Species recognition and reproduction

Organisms belong to the same species if they can reproduce together and produce fertile offspring. Fertile offspring can themselves reproduce. Species membership is therefore based on reproductive compatibility, not on appearance alone: looking similar does not necessarily mean that two organisms belong to the same species.

Courtship and species recognition

Before mating, organisms may perform courtship behaviour: actions or signals that help them identify potential mates and coordinate mating. Courtship signals are often specific to a species, so an organism can recognise a potential mate of the same species by its signals or responses.

These species-specific signals make mating between members of different species less likely. Courtship therefore helps bring together organisms that can reproduce successfully with one another.

Phylogenetic classification

Phylogenetic classification arranges species into groups according to their evolutionary origins and relationships. Species with a more recent shared evolutionary origin are placed in more specific groups together.

Each group in the classification is called a taxon; the plural is taxa. Taxa form a hierarchy: smaller groups are nested within larger groups. For example, a genus contains one or more species, and a family contains one or more genera.

Ranks in the phylogenetic hierarchy

From the broadest rank to the most specific, the hierarchy is:

Domain → kingdom → phylum → class → order → family → genus → species

Each successive rank divides organisms into more specific groups. A species belongs within a genus, that genus belongs within a family, and so on up to a domain.

Groups at the same rank do not overlap. For example, a species cannot belong to two different genera in the same classification. Different genera can, however, be nested within the same family.

Binomial nomenclature

Binomial nomenclature is a system that gives each species a universally used scientific name made of two parts. This provides a shared name for a species even when different common names are used.

The first part is the genus name. The second is the specific epithet, which identifies the species within that genus. Both parts together form the species’ binomial name.

The genus name begins with a capital letter, while the specific epithet begins with a lower-case letter. The full binomial is normally written in italics.

For example, the scientific name of humans is Homo sapiens: Homo is the genus and sapiens is the specific epithet.

Molecular evidence for evolutionary relationships

Organisms inherit molecules and the instructions for making them from their ancestors. By comparing these molecules, scientists can look for evidence of shared ancestry. Greater molecular similarity can indicate that two organisms share a more recent common ancestor and are therefore more closely related.

Immunological comparisons

Antibodies bind to specific features of molecules such as proteins. An antibody that binds to a protein from one organism may also bind to a similar protein from another organism. This cross-binding provides evidence that the proteins share molecular features.

Scientists can use such comparisons to investigate evolutionary relationships: greater similarity between corresponding proteins can suggest a closer relationship between the organisms. Antibody binding indicates similarity, but it does not show that the proteins are identical.

Genome sequence comparisons

Genome sequencing allows scientists to compare DNA sequences directly. They can compare corresponding regions of DNA from different organisms and identify similarities and differences in the order of their bases.

If the sequences are very similar, this can indicate that the organisms inherited them from a more recent common ancestor. Differences can arise as sequences change over generations. Molecular evidence can therefore help clarify relationships and may lead scientists to revise how organisms are grouped.