Diversity of organisms — IB Diploma Biology HL
IB Biology A3.1: variation, species concepts, binomial names, chromosome numbers, karyograms, genome diversity and whole genome sequencing.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Variation between organisms is a defining feature of life: no two individuals are identical in all their traits. Linnaeus grouped organisms into species by shared traits, the morphological species concept, and named each with a binomial: a capitalised genus name followed by a lowercase species name, for example Homo sapiens.
According to the biological species concept, a species is a group of organisms that can breed and produce fertile offspring. It is hard to apply to fossils, separated populations and hybrids, so competing definitions exist. Speciation is gradual, so deciding whether diverging populations are one species or two can be arbitrary.
Chromosome numbers and karyograms
PracticalChromosome numbers vary between plant and animal species: humans have 46 and chimpanzees have 48. Diploid cells have an even number because chromosomes are in homologous pairs. A karyogram shows a cell's chromosomes arranged in pairs by length, centromere position and banding pattern, and supports the hypothesis that human chromosome 2 formed by fusion.
Unity and diversity of genomes
PracticalThe genome is all the genetic information of an organism. Members of a species share most of their genome, with some diversity from single-nucleotide polymorphisms. Eukaryote genomes vary in size, set by the total amount of DNA, and in base sequence. Variation between species is much larger than within a species, and genome size does not match complexity.
Whole genome sequencing is finding the entire base sequence of an organism's DNA. It has become much faster and cheaper: the first human genome took about 13 years, but one can now be sequenced in about a day. It is used to research evolutionary relationships and could in future allow personalised medicine.
The biological species concept does not work well for organisms that do not breed sexually, or for bacteria, where horizontal gene transfer moves genes between species. Chromosome number is a shared trait within a species: crosses between closely related species with different chromosome numbers are unlikely to produce fertile offspring, because chromosomes cannot pair in meiosis.
A dichotomous key identifies organisms through a series of paired choices between observable features, each leading to another pair or a name. Environmental DNA is DNA that organisms shed into soil or water. Sequencing short barcode regions in a sample and matching them to a reference database identifies the species present, so habitat biodiversity can be investigated rapidly.
Exam questions
9 exam-style questions (29 marks), each with its mark scheme.
Answer the questionsFlashcards
24 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of organisms (unity and diversity) on one page, so you can see where this subtopic fits.
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Free PDFs to print or save.
Quick check questions
In a binomial, which part has a capital letter?
Show answer
The genus name (first part). The species name is all lowercase.
State the biological species concept.
Show answer
A species is a group of organisms that can breed and produce fertile offspring.
How many chromosomes do humans and chimpanzees have?
Show answer
Humans 46, chimpanzees 48.
What is a genome?
Show answer
All the genetic information of an organism.
HL only Why is a mule infertile?
Show answer
Its parents have different chromosome numbers, so its chromosomes cannot all pair in meiosis and viable gametes are not produced.
Frequently asked questions
What is the difference between the morphological and biological species concepts?
The morphological concept, used by Linnaeus, groups organisms into a species because they share traits. The biological species concept defines a species as a group of organisms that can breed and produce fertile offspring. The biological concept focuses on interbreeding, so it can separate species that look alike but cannot breed together.
Why is it hard to define a species?
Speciation happens gradually, so diverging populations pass through stages where they are partly different, and deciding whether they are one species or two can be arbitrary. Interbreeding also cannot be tested for fossils or separated populations, and some species produce fertile hybrids, so competing species definitions exist.
What is the evidence that human chromosome 2 formed by fusion?
Its banding pattern matches two chimpanzee chromosomes placed end to end, and it has telomere sequences near its middle, where the ends of two chromosomes joined. It also has the remains of a second, inactive centromere. This supports fusion of two ancestral chromosomes, reducing the number from 48 to 46.
Does a bigger genome mean a more complex organism?
No. Genome size is the total amount of DNA, and it does not correlate closely with complexity. Some plants, such as Paris japonica, have genomes dozens of times larger than the human genome. Much of a large genome is non-coding, repeated DNA rather than extra genes.
How is environmental DNA used to identify species?
HL only DNA shed by organisms is extracted from a water or soil sample, and a short barcode region is copied by PCR and sequenced. The sequences are matched to a reference database of barcodes from known species, so the species living in a habitat can be identified rapidly without seeing them.
Written and checked against the IB Biology HL specification · Updated October 2026