What is the biological definition of a species, and how is it applied?

A species is a group of living organisms that can interbreed to produce fertile offspring.

In biological terms, a species is the fundamental unit of classification. It is defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions. This definition of a species, known as the Biological Species Concept (BSC), emphasises reproductive isolation as the most important factor in species formation.

For instance, horses and donkeys are similar and can mate to produce mules, but mules are almost always sterile. Therefore, horses and donkeys are considered separate species. On the other hand, all dogs, from chihuahuas to German shepherds, are part of the same species because they can interbreed and produce fertile puppies.

However, the BSC is not without its limitations. It cannot be applied to organisms that reproduce asexually, such as bacteria and many plants. It also struggles with 'ring species', where populations are able to interbreed with their neighbours, but there are two 'end' populations in the series that are too distantly related to interbreed.

To address these issues, other definitions of species have been proposed. The Phylogenetic Species Concept (PSC), for example, defines a species as a group of organisms that shares an ancestor; a lineage that maintains its identity from other lineages. The Ecological Species Concept (ESC), on the other hand, defines a species in terms of its ecological niche, the role that it plays in its environment.

Despite these different definitions, the concept of a species as a group of interbreeding individuals remains the most widely accepted and applied in biology. It is a fundamental concept that underpins our understanding of biodiversity and evolution.