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

This guide teaches sub-topic A3.1, Diversity of organisms, from the IB Biology guide, first assessment 2025. It covers statements A3.1.1 to A3.1.11 for SL and HL, and A3.1.12 to A3.1.15, which are marked HL only wherever they appear. For condensed recall use the revision notes; to test yourself use the practice questions. The wider theme is in the Theme A study guide. Course hub: IB DP Biology. Printable list: checklist. Unsure where to start? Try the free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do	Level
A3.1.1	Explain why variation between individuals is a defining feature of life	SL and HL
A3.1.2	Describe a species as a group sharing traits (the Linnaean, morphological idea)	SL and HL
A3.1.3	Write binomial names correctly	SL and HL
A3.1.4	State the biological species concept and its challenges; know other definitions exist	SL and HL
A3.1.5	Explain why speciation makes species boundaries hard to draw	SL and HL
A3.1.6	Know that chromosome numbers vary; human 46, chimpanzee 48; diploid numbers are even	SL and HL
A3.1.7	Classify chromosomes in karyograms; evaluate the chromosome 2 fusion hypothesis	SL and HL
A3.1.8	Explain the unity and diversity of genomes within a species, including SNPs	SL and HL
A3.1.9	Compare genome size and sequence variation between eukaryotes	SL and HL
A3.1.10	Extract genome sizes from a database and compare them with complexity	SL and HL
A3.1.11	Outline current and possible future uses of whole genome sequencing	SL and HL
A3.1.12	Explain why the biological species concept fails for asexual species and for bacteria with horizontal gene transfer	HL only
A3.1.13	Link differing chromosome numbers to infertile hybrids	HL only
A3.1.14	Develop a dichotomous key for local species	HL only
A3.1.15	Use barcodes and environmental DNA to survey a habitat	HL only

Variation and the species idea (A3.1.1 to A3.1.2)

No two individuals are identical in every trait. Even clones raised in different places differ, and offspring of the same parents differ from each other. The patterns of this variation are complicated, and they are what allows biologists to recognise groups, name them and classify them.

Linnaeus defined a species by shared observable traits: a group of organisms that look alike in their structure. This is the morphological species concept. It is simple and works on preserved specimens and fossils. Its weakness is that individuals within one species can differ strongly (males and females, juveniles and adults), while different species can look almost identical.

Binomial names (A3.1.3)

Each organism has a two-part name. The first part is the genus and the second distinguishes the species. Species in one genus share many traits.

Rules: the genus starts with a capital letter; the species name is all lower case; both are in italics when printed, and underlined when handwritten. The species name is never used alone.

Worked example. Correct: Panthera leo (lion) and Panthera tigris (tiger). They share a genus, so they share many traits. Incorrect versions: panthera Leo (capitals wrong), Panthera Tigris (species capitalised), tigris (species name alone).

The biological species concept (A3.1.4 to A3.1.5)

The biological species concept defines a species as a group of organisms that can breed together and produce fertile offspring. “Fertile” matters: a horse and a donkey can breed, but their offspring cannot normally reproduce, so they are separate species.

Challenges:

• Many organisms reproduce asexually, so “can breed” has no meaning (see HL section).
• Extinct species known only from fossils cannot be tested for interbreeding.
• Populations separated by geography may never meet, so interbreeding cannot be tested.
• Some clearly different species do interbreed and produce fertile hybrids.

Other definitions exist, such as the morphological concept above, and concepts based on ecological role or on shared ancestry and DNA sequence. None is perfect, so the choice depends on the organisms.

Speciation is the process by which one species divides into two or several. It is usually gradual. Two populations stop exchanging genes, then accumulate different traits over many generations. At an intermediate stage they may differ somewhat, interbreed only partly, or be untestable. Whether they count as one species or two is then an arbitrary decision. That is a feature of how species form, not a mistake in the definition.

Chromosome numbers and karyograms (A3.1.6 to A3.1.7)

Different species have different chromosome numbers. You need two values: humans 2n = 46 and chimpanzees 2n = 48. You do not need other values, but you should know that a diploid number is even, because chromosomes come in homologous pairs (46 is 23 pairs; 48 is 24 pairs).

A karyogram is an image of the chromosomes from one cell, arranged in homologous pairs. The process of preparing it is karyotyping. Pairs are identified and matched by:

• length, usually arranged longest to shortest;
• centromere position, either near the middle, off-centre, or near one end;
• banding pattern after staining, which gives each chromosome a characteristic series of light and dark bands.

Sex chromosomes are normally placed last. Karyograms show an extra or missing chromosome, such as three copies of chromosome 21, and give the sex of the individual.

Worked example. A micrograph shows one chromosome 16 mm long at a magnification of ×2000. The short arm measures 3.5 mm.

• Actual length = image length ÷ magnification = 16 mm ÷ 2000 = 0.008 mm = 8.0 µm.
• Short arm as a fraction of the whole = 3.5 ÷ 16 = 0.22.
• The centromere is well away from the middle, so this chromosome is not one with a central centromere; it has a long arm and a clearly shorter arm.

Chromosome 2 and the fusion hypothesis

Great apes have 24 pairs of chromosomes; humans have 23. The hypothesis is that human chromosome 2 arose when two ape chromosomes (numbered 12 and 13 in the hypothesis the syllabus names) fused in a shared ancestor. Evidence:

• The banding pattern of human chromosome 2 matches the two ape chromosomes placed end to end.
• Sequences typical of telomeres, which belong only at chromosome ends, are found in the middle of human chromosome 2.
• A second, inactive centromere-like region lies on the long arm, as expected from a former second centromere.
• The numbers fit: 48 chromosomes less one fusion of two into one gives 46.

Nature of science. A testable hypothesis makes predictions that evidence could contradict. The fusion hypothesis predicted telomere sequences in the middle of chromosome 2; had they been absent, the idea would have been weakened. A non-testable statement, such as “the fusion happened so that humans would become intelligent”, predicts nothing that data could refute, so it is not scientific.

Genomes within and between species (A3.1.8 to A3.1.10)

A genome is all the genetic information of an organism. Members of a species share most of their genome, which is the unity. Diversity comes from differences such as single-nucleotide polymorphisms (SNPs), where individuals have different bases at one position.

Between two unrelated people, roughly 1 base in 1000 differs. For a genome of 3100 Mb (3.1 × 10⁹ bases) this is 3.1 × 10⁹ × 0.001 = 3.1 × 10⁶ differences, which is still tiny compared with the whole.

Eukaryote genomes differ in size (total amount of DNA) and in base sequence. Variation between species is much greater than variation within a species.

For A3.1.10 you must extract genome sizes from a database and relate them to complexity. Typical supplied values:

Organism	Genome size (Mb)
Baker’s yeast	12
Fruit fly	140
Human	3100
Flowering plant Paris japonica	149 000

Human genome ÷ fruit fly genome = 3100 ÷ 140 = 22.1. Paris japonica ÷ human = 149 000 ÷ 3100 = 48.1. The plant has about 48 times as much DNA as a human, yet nobody would call it 48 times more complex. Genome size therefore does not track complexity closely. Much DNA does not code for proteins, and the amount of repeated sequence varies widely.

Whole genome sequencing (A3.1.11)

Whole genome sequencing determines the base order of an entire genome. Speed has risen sharply and cost has fallen: the first human genome took more than a decade and billions of dollars, while a genome can now be sequenced in days for a small fraction of that. If a cost fell from ten million to one thousand US dollars, the fall is 10 000-fold.

Current use: comparing genomes to work out evolutionary relationships between species. Possible future use: personalized medicine, where treatment is chosen using a patient’s own genome.

HL only: further species ideas (A3.1.12 to A3.1.15)

A3.1.12 Asexual species and horizontal gene transfer (HL only)

The biological species concept needs sexual breeding. Asexual organisms never interbreed, so every lineage would be its own “species”. In bacteria, horizontal gene transfer moves genes between distinct species, blurring the boundaries the concept relies on.

A3.1.13 Chromosome number as a shared trait (HL only)

Members of a species share a chromosome number. If two closely related species have different numbers, a hybrid has chromosomes that cannot pair properly in meiosis, so fertile offspring are unlikely.

Worked example. Horse 2n = 64 gives gametes with 32. Donkey 2n = 62 gives gametes with 31. A mule has 32 + 31 = 63 chromosomes. An odd number cannot form pairs, so meiosis fails and the mule is usually sterile.

A3.1.14 Developing a dichotomous key (HL only)

A dichotomous key is a series of numbered pairs of contrasting statements, each leading to another pair or to a name. To build one for local species:

1. Collect or photograph the species and list observable features.
2. Choose features that are clear and do not vary with age or season; avoid size and colour alone.
3. Split the set in two at each step. For n species you need n − 1 pairs.
4. Test the key with someone who has not seen the organisms, and revise.

Worked example (five tree leaves): 1a. Leaf is a needle … pine 1b. Leaf is flat … go to 2 2a. Leaf divided into leaflets … ash 2b. Leaf is one piece … go to 3 3a. Margin spiny … holly 3b. Margin not spiny … go to 4 4a. Margin deeply lobed … oak 4b. Margin smooth or gently waved … beech

Five species give four pairs, as expected.

A3.1.15 Barcodes and environmental DNA (HL only)

A DNA barcode is a short, standard stretch of DNA that varies between species but is similar within one. Environmental DNA (eDNA) is DNA shed into water, soil or air in skin, mucus and faeces. A sample is taken, DNA is extracted and copied by PCR, the barcode regions are sequenced and each sequence is matched to a reference database. This allows the biodiversity of a habitat to be assessed rapidly without catching every organism. A limitation: a species missing from the database cannot be named.

Common errors

• Writing Panthera Leo or giving only the species name.
• Saying the biological species concept needs only “can breed”; offspring must be fertile.
• Calling a fusion hypothesis “just a theory” rather than testable.
• Confusing a larger genome with a more complex organism.
• Stating that diploid numbers can be odd.

Official syllabus

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