There is not a consensus currently on a universally applicable definition of a species. Instead, there are a number of definitions based on various inclusive criteria. For instance, one definition may apply to sexually reproducing organisms, but does not address asexually propagating organisms. The definition of species does not imply that organisms from the same species must be in the same population; in biological terms, a population is a group of organisms that interbreed and live in the same geographical location at the same time. After all, a fertile human can travel anywhere in the world and have children with another fertile human. All humans on Earth are members of the same species. However, all members of a given species are reproductively separate from members of all other species. They cannot interbreed with members of another species and produce viable, fertile offspring. This reproductive separation, or reproductive isolation, is the key to speciation. Reproductive isolation is a state in which reproductive barriers exist to prevent the formation of healthy, fertile offspring from individuals of two different species. Biological barriers can take a variety of forms, including differences in the size of species or entirely different ways of reproducing.
The biological species concept is the idea that a species is defined in terms of their ability to reproduce in nature to form healthy, fertile offspring. Although the biological species concept is practical for sexually reproducing organisms, it has limitations. For instance, it excludes all asexually reproducing organisms, including prokaryotes. The concept also excludes extinct organisms whose reproductive isolation cannot be evaluated. Considering that two of the three domains of life forms are asexually reproducing prokaryotes, meaning that they reproduce by binary fission or exchange genetic material by transformation, transduction, or conjugation, alternative definitions of species are necessary. One of the many alternative concepts is the morphological species concept, the idea that a species is defined and characterized by body shape and structure. This can be useful among species that show much variation in appearance. Consider, for example, that outward appearances in the human species vary but the basic body structure is the same for all. However, the choice of what constitutes important morphological traits is somewhat subjective. Additionally, it can be problematic when defining species that are morphologically indistinguishable yet do not interbreed. For example, there are three species that belong to the beetle genus Epicauta (E. chinensis, E. dubia, and E. sibirica) that appear morphologically indistinguishable.
Another way to define a species is by its role within an ecosystem, which consists of all organisms living in one geographic region and all the environmental factors that affect those organisms. The ecological species concept is the idea that a species is defined based on its ecological niche, which includes all the biotic and abiotic factors in the environment. This definition of species can be applied to asexually reproducing species, which are precluded from the biological species concept by a lack of interbreeding. Bacteria and other asexual microbes with distinct roles within an ecosystem, such as nitrogen fixation or photosynthesis, can be categorized using this concept.