As with all terms, ‘species’ is a word created by human, and so is its underlying concept. Not surprisingly, a single definition cannot be accurately applied to all situations. It should be noted that the term was invented to allow us to organize the huge numbers of different life forms on Earth.
The species concept has an important role in several societal issues such as nature conservation and environmental law; therefore a proper understanding of species is warranted.
Biological Species Concept
When people refer to ‘species’, they often follow the ‘biological species concept’ that supposes groups of individuals are of the same species if they are capable of mating and producing fertile offspring, but belong to different species if they are unable to mate or the mating results in infertile offspring. Different species evolve because the groups are reproductively isolated from each other over a long period of time by factors (physical and/or genetic barriers) that prevent interbreeding.
Defining a species this way allows us to unequivocally distinguish between incompatible sexually reproducing organisms, such as a dog from a cat. However, the definition does not apply to organisms that reproduce asexually (e.g. bacteria, single-celled organisms) or those that are extinct or whose reproductive physiology has not been investigated.
Morphological Species Concept
A long tradition is to distinguish species by what they look like. If the structure of two individuals is rather dissimilar, then the individuals are considered different species. This method of discerning species can still be useful, such as deduction from fossilized structural imprints of extinct species whose physiological data are unavailable. Even for extant (living) organisms whose biology is not well known, classification via morphological traits may be the only option.
But an obvious drawback is deciding whether the morphology in two individuals is sufficiently large overall to consider them separate species. Many organisms look very alike to the human eye, but can in reality be unrelated (e.g. many microorganisms), and some may look very dissimilar but are actually the same kind (e.g. caterpillars and butterflies, male and female birds with contrasting plumage coloration).
Genetics/Genealogical Species Concept
An increasing number of biologists are adopting a genealogical species concept that uses genetic data to determine the degree of divergence (deviation) in the genomes of organisms. A set of organisms is identified as one species if they share a unique genetic history. Genetic analysis can also reveal the time of divergence between taxonomic groups, reinforcing its value in defining species. However, it strikes a problem in organisms whose DNA is or cannot be obtained (e.g. many fossilized forms).
Other Species Concepts
Other ways of describing species exists. Examples include ecological species concept which is based on an organism’s ecological niche (the set of environmental resources an organism uses) and pluralistic species concept that considers both ecological niche and reproductive isolation depending on the situation.
Which Definition is Appropriate?
Usage of each species concept depends on the situation and the types of answers we are seeking.
Thus, although measuring the degree of morphological differences can be carried out for almost all forms of life, it does not explain how species are derived. This question cannot be answered either by the genealogical species concept, which is instead more suited to evolutionary inquiries. The biological species concept does determine how species originate, but perceptions about adaptation are more in the interests of the ecological species concept.
To the question of distinguishing whether two individuals are the same or different species, it appears combining the foundation concepts of reproductive incompatibility and morphological estimation should suffice in most cases.
Reading
Ridley M. (2003) Evolution. 3rd Ed. Wiley-Blackwell.
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