The species problem consists of finding criteria for drawing clear boundaries between different species.
The problem arises because evolution is a continuous process while biological classification uses discrete categories (such as species).
There is often a broad and continuous range of phenotypes and genotypes in a population. This makes it somewhat arbitrary to set a cut-off point where a new species begins.
Definitions that work well for one set of organisms may not be applicable to others.
A species definition based on the ability of two individuals to interbreed (biological species concept) has no meaning for organisms that reproduce asexually (such as bacteria).
As a result, several different definitions of species are needed.
According to the biological species definition, a biological species is a group of organisms that could interbreed and produce fertile offspring.
All horses are able to mate with any other horse of the opposite sex and produce fertile offspring. As a result, all horses are considered to belong to the same biological species.
Conversely, donkeys and horses are defined as separate species because their offspring, mules, are infertile.
The biological species concept only applies to species that reproduce sexually. For example, this concept does not help in identifying species of Salmonella.
Two members of a species with extreme variety are sometimes unable to breed together. However, both would be able to breed with more typical members of the population. An example of this phenomenon is the domestic dog.
A ring species is a population with sub-groups that have partially overlapping habitats.
Organisms from subgroups with overlapping habitats are able to interbreed because gene flow continues between neighbouring subgroups. By contrast, two organisms from different subgroups that are separated completely are unable to breed.
Ring species are an example of the difficulties of specifying clear boundaries between species.
The biological species concept is considered to be the most rigorous for classifying animals, but there are many cases where alternative definitions make more sense or are also taken into consideration.
There are several alternative definitions of a species:
The morphological species concept defines a species based on morphological similarity (similarity in physical characteristics).
The ecological species concept defines a species by its occupation of a certain ecological niche (i.e. feeding on the same food in a specific manner or exhibiting the same patterns of behaviour).
The phylogenetic species concept groups organisms that share a unique common ancestor.
In tree 1 below, all the organisms share a most recent common ancestor, so they must be classed as one species or four separate species.
In tree 2, organisms 1 and 2 can be classified as a single species while excluding 3 and 4. This is because organism 1 has a more recent ancestor in common with 2 than with 3 or 4.