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Species Concepts and Reproductive Isolating Barriers

Author(s): Tadzia GrandPré, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Limitations of the Biological Species Concept

While the biological species concept has had a profound influence on the development of current evolutionary theory, it has limitations. For example, because the biological species concept defines species in terms of interbreeding, it cannot be used to determine if similar populations that are geographically separated belong to the same species, nor is it useful for the classification of species in extinct populations. In addition, the biological species concept is most applicable to organisms that reproduce sexually and is less useful for the classification of organisms that reproduce asexually, including single-celled organisms like bacteria, many plant species, and even some vertebrates. 

The biological species concept also can be problematic when reproductive isolating barriers are incomplete. Biologists would consider this strong evidence of reproductive isolation. For example, some estimates suggest that 10% of bird species hybridize. Hybridization is even more common among flowering plants. These limitations of the biological species concept have led scientists to propose alternative species concepts.