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. Variation between individuals is considered a defining feature of living organisms because:
- all members of a species are genetically identical
- no two organisms are exactly alike
- only different species show any variation
- variation occurs only in sexual reproducers
Show the answer
Answer: B. Variation exists both between species and within a species; differences between individuals are a universal feature of life.
Question 2. A horse and a donkey can mate to produce a mule, but the mule is sterile. This shows that horses and donkeys are:
- the same species, since offspring are produced
- the same species, because they look similar
- different species — the offspring is sterile
- different genera that can never interbreed
Show the answer
Answer: C. Under the biological species concept the key test is fertile offspring. Because the mule is sterile, horse and donkey are separate species.
Question 3. Around a mountain, six neighbouring salamander populations form a ring. Each population can interbreed with its immediate neighbours, but the two populations that meet where the ring closes cannot. This is an example of:
- a ring species, awkward for the BSC
- two clearly separate species, unconnected
- a single population showing no variation
- chronospecies changing over geological time
Show the answer
Answer: A. In a ring species a continuous chain of interbreeding populations exists, yet the two populations that meet where the ring closes behave as separate species, so no single boundary line can be drawn.
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? (see A3.2)
- How does variation within a species provide the raw material for natural selection and the formation of new species? (see A4.1, 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, A3.2)
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.