Classification of Living Organisms
Scientists classify organisms into different groups. Two closely related species will be classed in more of the same groups than two species which are more distantly related. The relationships between difference species are represented on evolutionary trees.
Classification
Traditionally, living organisms have been placed into groups depending on their structure and their characteristics in a system developed in the 18th century by Carl Linnaeus.
Linnaeus classified living things into a series of categories, starting with a broad category and then classifying them into smaller and more specialised subcategories. These categories are known as kingdom, phylum, class, order, family, genus, and species. The first group, domain, is the broadest category whereas the last group, species, is the most specific category. These groups do not overlap with one another, so an organism which is placed in the kingdom ‘animals’ will not share the same categories as an organism which belongs to the plant kingdom.
It’s useful to use a mnemonic to help you remember the order of the taxonomic groups. The classic one is ‘Dear King Phillip Came Over For Good Soup’ but my favourite is ‘Delicious King Prawn Curry Over Fatty Greasy Sausages’.
Binomial System
In the past, organisms were referred to by local and colloquial names and there were often hundreds of different names used to refer to the same organism. To make things more straightforward, scientists now use the binomial system for naming organisms. This two name system uses first the genus name and then the species name to refer to each organism. The scientific name for human beings is Homo Sapiens since ‘homo’ is the genus name and ‘sapiens’ is the name of our species. Likewise, the scientific name for the chimpanzee is Pan Troglodytes, which makes ‘pan’ the name of the genus and ‘troglodytes’ the name of the species.
After the invention of the microscope, scientists were finally able to see the internal structure inside cells. Together with improved understanding of biochemical processes, scientists were able to propose new and better ways of classifying organisms.
For example, due to evidence available from chemical analysis, there is now a three-domain system which was proposed by Carl Woese. In this system, organisms all organisms are divided into one of three domains, which now makes up the broadest category of classification. The three domains are:
Archaea – primitive bacteria usually living in extreme environments
Bacteria – true bacteria
Eukaryota – these include protists, fungi, animals and plants
Evolutionary Trees
Evolutionary trees are used to show the relationships between organisms over time.
Branch points indicate where one species develops into two new species, i.e. where speciation has occurred. If a branch point occurs between two species fairly recently, this means the two species share a recent common ancestor and are more closely related than two species with a branch point which has occurred further back in time.
Scientists draw evolutionary trees from a number of different sources, such as genetic evidence (e.g. looking at DNA sequences) and fossil records.
