Understanding Species and Populations
In the 20th century the modern concept of a biological poplation was developed. Biological populations are interbreeding communities of organisms that share in the evolutionary process. However populations are not always easy to study or identify.
At or above the level of populations, biologists also work with species. The idea of a species, or a kind of organism, has been around for a very long time - perhaps into the very roots of human language.
Paradoxically one of the most pernicious uncertainties in evolutionary biology is the meaning of the word "species". This question, and the general absence of consensus on the best methods to identify species, have been called the species problem. The debate has raged at various levels and on various fronts for several decades, and it ranges over very practical applied aspects as well as over very theoretical and philosophical aspects. The debate is also played out on two very different fronts: the systematic front, where researchers are most concerned with methods of identification and classification; and the population biology front, where researchers are most concerned with a species as a kind (what kind?) of natural group- a level of biological organization.
The species problem is not just an academic debate, for it has a large effect on the way that biologists work to discover and preserve biological diversity. In recent years, discussions over how to identify species and define "species" have come to the fore in the literature on biological conservation. Nor will they dissipate without some widespread recognition of the basic causes of our uncertainty.
Genes, Categories and Species Jody Hey (2001) Oxford University Press