The key characteristic used to identify a group as a "biological species" is:

another as closely because they share fewer genetic characteristics ... Similarly, we cannot use the biological species concept species concept is that biologists can apply it to any group of organisms, including species that have long. flow between them-one of the key requirements of the biological species definition.

The biological species concept gives an explanation of how species form (speciation). A biological species is a group of individuals that can breed together (panmixia). However, they cannot breed with other groups. In other words, the group is reproductively isolated from other groups.

"The words 'reproductively isolated' are the key words of the biological species definition".

According to Ernst Mayr, a new species forms when an existing species splits. A similar idea had been suggested in the 19th century by Moritz Wagner. Dobzhansky described the role of reproductive isolation in the formation of new species. Once a species lives in two different areas, the geographical isolation makes breeding between the groups reduce or stop. Each group develops features which make breeding between them work less well. Eventually, each group becomes a 'good' biological species, because the two species do not reproduce with each other even when they are together.

This is still considered the most common reason for species splitting, and has the technical name of allopatric speciation. It is contrasted with sympatric speciation where speciation takes place even though all the members live in the same area.