A species is a group of similar organisms able to reproduce to give fertile offspring. Each species is given a binomial name. Courtship behaviour helps members of a species to recognise each other and is used in classification. Phylogenetic classification arranges species into a hierarchy of groups that share a common ancestor, and the taxa from domain to species reflect evolutionary relationships.

What this dot point is asking

AQA wants you to define a species, use the binomial naming system correctly, explain the phylogenetic basis of modern classification and the hierarchy of taxa from domain to species, and explain the role of courtship behaviour in species recognition and classification.

What a species is

The word "fertile" is doing the work here. A horse and a donkey can mate to produce a mule, but the mule is infertile, so horses and donkeys are different species. This definition is hard to apply to organisms that reproduce asexually or are extinct, which is one reason classification also uses genetic and biochemical evidence.

The binomial naming system

Each species is given a two-part Latin name, the binomial:

• The first word is the genus (capitalised).
• The second word is the species (lower case).
• Both are written in italics (or underlined when handwritten).

For example, humans are Homo sapiens and the brown bear is Ursus arctos. The binomial is universal, so scientists worldwide refer to the same organism without ambiguity from local common names.

Phylogenetic classification

Modern classification is phylogenetic: it groups organisms according to their evolutionary relationships and common ancestry, rather than just superficial similarity.

Organisms are arranged in a hierarchy of groups called taxa. Each group is contained within a larger group, with no overlap between groups at the same level. Groups that share a more recent common ancestor are placed closer together.

The taxonomic hierarchy, from largest to smallest, is:

Evolutionary relationships can be shown on a phylogenetic tree, where branch points represent common ancestors. The more recently two species share a branch point, the more closely related they are.

Evidence used in classification

Phylogenetic relationships are increasingly judged using molecular evidence, which is more objective than appearance:

• DNA base sequences - the more similar the base sequences, the more closely related the species (and the more recent their common ancestor).
• mRNA base sequences, which reflect DNA sequences.
• Amino acid sequences of proteins - similar sequences indicate similar genes.
• Immunological comparisons - similar proteins bind the same antibodies.

These methods can confirm or revise classifications based on anatomy alone.

Courtship behaviour

Courtship behaviour is a sequence of actions performed before mating. It has several functions:

1. Species recognition - individuals identify members of their own species, so they do not waste gametes producing infertile hybrids.
2. Sex and maturity recognition - it identifies a mate of the opposite sex that is sexually mature.
3. Fertility and receptiveness - it signals that a partner is able to breed and ready to mate, often synchronising the timing of mating.
4. Mate quality and pair bonding - displays let an individual assess a partner's quality and form a stable pair to raise offspring.

Because courtship behaviour is often species-specific, the more closely related two species are, the more similar their courtship behaviour tends to be. This makes courtship a useful tool in classification, helping to confirm how closely species are related.

Common mistakes

Try this

Q1. Explain why a horse and a donkey are classified as different species. [2 marks]

• Cue. They can mate but their offspring (a mule) is infertile; a species must produce fertile offspring.

Q2. List the eight taxonomic groups in the hierarchy, from largest to smallest. [2 marks]

• Cue. Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Q3. Explain how DNA base sequence comparison can be used to determine how closely two species are related. [3 marks]

• Cue. The more similar the base sequences, the more recently the species shared a common ancestor and the more closely related they are; large differences indicate a distant common ancestor.