When is a species not a species? It might seem that all living things are neatly divided into separate types and each is distinct from all others, but the boundaries are actually much more blurry than this. A fascinating example is shown by what are called ring species and there are two types of gull in the northern hemisphere that provide a classic case.
Two of the gulls occurring in Britain are the herring gull and lesser black backed gull. They are quite distinct from each other. They do not interbreed with one another and they look quite different – herring gulls have light grey backs and black backs have, er well, very dark grey backs. If you now take a quick trip round the world, first you will find that in North America there are herring gulls but no lesser black backs. Then as you journey round you will notice that these gulls look progressively less like herring gulls and more like lesser black backed gulls. Adjacent populations are mixing and interbreeding as they are virtually identical. But as you go round the world through Russia and back to Europe, the population has now become fully lesser black backed gull and does not breed with the herring gulls there. As you travelled round the ‘ring’ one species ever so gradually morphed into the other until you get back to the start where they exist together as two separate species.
This is showing two interesting things. Firstly it’s a great example of evolution in action – one species is splitting into two in front of our eyes. For a dichotomy to occur a population needs to be physically divided into two to prevent genetic mixing so that evolution will take each of them off in their own independent direction. In the case of these gulls the populations have not been fully isolated so the genetic mixing that is happening over part of the range is blurring the concept of species. The other thing it does is give a superb insight into the gradual nature of evolution. Here we can see a gradual transition over a geographical range, but exactly the same thing happens over time as well. As things evolve the changes taking place are incredibly gradual – at no point does a species suddenly leap from being one into being another. It’s a little like counting. If we define 5 as a small number and 1000 as a large number, when we count from 5 to 1000, at what point does the number become large? It’s impossible to define, and so is the change from species to species in evolution.
There is a natural desire to define things and to put them in different boxes, but the nature of evolution tends to make this a difficult and rather artificial task. Another area which tends to confound the desire for strict definitions is that of grouping and classifying living things. Seeking to put everything into it’s own neat little box is always going to lead to problems – but I’ll probably come back to that in another post.