The biological species concept fits bacteria poorly because they reproduce asexually and can exchange genes across species boundaries through horizontal transfer.
Coverage 2025–2025 · Updated 15 Jul 2026
The biological species concept is based on sexual reproduction, so it does not work well for asexual organisms or for bacteria that exchange genes horizontally.
Bacteria gain diversity through mutation and horizontal gene transfer, including transformation, plasmid transfer and recombination. Genes can therefore cross lineage boundaries without the organisms breeding as animals or plants do.
In many sexually reproducing species, haploid and diploid chromosome numbers are shared traits because meiosis and fertilization conserve chromosome number across generations. If closely related parents have different chromosome numbers, homologous pairing in a hybrid's meiosis can fail, reducing fertility.
A horse and donkey can produce a mule, but mismatched parental chromosome sets disrupt homologous pairing during meiosis, so the hybrid is usually infertile.
Chromosome number can support a species distinction but is not a universal definition: different species can share a chromosome number, and bacterial boundaries require evidence other than interbreeding.
This objective is assessed through structured response, commonly using Outline.
Outline
Build the answer around this relationship: Bacteria usually reproduce asexually by binary fission.
Representative question
Outline a difficulty in applying the biological species concept to bacteria.
One additional mark is available for the construction of your answers for each question.
The HL extension connects two problems: species boundaries and species identification. Bacteria challenge the biological species concept because they are asexual and exchange genes horizontally. Chromosome number can help define boundaries in many sexual species because meiosis conserves it and mismatches can produce infertile hybrids. Dichotomous keys identify visible organisms using paired choices. DNA barcoding and eDNA identify species using short sequences, PCR, and databases when visible traits are limited.
