Taxonomists identify a species by comparing its observable characteristics and genetic evidence with established descriptions or reference data. Dichotomous keys use paired choices, while DNA surveys compare genetic sequences or detect DNA left in the environment.
This is SL and HL content in Subtopic 2.1: Individuals, populations, communities and ecosystems, particularly understanding 2.1.4 on tools used to identify organisms.
How the Methods Work
A dichotomous key presents two contrasting statements at each step, such as “leaves are opposite” or “leaves are alternate.” Each choice directs the user to another pair until a taxon or species is identified. Accuracy depends on consistent characteristics and a key designed for the correct region and organism group.
In a DNA survey, scientists collect DNA from an organism or environmental DNA (eDNA) from water, soil or air. A selected sequence may be amplified and compared with sequences in a reference database. A close match supports identification, while eDNA can reveal a species without direct observation or capture.
| Method | Strengths | Limitations |
|---|---|---|
| Dichotomous key | Cheap, rapid and suitable for fieldwork | Damaged, juvenile or variable specimens may be misidentified |
| DNA survey | Distinguishes visually similar species and detects rare or hidden organisms | Requires specialist equipment and reliable reference sequences; contamination may cause errors |
For example, eDNA surveys detect the protected great crested newt in UK ponds. This can inform conservation decisions, but detecting DNA does not prove that a large or breeding population is present.
A common misconception is that DNA always provides a definitive answer. Incomplete databases, contamination, hybridization and closely related species create uncertainty, so taxonomists often combine genetic, morphological, ecological and geographical evidence.
Exam Technique
For an explain question, outline the steps and show how they produce an identification. For compare and contrast, refer to both methods throughout and include a strength and limitation of each rather than describing them separately.
