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 , 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.