Species, Taxonomy and Phylogeny

The biological species concept groups organisms capable of interbreeding to produce fertile offspring. Courtship behaviour can help members recognise suitable mates and coordinate reproduction, contributing to reproductive isolation between species.

This concept is useful for many sexually reproducing organisms but cannot be applied straightforwardly to fossils, asexual organisms or populations whose reproductive compatibility cannot be tested. Classification therefore also draws on other evidence. A similar appearance alone does not guarantee shared species membership.

A hierarchy can include domain, kingdom, phylum, class, order, family, genus and species, with increasingly specific groups nested within broader ones. Each named group is a taxon. In a binomial such as Homo sapiens, Homo is the genus and sapiens the specific name; the genus begins with a capital letter and the species name with lowercase.

A phylogenetic classification aims to reflect common ancestry. A branching point on a tree represents a proposed common ancestor, and relatedness depends on the recency of shared ancestry, not on which tip happens to be drawn highest or farthest right. The left-to-right order of tips can often be rotated without changing relationships.

Comparing homologous DNA, RNA or protein sequences can reveal similarities and differences useful for inferring relationships. Comparable regions and adequate sequence length matter. One highly conserved short protein may not distinguish close relatives, while one rapidly evolving region may need careful alignment and analysis.

Immunological comparisons and genome sequencing have revised some groupings based mainly on appearance. Similar features can arise independently through similar selection pressures, so convergent resemblance need not indicate especially recent ancestry. Focus on hierarchy, evidence and the taxonomic systems named by this specification.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.