Species and taxonomy — AQA A-Level Biology

The biological species concept defines a species by reproductive compatibility, not by appearance. Two organisms belong to the same species only if they can interbreed and the offspring are themselves fertile, so that alleles keep flowing between them and they share a single gene pool. A horse and a donkey interbreed and produce a mule, but the mule is sterile because its odd chromosome number prevents homologous chromosomes pairing in meiosis; horse and donkey are therefore separate species. The word fertile is what carries the definition, because offspring that cannot themselves reproduce are an evolutionary dead end and the two parent populations continue to evolve apart.

Courtship is a sequence of behaviours performed before mating, and each action by one animal is the stimulus for the next response by its partner, so the pair works through a chain of stimulus and response. The chain has to be completed before mating takes place, which is what makes it a necessary precursor. It does several jobs at once. It allows an animal to recognise a member of its own species, so that gametes are not wasted on a mating which could not produce fertile offspring. It allows an individual to recognise the opposite sex, and to judge whether a partner is sexually mature, fertile and receptive, because an animal that does not give the correct response is not ready to breed. It synchronises mating so that sperm and egg are released at the same time, raising the probability of fertilisation, and in many species the display itself triggers ovulation or sperm release.

Courtship displays are species-specific. The pattern of movement, sound, or scent is characteristic of one species, and only a member of that species gives the correct response. Because courtship works as a chain of stimulus and response, an individual of another species fails to respond correctly, stopping the chain and preventing mating. This is crucial because interspecific mating often produces no offspring or infertile hybrids (like mules). Therefore, gametes, time, and energy would be wasted. Species recognition prevents this waste, helping to keep the gene pools of closely related species separate. It acts as a mechanism of reproductive isolation.

A phylogenetic classification groups species by evolutionary history: organisms are placed together because they share a common ancestor. The more recently they shared that ancestor, the more closely related they are and the smaller the group they share. This differs from grouping by appearance alone, because similar features can evolve independently in unrelated organisms exposed to similar conditions. Phylogeny is usually drawn as a branching tree in which every branch point (node) is a common ancestor and the branches show the order in which lineages separated. Reading such a diagram is a key skill: relatedness is read from the position of the branch points, not from how close two names happen to sit on the page. Living taxa at the tips of the branches cannot be described as having evolved more or less recently than one another.

A hierarchy is a set of groups nested inside larger groups, like boxes within boxes. In classification the largest group is the domain, and each level below it divides its members into smaller, more closely related groups until the smallest group, the species, is reached. Two rules follow. First, every member of a small group is also a member of every larger group containing it, so one species belongs to one genus, one family, one order and so on all the way up. Second, there is no overlap: a species cannot sit in two genera, and a genus cannot be shared between two families, so at every level an organism has exactly one place. Because the system is phylogenetic, the nesting mirrors evolutionary branching, and organisms sharing a small group also share a recent common ancestor.

A taxon is any one group within the classification hierarchy, at any level. A domain is a taxon, a class is a taxon, and a species is a taxon; the word names the group, not the rank. The plural is taxa, used when more than one group is meant, as in 'six taxa of shade-tolerant plants'. Taxonomy is the study of how organisms are placed into these groups, and a taxonomist uses evidence about ancestry to decide which taxon an organism belongs to. In data questions, a value given for a taxon may be an aggregate or mean for the group, so it need not apply to every species it contains; comparisons should therefore be written between named taxa rather than between individual organisms, for example 'the ferns produce less than the asterids'. Read the plural carefully, since questions often refer to several taxa at once.

The order runs domain, kingdom, phylum, class, order, family, genus, species, from the largest and most inclusive group down to the smallest. Learn the eight names in sequence, because classification tables may ask you to fill gaps. In such a table each column is one organism and each row one level, so a missing entry is found by reading across the row to see which level you are in and down the column to keep the organism consistent. The domain sits above kingdom and was added when sequence data showed that prokaryotes fall into two deeply separated groups. The last two levels, genus and species, are the ones used in the organism's name, so a column that ends with a genus and a species name has ended in the right place.

Every species has a two-word Latin name, the binomial: the first word is the genus and begins with a capital letter, the second is the species and does not, as in Homo sapiens or Luscinia svecica. In print the binomial is italicised; in handwriting it is underlined. After the first use the genus can be shortened to its initial, which is why exam papers write C. difficile and E. coli. The system is universal, so a species carries the same name in every country and every language, which removes the confusion caused by common names: one common name may be used for several species, and one species may have several common names in different regions. Because the first word is the genus, the binomial also carries classification information, so two species sharing the first word sit in the same genus and are closely related.

Classification used to depend on what could be seen. Bacteria, for example, were once grouped by the number, position and shape of their flagella viewed with an optical microscope, a method that was unreliable because flagella are fragile, hard to stain and can project from the cell at any angle, and because unrelated species can share a cell shape. Two kinds of evidence have replaced it. Genome sequencing reads the base sequence of DNA directly, and sequences can be compared base by base: the more similar the sequences of two organisms, the more recently they shared a common ancestor. Amino acid sequences of the same protein can be compared in the same way, since the base sequence determines the amino acid sequence. Immunology provides a third line of evidence: antibodies bind to similar proteins from another species, and the more precipitate formed, the closer the relationship.