Species and taxonomy (3.4.5)
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Taxonomic hierarchy is a system of classification that organises living organisms into progressively smaller and more specific groups that do not overlap.
Each level in the hierarchy is known as a taxon (plural: taxa). The main levels of the taxonomic hierarchy are:
- Domain
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species.
The binomial naming system is a universal system for naming organisms. Every species is given a unique two-part Latin name consisting of its genus and species, e.g., Homo sapiens.
The genus name begins with a capital letter, the species name is lowercase, and both are written in italics (or underlined if handwritten).
The binomial system provides a standardised, internationally recognised name for each species, avoiding confusion caused by different common names, and reflects relationships between organisms through shared genera.
Phylogeny refers to the evolutionary history and relationships among species. It shows how species have evolved and diverged from common ancestors over time.
Phylogenetic trees (or evolutionary trees) are diagrams that represent these relationships.
Modern classification systems are increasingly based on phylogenetic relationships, using evidence from DNA, RNA, proteins, and other biological molecules to reflect evolutionary history rather than relying only on observable characteristics.
A species is the smallest unit of taxonomic classification.
How a species is defined has changed over time. Typically, individuals are considered to come from the same species if they can breed to produce fertile offspring.
Courtship behaviour involves specific actions or signals (e.g., dances, calls) that help individuals identify suitable mates. It increases reproductive success by:
- Ensuring species recognition: only members of the same species mate.
- Assessing sexual maturity and fertility.
- Synchronising mating: ensures gametes meet at the right time.
- Forming pair bonds: vital for species that raise offspring together.
- Preventing inbreeding: maintains genetic isolation and species integrity.
Immunology can be used to study evolutionary relationships by comparing the proteins of different species.
Closely related species have proteins with more similar antigenic sites (epitopes). Antibodies raised against the proteins of one species will bind more strongly to the proteins of closely related species because their antigenic sites are more similar.
The stronger the antigen–antibody binding, the more closely related the species are likely to be, indicating they share a more recent common ancestor.
By comparing gene sequences and entire genomes, scientists can determine the evolutionary relationships between organisms at the genetic level.
Highly conserved genes, such as the genes that code for ribosomal RNA (rRNA), provide a powerful tool for classification because they change very slowly over time.
The more similar the DNA sequences of two species, the more closely related they are, as they share a more recent common ancestor. This can lead to changes in classification and phylogenetic trees.