Most people will have a pretty clear intuition of what constitutes a biological species. They will look around and see types of animals that look and behave very differently and consider these types as different species. However, the problem of defining the concept of species scientifically is much trickier than it appears. This article looks at the problems that have been encountered over the years and current attempts at the definition.
The species is the most basic taxonomic rank in the modern biological classification system. Indeed the latin names of familiar living things consist of the the genus, which is the rank above species, followed by the species name. An example of this binomial classification is the scientific name for human beings, Homo sapiens. But how do you decide where to draw the line between species in your classification system?
This is a problem that would have had its roots in the prehistoric world as languages themselves were developing. People had to agree on common terms for creatures just for the sake of basic communication about their environment. But it was with the birth of Philosophy, in ancient Greece, that attempts at providing a systematic taxonomy of living things began in earnest. Aristotle, in particular, provided a system that would prove influential right up to the birth of modern biology.
In modern science the typical way of classifying species has been on the basis of which animals can breed with each other and produce fertile offspring. One problem with this definition is that not all perceived species are completely isolated from each other genetically, especially in cases where new species are still in the process of diverging from a common ancestral species. There may be some degree of overlap, with some members of the two species being suggested, Species A and Species B, able to cross-breed with each other whilst other members of A and B are not able to cross-breed with each other.
As new techniques have become available and more evidence has come in scientists have been able to provide a more accurate picture of the relationships between the different types of living things. But the term species appears to have become weaker, being used more like one more rank of the classification system. Genetically, every creature, whether of the same species or different species, can be linked by a sequence of mutations. Animals appear to come in distinct groupings that can be called species only because there are no extant intermediate forms.