Classification
With the amount of species (both past and present), it makes sense to organise them into groups to avoid confusion.
The grouping together of organisms is described as classification.
The theory and practice of biological classification is called taxonomy.
Phylogenetic classification
Phylogenetic classification is based on evolutionary differences between organisms and their ancestors.
It classifies species into groups using shared features derived from ancestors and arranges them into a hierarchy in which the groups are contained within larger complex groups with no overlap.
You can look at homologous characteristics to determine how closely related organisms are to each other.
Homologous characteristics have similar evolutionary origins regardless of the function in the adult species.
An example of homologous characteristics are the wing of a bird, the arm of a human and the front leg of a horse.
Each group within a natural biological classification is called a taxon. The plural is taxa.
Taxonomy is the study of these groups and their structure in a hierarchical order in which they are known as taxonomic ranks.
The groups are based on evolutionary line of descent and are categorised like this:
Domain: This is the broadest category. It includes bacteria, archaea and eukaryota.
Kingdom: Kingdom is the second highest taxonomic rank.
Phyla: Phyla is the largest group within kingdoms. Organisms in each phylum have a body plan different from organisms in any other phylum.
Classes: classes are a result of diversity within each phylum.
Order: each class is divided into orders of organisms that have additional things in common.
Families: orders are divided into families. At this level, differences are less obvious.
Genera: each family is divided genera
Species: each genus is divided into species
Each species has a binomial name which indicates its genus and species.
Evolutionary relationships between different organisms are referred to as phylogeny.
Phylogeny reflects the evolutionary branch that leads up to a species.
Species
A species describes a group of organisms that shares similarities as well as differences from other species and can produce fertile offspring.
Species are not fixed forever, they can change and evolve over time and some individuals develop into new species.
There can be considerable variation within a species. For example, all dogs belong to the same species, but because of artificial selection there are now various dog breeds.
Many species are now extinct and many have left no fossil record or no fossils were discovered.
Some species, rarely, if ever reproduce sexually.
Groups of organisms may be isolated from each other and never meet to interbreed