The biological species concept cannot be applied reliably to bacteria because bacteria reproduce asexually rather than interbreeding to produce fertile offspring. In addition, horizontal gene transfer can move genes between bacteria classified as different species, so reproductive isolation does not provide a clear boundary.
This is AHL content in A3.1 Diversity of organisms.
Under the biological species concept, a species is a group of organisms that can breed with one another and produce fertile offspring. Members of different species are expected to be reproductively isolated, meaning that gene flow between their populations is prevented.
Bacteria do not fit this definition for two main reasons:
| Feature | Sexually reproducing eukaryotes | Bacteria |
|---|---|---|
| Reproduction | Gametes from two parents fuse during fertilization | Cells usually reproduce asexually by binary fission |
| Test for membership of the same species | Individuals can interbreed and produce fertile offspring | Interbreeding and offspring fertility cannot be tested |
| Gene transfer | Gene flow usually occurs through sexual reproduction within a species | Horizontal gene transfer can move DNA between unrelated cells, including cells assigned to different species |
| Species boundary | Reproductive isolation can separate gene pools | Genetic material may cross proposed species boundaries |
Horizontal gene transfer is not the same as reproduction. A bacterium receiving DNA does not necessarily produce a new organism; instead, it acquires genetic information from another cell. This can rapidly spread characteristics such as antibiotic resistance and produce bacterial genomes containing genes from different evolutionary sources.
A common misconception is that bacteria cannot be classified into species at all. They can be grouped using characteristics such as morphology, metabolism, and DNA sequence similarity, but the biological species concept itself is unsuitable because it depends on sexual reproduction and reproductive isolation.
Exam technique: For an AHL “explain” question, link both points explicitly: bacteria reproduce asexually, so interbreeding cannot be assessed; horizontal gene transfer permits gene flow across classified species, so reproductive isolation is not a reliable criterion.
