IB DP Biology: Diversity of organisms: species, naming, karyotypes and genomes (DP Biology) – Revision Notes

These notes condense sub-topic A3.1, Diversity of organisms, from the IB Biology guide, first assessment 2025 (statements A3.1.1 to A3.1.11 for SL and HL; A3.1.12 to A3.1.15 are HL only and labelled). For full explanations read the study guide, then test yourself with the practice questions. Related: the Theme A revision notes, the course hub, the checklist and the free 10-minute diagnostics.

Key definitions

Term	Meaning
Variation	Differences in traits between individuals; no two are identical in every trait
Species (morphological)	Group of organisms sharing traits; Linnaeus’ original idea
Species (biological)	Group that can breed and produce fertile offspring
Genus	First part of a binomial name; capital letter
Speciation	Division of one species into two or several, usually gradual
Karyotyping	Preparing a karyogram of an organism’s chromosomes
Karyogram	Chromosomes of one cell arranged in homologous pairs
Genome	All the genetic information of an organism
SNP	Single-nucleotide polymorphism: a base that differs between individuals at one position
Whole genome sequencing	Finding the base order of an entire genome
Dichotomous key (HL only)	Numbered pairs of contrasting statements leading to identification
eDNA (HL only)	DNA shed by organisms into water, soil or air

Names and species concepts (A3.1.1 to A3.1.5)

• No two individuals are identical; this variation characterises life and underlies naming and classifying.
• Binomial: Genus species. Genus capital, species lower case, italics in print, underlined by hand. Same genus means similar traits.
• Biological species concept: breed together and produce fertile offspring.
• Challenges: asexual organisms, fossils, populations that never meet, fertile hybrids between clearly different forms.
• Other definitions exist (morphological, ecological, shared ancestry and DNA sequence).
• Speciation is gradual, so a boundary between two diverging populations may be an arbitrary decision.

Must-know distinctions

Morphological concept	Biological concept
Shared observable traits	Interbreeding with fertile offspring
Works for fossils and preserved specimens	Needs living, sexually reproducing organisms
Risk: variation within a species, look-alike species	Risk: untestable where populations are isolated

Method to answer “is it one species or two?”: (1) state the definition you apply; (2) apply it to the evidence given; (3) note a limit of that definition; (4) conclude.

Chromosomes and karyograms (A3.1.6 to A3.1.7)

• Humans 2n = 46 (23 pairs); chimpanzees 2n = 48 (24 pairs). Other values are not required.
• Diploid numbers are even because chromosomes exist as homologous pairs.
• Classify chromosomes by banding pattern, length and centromere position.
• Magnification: actual size = image size ÷ magnification. Check units (1 mm = 1000 µm).

Method: pairing chromosomes in a karyogram

1. Find chromosomes of similar length.
2. Check the centromere position.
3. Match the banding pattern.
4. Arrange in pairs, longest first; sex chromosomes last.
5. Count: extra or missing chromosomes show as a pair with three or one.

Chromosome 2 evidence

• Banding of human chromosome 2 matches two ape chromosomes end to end.
• Telomere-type sequence in the middle of chromosome 2.
• A remnant of a second centromere on the long arm.
• 48 − 1 fusion = 46.
• Testable: it makes predictions evidence could contradict. Non-testable: gives no prediction that data could refute, for example a claim about purpose.

Worked reminder. Chromosome 21 mm long at ×3000: 21 ÷ 3000 = 0.007 mm = 7.0 µm.

Genomes (A3.1.8 to A3.1.11)

Point	Content
Within a species	Most of the genome is shared; SNPs give diversity
Between species	Size and base sequence differ; much more than within a species
Size	Total amount of DNA, often in Mb
Size and complexity	No close match: large genomes in some plants, small genomes in some complex animals
Database task	Extract sizes for taxonomic groups, then compare with complexity

Worked reminder. Roughly 1 base in 1000 differs between two people. Genome 3100 Mb: 3.1 × 10⁹ × 0.001 = 3.1 × 10⁶ differences.

Worked reminder. Comparing sizes: 3100 Mb ÷ 140 Mb = 22.1 times larger.

Whole genome sequencing

Trend	Detail
Speed	Much faster than the first human genome, which took more than a decade
Cost	Fallen enormously; falls are best stated as a ratio
Current use	Research into evolutionary relationships
Potential future use	Personalized medicine

HL only (A3.1.12 to A3.1.15)

• A3.1.12: The biological species concept fails for asexual species (no breeding) and bacteria (horizontal gene transfer moves genes across species).
• A3.1.13: Different chromosome numbers make fertile hybrids unlikely. Mule: 32 + 31 = 63, odd, pairing fails in meiosis.
• A3.1.14: Dichotomous key. Use clear, constant features; two contrasting statements per step; n species need n − 1 pairs; test it on someone else.
• A3.1.15: Barcode = short standard DNA region. eDNA method: sample, extract, amplify by PCR, sequence, match to a reference database. Fast and non-invasive. Limits: incomplete databases, DNA that has drifted from where the organism lives.

Writing good answers on this unit

Evaluating evidence for a hypothesis (A3.1.7)

For a command such as “evaluate the evidence”, give a short list of evidence, say what each piece predicts, then weigh it.

• Evidence that supports: matching bands, telomere-like sequence in the middle of chromosome 2, a second centromere remnant.
• Weighing: independent lines of evidence agreeing make the hypothesis stronger.
• Limit: a past event cannot be observed, so the conclusion is an inference from the evidence, not a direct observation.
• Close with a clear judgement: the evidence strongly supports fusion.

Comparing genome data (A3.1.10)

1. Read the unit and the column headings.
2. Put sizes in the same unit before dividing.
3. State the comparison as a ratio (“22 times larger”) or a difference.
4. Compare with what you know of the organism’s complexity.
5. Conclude carefully: a few data points show a pattern or an exception; they do not prove a rule.

Quick HL reminders

Dichotomous key check. Five species need four pairs of statements. Each ending names exactly one species, and no statement uses a feature that changes with age.

eDNA steps in order. Collect sample, filter or extract, amplify barcode region by PCR, sequence, compare with reference database, list species present.

Why hybrids are infertile. Different chromosome numbers in the two parents give an unpaired set in the hybrid. Meiosis needs homologous pairs, so balanced gametes are unlikely.

Phrases that earn credit

• “Fertile offspring” in every definition of the biological species concept.
• “Gradual” when describing speciation, with a reason: populations slowly become more different.
• “Banding pattern, length, centromere position” as the three karyogram criteria.
• “Variation within a species is small compared with variation between species.”

Quick self-test

1. Write the correct binomial name for a lion given the genus Panthera and species leo.
2. State the definition of a species in the biological species concept.
3. Give the diploid number of a chimpanzee and the number of homologous pairs.
4. Name three features used to classify chromosomes in a karyogram.
5. A chromosome is 18 mm long on a micrograph at ×1500. Calculate its actual length in µm.
6. Give one piece of evidence that human chromosome 2 formed by fusion.
7. Explain why variation between species is greater than within a species.
8. A genome of 3100 Mb has 0.1% SNP difference between two people. How many bases differ?
9. State one current and one potential future use of whole genome sequencing.
10. (HL only) A hybrid has 2n = 39. Explain why it is likely to be infertile.

Answers

1. Panthera leo (genus capital, species lower case, italics).
2. A group of organisms that can breed together and produce fertile offspring.
3. 48 chromosomes, 24 pairs.
4. Any three of: banding pattern, length, centromere position.
5. 18 ÷ 1500 = 0.012 mm = 12 µm.
6. Any one: banding matches two ape chromosomes end to end; telomere sequences in the middle; a remnant second centromere.
7. Species have accumulated separate changes over time, with no gene exchange to keep them alike; members of one species share a gene pool and so differ little.
8. 3.1 × 10⁹ × 0.001 = 3.1 × 10⁶ bases.
9. Current: research into evolutionary relationships. Future: personalized medicine.
10. 39 is odd, so chromosomes cannot all pair in meiosis, so viable gametes are unlikely.

Where marks are usually lost

• Writing species names with a capital, or without italics or underlining.
• Defining a species without the word fertile.
• Saying a diploid number could be odd.
• Forgetting to convert mm to µm in magnification questions.
• Quoting a genome size with no unit, or mixing Mb and Gb in one comparison.
• Saying “chromosome 2 proves” the fusion, rather than evaluating evidence for a hypothesis.
• Answering that bigger genomes mean more complex organisms.
• Describing eDNA as “finding the animal” rather than detecting its DNA in a sample.
• Mixing up the genus and the species when explaining the name.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Sub-topic A3.1, Diversity of organisms.