AQA A-Level Biology: Species and Taxonomy

Biologists need a consistent way to define, name and group the millions of species on Earth. This guide covers what a species is, how organisms are classified by their evolutionary relationships, and the modern methods used to work those relationships out.

What a species is

A species is a group of organisms that can interbreed to produce fertile offspring. The word fertile is important. Two different species sometimes produce offspring, but those offspring are usually infertile, because the parents had different chromosome numbers. With an odd or unmatched set of chromosomes, homologous pairs cannot form, so meiosis cannot take place to make gametes.

This gives a practical test for whether two separated populations are the same species: breed individuals from the two populations together, and if they produce fertile offspring, they are the same species.

Classification and the taxonomic hierarchy

A phylogenetic classification system arranges species into groups, called taxa, based on their evolutionary origins and relationships, that is, on how closely they share a common ancestor. It is a hierarchy: smaller groups sit inside larger groups, with no overlap between groups at the same level.

The taxa run from the broadest to the narrowest:

• Domain
• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

Naming species

Every species is given a universal, two-part name using the binomial system: the first name is its genus and the second its species, for example Homo sapiens. The advantage is that the name is universal, so scientists everywhere refer to the same organism, avoiding the confusion caused by common names, which vary between languages and regions and where one organism may have several.

Clarifying evolutionary relationships

Classification used to rely on observable features, but two modern methods now let biologists compare organisms far more precisely.

Genome sequencing compares the DNA base sequences of different species. Because mutations build up in DNA steadily over time, two species with more differences in their base sequences are more distantly related, having shared a common ancestor longer ago.

Immunology compares proteins, such as albumin, using antibodies. If an antibody raised against a protein from one species binds strongly to the same protein from another species, the two proteins have a similar amino acid sequence and tertiary structure, which means the species are closely related and share a recent common ancestor. Fewer mutations have had time to build up between them.

These comparisons are often summarised in a phylogenetic tree, where each branch point represents a common ancestor and each branch an evolutionary path. Two species that share a more recent common ancestor, meeting at a branch point closer to the tips, are more closely related.

Courtship behaviour

Before mating, many animals perform courtship behaviour, and it serves several purposes that make successful reproduction more likely. It allows individuals to recognise members of their own species, so that mating produces fertile offspring rather than being wasted on another species. It lets them recognise and attract a member of the opposite sex, indicates that a partner is sexually mature and fertile, helps to synchronise mating so that gametes are released at the same time, and can establish a pair bond to raise the young.