Investigating diversity

Biologists have named and described over a million different species. To organise this diversity, biologists use a phylogenetic classification system based on evolutionary relationships. This system arranges organisms into a hierarchical structure of groups, called taxa (singular: taxon). Smaller groups are nested within larger groups, and there is no overlap between groups. Each species appears only once within this hierarchy.

The classification hierarchy, from the broadest to the most specific, consists of the following levels: Domain - Kingdom - Phylum - Class - Order - Family - Genus - Species

Each species is universally identified by a binomial nomenclature, consisting of its genus (capitalised) and species (lowercase) names, e.g., Homo sapiens.

Evidence for relationships

To determine these relationships, taxonomists use several methods:

• The frequency of measurable or observable characteristics: such as mass, number of legs, or number of stripes.

• DNA Sequencing: DNA can be extracted from different organisms, sequenced, and compared. Over time, DNA accumulates mutations, so the more similar the DNA sequences between two species, the more closely related they are. mRNA sequences can also be used similarly.

• Protein Sequencing: Since the amino acid sequence of proteins is determined by DNA, protein comparisons can also indicate evolutionary relationships. Proteins commonly used for this purpose include haemoglobin (found in animals) and cytochrome c (an enzyme in respiration found in all eukaryotes). The more similar the amino acid sequences, the more closely related the species.

• Immunological Comparisons: In this method, proteins from one species (e.g., Species A) are injected into a rabbit, causing the rabbit to produce antibodies against those proteins. The rabbit’s blood serum is then mixed with proteins from other species. The more similar the proteins, the more antibodies bind and form a precipitate. A greater amount of precipitate indicates a closer evolutionary relationship.

Advances in immunology and genome sequencing have enhanced our understanding of evolutionary relationships by providing direct comparisons of genetic material. Previously, genetic diversity was inferred from observable characteristics, but gene technology now allows for precise DNA analysis, providing more accurate insights.

Definitions