Taxonomy is the scientific study of the diversity of living organisms. A classification system places order and a general plan upon the immense diversity of life. When describing living organisms and determining if they have a new species, taxonomists look for differences and similarities between them: things like their physical features, methods of reproduction and molecular sequences.
What is a species?
There are about 20 different definitions for a species. Multiple definitions of species are required because some species definitions/concepts cannot be applied to organisms that do not reproduce sexually. For example, many organisms reproduce asexually (producing clones), undergo parthenogenesis or can transition between sexual and asexual.
Some examples of species concepts:
Morphological definition:
Differences in the form and structure of organisms is used to divide organisms into species. This definition can be applied to both sexual and asexual organisms and can also be useful when considering species in the fossil record.
Problems: very subjective, there can often be lots of variation within groups, and it also needs to take into account sexual dimorphism.
Biological definition:
Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups that produce fertile offspring.
Problems: inapplicable to organisms that reproduce asexually, and also problematic when you have geographically isolated populations.
Interspecific hybrids occur when closely related species breed together, but the offspring are not fertile. An example of an interspecific hybrid is a mule. However, note that although the syllabus states a binomial name for the mule (Equus mulus), this is considered invalid and cannot be used under the ICZN Code.
Ecological definition:
Defines a species by the resources that the particular group of organisms uses, and depends on the specialised ecological niche the group occupies.
Problems: it can be difficult to determine the degree at which different organisms are occupying the same ecological niche. Many species naturally have overlapping niches.
Phylogenetic definition:
Defines a species as a group of organisms that share an ancestor and have a shared and unique evolutionary history.
Problems: it is impossible to definitively know the past evolutionary pathway of a species.