Species concepts

Biologists are often asked what the definition of a species is. Before dealing with this question it is important to consider the differences between what a species is and how species come into being. The ‘what’ question deals with the main subject of this essay, how do you define a species. The ‘how’ question deals with processes and mechanisms. I would like to write about this part first; partly because it is the really contentious and interesting part of the question and partly to clear up any misunderstanding between definitions (what) and mechanisms (how).

Species, at their most abstract, can be thought of individual historical entities. This is to say, like the most basic story outline, they have a beginning, middle and an end. Or, if you prefer, a birth, lifespan and a death. Just as all people do. So species, like people, exist at certain places and certain times. 

How the birth of species comes about is a question of mechanism(s) that I will not go into in very much detail, but the key image to have is the notion of a single homogeneous population (group of individuals) that eventually gives rise to two distinct populations that can no longer combine their genetic material to give rise to more of their kind. The most widely accepted method by which this can come about is the subdivision of the single population into two subpopulations by the appearance of a geographic barrier (a mountain range, a new river) that can no longer exchange genes. As time passes then the genetic make-up of the two subpopulations could evolve enough to generate sufficient differences that would mean they are no longer two subpopulations, but are in fact different species. It should be clear that at least some time has to pass for enough differences to accumulate for the two subpopulations to become separate species, but the amount of time varies, due to a wide range of factors. 

Despite what many people believe scientists HAVE witnessed the evolution of new species (http://www.talkorigins.org/faqs/faq-speciation.html), but the difficulty in communicating these findings to non-specialists is that the MECHANISM is often quite difficult to understand (e.g. self-fertilization and chromosome duplication in plants) or involves organisms such as bacteria that don’t conform to most peoples’ ideas of animal species that they are familiar with.

Which species concept is best?

Species concepts are best thought of as tools for a job. As an example, think about your five favourite books. This is a meaningful way of categorizing books, but is relevant to you and has nothing to do with the intrinsic properties of the books themselves. Most scientists contend that species are entities that have a real existence outside of human thought, and our scientific concepts should correspond to real entities in nature that would exist whether or not humans had names for them. Thus concepts such as the biological species concept attempt to mirror nature. Conversely, species could be erected using morphological or genetic data that do not reflect shared evolutionary heritage. An example would be old classifications that included whales and dolphins (mammals) within fish. They do indeed share some similarities with fish (fins, living in water) but not as the result of having a shared evolutionary history. Nonetheless, such classifications can be useful for identifying organisms, but they do not take evolutionary history into account. 

Now we will go on to the various species definitions used by different areas of biology. That’s right, definitions plural. Different areas of biology use different species concepts, depending on the organisms and data available . Each of the major species concepts is explained below, along with the advantages and disadvantages of the species concept and which areas of biology use the definition. In several cases below I have combined several finer distinctions of species concepts into a single definition, as I think the distinctions are not so important for an introductory essay. For those who want to know more, some suggest