Despite the BSC being a commonly universal definition of species, there are some problems with the concept. Firstly, this concept can only be applied to sexual reproductive organisms and therefore can not be applied to asexual organisms or fossilised extinct organisms:
"Where we have sexual reproduction a species can be objectively defined as a group of organisms which reproduce sexually amongst themselves but don't reproduce with members of other species.
Where we don't have sexual reproduction - as in asexual species, or in fossils where we have no idea of how they reproduced - then there is no objective definition of the species, and the species just becomes like the genus, the family or the class. It's subject to arbitrary human decision."
(Richard Dawkins (2005))
Also the BSC does not take fossils into account as there is no way of knowing how these extinct taxa reproduced and therefore which organisms were reproductively isolated from one another.
Furthermore, this concept assumes that individuals can only mate with members within their own species. However, there are a few cases of interspecies reproduction that occur and produce hybrid offspring which in some cases are fertile. For example, in 1985 at Sea Life Park in Hawaii a mating between a female bottlenose dolphin, Tursiops truncates, and male false killer whale, Pseudorca crassidens, resulted in a hybrid female calf known as a ‘wholphin.’ Not only was the hybrid calf healthy but she also recently confirmed her fertility by mating with a male bottlenose dolphin and giving birth to a female calf:
The only whale-dolphin mix in captivity has given birth to a playful female calf, officials at Sea Life Park Hawaii said Thursday. The calf was born on Dec. 23 to Kekaimalu, a mix of a false killer whale and an Atlantic bottlenose dolphin…The young as-yet unnamed wholphin is one-fourth false killer whale and three fourths Atlantic bottlenose dolphin.
(Lee, J.J. (2005))
The BSC does not account for these rare hybrid occurrences, such as the previous example. Some people may argue that this interspecies mating was somehow forced since the animals were in captivity however “there have been reports of wholphins in the wild.” (Lee, J.J. (2005)). This natural example of successful hybridization along with a few more selected cases of interspecies mating that occur represent a group in which the term species cannot be defined in terms of interbreeding and hence the BSC cannot be applied.
An alternative species concept to BSC is the Morphological Species Concept (MSC). According to this concept, "a species is a community, or a number of related communities, whose distinctive morphological characters are, in the opinion of a competent systematist, sufficiently definite to entitle it, or them, to a specific name" ()). In other words species are defined as groups of individuals that have many morphological similarities that are clearly distinguishable from other groups. This is how, in practice, taxonomists recognise species. An advantage it has over BSC is that it can be applied to asexual organisms and fossils. However, the MSC does have some difficulties of its own. For example, some organisms use a mechanism known as Batesian mimicry. This is where one innocuous species (mimic) evolves morphological similarities to resemble a noxious or dangerous species (model) in order to survive:
“The classic example would be the perfectly harmless Viceroy butterfly which, as seen in the image, closely resembles the poisonous Monarch butterfly. Monarch butterflies are almost completely free from attacks by birds, presumably because of their bad flavor attributed to the fact that its larvae feed exclusively on milkweeds. Viceroy butterflies on the other hand are a completely different family of butterflies whose larva feed on the leaves of cottonwood and willow trees, and who are perfectly tasty. Most birds however avoid eating Viceroys just because of its similar looks to the Monarchs.”
(Skorucak, A. (2005))
These two types of insects “are a completely different family of butterflies” (Skorucak, A. (2005)) yet the MSC would categorise them into the same species since they share almost identical morphological characteristics.
Another problem with the MSC is that it is very subjective with respect to how the individual interprets similarity and distinctiveness of morphological characteristics. As a consequence of this, the concept will result in a range of arbitrary species definitions dependent on what interpretation of the concept is used. For example, the similarity used by an individual may be so narrow that differences which occur in some groups of organisms between females and males due to sexual dimorphism may result in the two sexes being assigned to different species.
Another species definition that has been proposed is phylogenetic species concept (PSC). This concept defines a species as a group sharing a unique evolutionary history: PSC is “The concept of a species as an irreducible group whose members are descended from a common ancestor and who all possess a combination of a certain defining, or derived, traits.” (Oxford Dictionary of Biology (2004)) This concept is not as limited as the BSC since interspecies mating is not excluded. However the PSC has limitations of its own; this concept will tend to result in extreme division of species into many small groups, since variation is present in almost every group of organisms.
In conclusion, no specific definition of a species can be given, but in fact there are several, with each depending on what concept is being used. The BSC defines species in terms of interbreeding. The MSC defines species in terms of morphological similarities. PSC defines species in terms of evolutionary history. There are in fact more species concepts but, like the ones mentioned, each concept has faults and no concept can be applied constantly to every situation. Each concept is similar in principle. They all define species as distinct groups that differ according to one particular factor. “No one definition (of species) has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.” (Darwin, C. (1859)). Darwin wrote this in 1859 yet it is still true today. The definition of species therefore, is the one which best suits the context in which it is being used.
References:
Campbell, N. and Reece, J. (2005). Biology. 7th edition. San Francisco: Benjamin Cummings.
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection, or the
Preservation of Favoured Races in the Struggle for Life. 1st edition. John Murray,
London.
Dawkins, R. cited in Ridley,M. (2005). Evolution. 3rd edition, [online]. Available:
[25 Oct. 2005]
Lee, J.J. (2005) Whale-Dolphin Hybrid Has Baby ‘Wholphin’, [online]. Available: [25 Oct. 2005]
Oxford Dictionary of Biology. (2004) 5th Edition. Oxford: Oxford University Press
) cited , E. (2004) Species Diversity, [online]. Available:
[29 Oct. 2005]
Ridley, M. (2004). Evolution. 3rd edition. Oxford: Blackwell Science Ltd.
Skorucak, A. (2005) Batesian mimicry, [online]. Available: [27 Oct. 2005]