A species is defined as a group of living organisms capable of interbreeding to produce fertile offspring.

In biological terms, a species represents the fundamental unit of classification. It is characterized as a group of individuals that can or do interbreed naturally. This definition implies that a species encompasses the largest possible gene pool under natural conditions. Known as the Biological Species Concept (BSC), this definition emphasizes reproductive isolation as a critical factor in the formation of species.

For example, horses and donkeys, while similar and capable of mating, produce mules, which are almost always sterile. This sterility leads to the conclusion that horses and donkeys are distinct species. Conversely, all domestic dogs—ranging from chihuahuas to German shepherds—belong to the same species since they can interbreed and produce fertile puppies.

However, the BSC has its limitations. It is not applicable to organisms that reproduce asexually, such as bacteria and many plants. Additionally, it encounters challenges with ‘ring species,’ where populations can interbreed with their neighboring populations, yet two end populations in the series are too genetically distant to interbreed.

To address these limitations, alternative species definitions have been proposed. For instance, the Phylogenetic Species Concept (PSC) defines a species as a group of organisms that share a common ancestor, forming a lineage that retains its identity from other lineages. Another approach, the Ecological Species Concept (ESC), defines a species based on its ecological niche, focusing on the role it plays within its environment.

Despite these various definitions, the notion of a species as a group of interbreeding individuals remains the most widely accepted and utilized concept in biology. This foundational idea is essential for understanding biodiversity and the processes of evolution.