Limits of the biological species conceptSpec A3.1.12, A3.1.13
In short
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.
Asexual reproduction
The biological species concept depends on interbreeding. Organisms that reproduce only asexually produce offspring from a single parent, so there is no interbreeding to test. Every individual is reproductively isolated, so by this definition each clone could count as a separate species, which is not useful.
Bacteria and horizontal gene transfer
Bacteria reproduce asexually by binary fission. They can also gain genes from other bacteria, even of other species, by horizontal gene transfer: for example plasmids passed from cell to cell. Genes therefore move between groups that the concept would treat as separate species, so the boundaries between bacterial species are blurred. Bacterial species are usually defined by similarity of base sequences instead.
Chromosome number as a shared trait
Members of a species have the same chromosome number. Cross-breeding between closely related species is unlikely to produce fertile offspring if the parents have different chromosome numbers.
- A horse (64 chromosomes) and a donkey (62) can produce a mule, with 63 chromosomes.
- In meiosis, the mule's chromosomes cannot all form homologous pairs, so meiosis fails and few or no viable gametes form.
- The mule is therefore infertile, which confirms that horses and donkeys are different species.
Linking question: what might cause a species to persist or go extinct? Reproductive isolation, such as a difference in chromosome number, keeps a species' gene pool separate.
Written and checked against the IB Biology HL specification · Updated October 2026