Classification Processes – Organisms

Classification by the Similarity of Physical Features (Linnaean System)

Organisms can be classified based on what they look like. Their physical or morphological differences are used to divide organisms into appropriate groups. This is called the Linnaean system of classification, as it was first developed by Swedish botanist, Carl Linnaeus (1707-1778).

Linnaeus initially intended only to give each plant a genus name and a number (Convolvulus 1, Convolvulus 2, and so on), but he soon realised that that was unsatisfactory and developed the binomial arrangement that is still used today.

His binomial system is part of a classification system that is based on large groups being divided up into progressively smaller groups by their physical characteristics. This is hierarchical rank based structure of classifying organisms. A taxon (pl. taxa) is any group within a system of classification and is a collection of organisms sharing some basic features. At any level, any organism only belongs to one taxon and no other.

This system involves three domains, divided into kingdoms, phyla, classes, orders, families, genera and species. With an additional rank below species (subspecies).

This method of classification uses morphological features of organisms to create groups according to their similarities/ features of the groups that become more similar at each lower taxon and it allows each particular organism have its own unique scientific (binomial) name. Binomial names provide consistency worldwide as they are precise, concise and allow better communication between scientists.

It also allows biologists to recognise that that two species are closely related, e.g. Panthera leo (lion) and Panthera tigris (tiger), or Equus ferus caballus (domesticated horse) and Equus zebra (mountain zebra).

What are the rules for writing a binomial name?

The genus is always first with a capital letter. The species name is second and all lowercase. The name needs to be written in italics (e.g. Pteropus natalis).

However, this method of classification doesn’t just rely on physical features. Some organisms do not look like each other, but developments in the understanding of evolution allow us to classify based on the phylogeny of organisms, leading to evolutionary taxonomy.

For example, hyraxes are in the same clade (Paenungulata) as elephants, dugongs and manatees.

Describe four features of the Linnaean system of classification.

1. Hierarchical rank-based structure of classifying organisms.

2. Three domains, divided into kingdoms, phyla, classes, orders, families, genera and species. With an additional rank below species.

3. Uses binomial nomenclature to name classified organisms at the Genus species level.

4. Uses morphological features of organisms to create groups according to their similarities/features of the groups that become more similar at each lower taxon.