation between and within species? (see D1.3, A1.2)
- How does comparing DNA base sequences help resolve cases where the biological species concept is hard to apply? (see A1.2)
Practise A3.1
Study notes, every question and full markschemes for A3.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.