Biology – Classification | e-Consult

The classification of organisms within the Eukarya domain is not simply a system of grouping based on superficial similarities. It is fundamentally based on evolutionary relationships, reflecting the history of life on Earth. This is achieved through the concept of phylogeny.

Phylogeny refers to the evolutionary history and relationships among a group of organisms. Modern classification systems, particularly those used in molecular biology, aim to reflect phylogeny as accurately as possible. This is done using data from various sources, including:

• Morphological data: Similarities in anatomical structures (e.g., bones in vertebrate limbs) suggest common ancestry.
• Fossil record: Fossils provide evidence of past life forms and help to trace evolutionary lineages.
• Molecular data: DNA and RNA sequences are compared to determine the degree of genetic similarity between organisms. The more similar the sequences, the more closely related the organisms are likely to be. This is particularly powerful in modern classification.

Modern classification systems, such as those based on cladistics, use these data to construct phylogenetic trees (also called evolutionary trees). These trees depict the evolutionary relationships among organisms, showing how they have diverged from common ancestors. Organisms that share a more recent common ancestor are grouped together in closer taxonomic ranks. Therefore, the classification reflects a nested hierarchy of lineages, with each branch representing a distinct evolutionary path. This means that organisms within the same taxonomic rank are more closely related to each other than to organisms in other ranks.