A species is defined as the group of individuals that form an interbreeding population such that the gene flow occurs freely within the species but do not occur between the species. Mayr in 1942 proposed this theory and his study is now referred to as the biological species concept and is summarized as follows: The species are the group of the interbreeding population that are reproductively isolated genetically from the other groups by isolating mechanisms as hybrid sterility or mate acceptability. The central issue is whether the species is viewed as the units of the evolution or as potentially inbreeding sets of populations that represent some coherence from the population. Reproductive isolation does not mean that reproduction is not possible. The biological species are the population that have the sufficient cohesive capacity to preclude indefinite divergence within the species but not in between the species.

Concept for Sexual Organisms:
The one of drawbacks of the biological species concept is that it is not applicable to non-sexual organisms. The concept emphasizes reproductive isolation, a byproduct of genetic divergence. Burger in 1975 studied the American Oaks and concluded that genetic discreteness differs from the genetically discrete species. Single or point mutation is sufficient to carry out the barriers to gene exchange. Gene exchange takes place between sympatric and allopatric species and forms the intermediates. Therefore, it is difficult to apply taxonomic and biological species concept to the genus. The syngameon species are capable of exchanging the genes, and the species that is involved is known as semispecies. Other scientists used the term multispecies to designate the genetically discrete but not the genetically isolated species groups. A species has multiple origins from different intermediates. Interbreeding is possible as the species lack reproductive isolation. The other term is compilospecies, which have apparently originated from the intermediates. For example, Bothriochloa intermedia, which has absorbed genes from several other species of Bothriochloa and related genera of Dichanthium and Capillipedium. The introgression is in the direction of the B. intermedia, without which all other species remain reproductively isolated and it is suggested that the species that contribute to the variation in the compilospecies is recognized as the distinct species.

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