Variation, species and binomial namesSpec A3.1.1, A3.1.2, A3.1.3
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.
Variation between organisms is a defining feature of life. No two individuals are identical in all their traits, even identical twins or clones differ slightly. The patterns of variation are complex, and they are the basis for naming and classifying organisms.
Some differences are small and found within a group (eye colour in humans). Others are large and consistent and separate one group from another (the number of legs in insects and spiders). Biologists use these consistent patterns to recognise species.
Species as groups with shared traits
The original idea of a species, used by Linnaeus in the 18th century, was morphological: a species is a group of organisms with shared traits. Organisms that look alike in their structure (morphology) are placed in the same species, and organisms with consistently different traits are placed in different species.
This works well for many organisms, but appearance can mislead. Males and females of one species can look very different, and different species can look almost identical.
The binomial system
Every species has a two-part scientific name, its binomial. The first part identifies the genus. The second part distinguishes the species within that genus. Species in the same genus have similar traits.
- The genus name has an initial capital letter; the species name is all lowercase: Homo sapiens, Pan troglodytes.
- Binomials are printed in italics or underlined when handwritten.
- After first use, the genus can be shortened to its initial: H. sapiens.
- Binomials are international, so scientists everywhere know exactly which organism is meant, whatever its local common name.
| Binomial | Common name | Genus | Species name |
|---|---|---|---|
| Panthera leo | lion | Panthera | leo |
| Panthera tigris | tiger | Panthera | tigris |
Writing homo Sapiens or Homo Sapiens loses the mark. Only the genus has a capital letter.
Linking question: how do species show both continuous and discontinuous variation? Traits vary continuously within a species, yet species themselves are discrete groups.
Written and checked against the IB Biology SL specification · Updated October 2026