The idea of a species is fundamental to gardening and botany, but what is a species? A dictionary will give you a definition but not one which allows you to say this group of plants forms a species, as opposed to say a subspecies. The Cambridge Illustrated Glossary of Botanical Terms describes a species as,
A group of closely related, mutually fertile individuals, showing constant differences from allied groups, the basic unit of classification
As it is this is a nice definition but does really tie down what a species actually is? No, the truth is a species is indefinable. I realise this is probably about as helpful as a fridge in an igloo but once you appreciate what a species actually is it makes complete sense.
So how did we end up in this situation? Well, if anyone is to blame its probably Charles Darwin and Alfred Russel Wallace. These two men independently came to the same conclusion, though they both arrived at it from observing the plants and animals on small islands.
Prior to this people saw species as distinct groups:
The problem with this was two fold. One where did the species come from originally and second where did the fossils come from people kept finding. The fossils were clearly very old: some similar to living things, while others where very different but still recognisable as plants or animals with characteristics at least similar to living examples.
By the 1850’s when Darwin first published his ideas on the origin of species the notion that living things appeared by evolution was widely accepted but the mechanism which made it work was unknown. What Darwin and Russel suggested was that species changed into new ones by a very gradual, survival driven, process of small changes. Each step in this evolution is so small as to be unnoticeable at the time and could only be appreciated with the benefit of hindsight. This process applied to all the species all of the time; but over many years and generations. The only reason we see separate species is because all of these intermediates have been lost, except for the very rare fossil.
If this is correct, and all the evidence we have found to date supports this, then there is no real starting point to a species and the only end point can be extinction. What we call a species is merely the extant remnants in a chain of gradual changes.
For this reason a species is rather like fog, you can see where it is, you can say you are in it or not, but it’s impossible to say exactly where it stops and starts. This is why no one has yet come up with a strict definition of what a species is, and in all likelihood they never will.