I was listening to a podcast the other day (The Skeptics’ Guide to the Universe – I recommend it) which featured an item about the number of species that have been described. This got me thinking about that age-old headcracker: What is a species?
People’s first introduction to the idea of a species usually includes a definition relating to interbreeding. That is, species are defined as groups of organisms that can or do interbreed. Groups of organisms that do not interbreed, or where the product of any interbreeding is not fertile (such as mules and ligers), are separate species.
But when you remember that not all species are fluffy mammals, this definition becomes problematic. There are issues such as hybrids, which result when groups that are quite different usually do not interbreed, but occasionally do. Are such groups different species or not? But the main issue to my mind is that of the huge numbers of organisms that reproduce asexually – things like bacteria and fungi and many plants. Clearly a definition of a species related to interbreeding will not work for organisms that do not breed at all.
As a biochemist, I am enamoured with genetic approaches, which look at differences in the genomes of different populations to get an idea of which groups form distinct species. Such approaches may operate by construction of ‘trees of life’, and finding points where genetic changes resulted in the creation of two lineages. Such lineages thus form separate species. But this is still a problematic approach, as lineages that appear to be splitting can still interbreed (think of evidence suggesting that ancient humans interbred with Neanderthals and other hominids). There can also be no rule as to how much genetic difference constitutes a species ‘borderline’ – there is not necessarily a correlation between the extent of differences in genetics, and differences in appearance or behaviour (traits), aspects that are central to our thinking on what constitutes a species.
The genetic route also cannot always be followed. Think of palaeontology; often all we have to go on to study organisms that have existed in the past is the fossil record, which provides no genetic material. Here, judgements about species are made based on what the organism looked like, where it fits in wider fossil record, and any clues about behaviour that can be picked up from where the fossil is found. There are clear potential issues with such judgements. Imagine palaeontologists of the future finding remains of a chihuahua and a great dane; they would instantly classify these finds as two different species, while we consider them both dogs.
In thinking about this, I realised that the ‘species problem’, as it is known, is an excellent example of how science is not the objective gathering of knowledge that we are so often taught to think it is. The existence of certain species is not a ‘fact’ of the world that is waiting to be identified by us. Rather, the idea of species is an imposition of our need to classify, to label, to impose an idea that helps us to better understand and describe the world in which we live. There are indeed very different organisms out there, and it is helpful to look at the differences and what they mean, and to attach different names to different things so we know what we are talking about. But underneath, we must remember that these borderlines that we think of in fact do not really exist. As is blindingly obvious when you think about the process of evolution, life is really just one thing, based on the same principles everywhere on earth (as far as we know), and expressing itself in a glorious variety of ways. Over time, changes have occurred that produced this variety, but these changes have been and still are gradual, so that it is nonsense to think that there are lines that could be drawn to show where one species became another. Indeed, any such lines would be completely arbitrary, and a result only of our purely human perspective.
I often think of this quote from Ernest Rutherford (possibly): “All science is either physics or stamp collecting.” I was instinctively annoyed by that quote when I first heard it, and it occasionally resurfaces in my head to bug me. Pondering on the question of species has made me smile, through the realisation that the classification of organisms into species – pretty much the most ‘stamp collecting’ aspect of science I can think of – has ultimately led to this realisation that it is the underlying processes of evolution and speciation that is the real point of interest, showing how biology is every bit a dynamic, investigative science that is concerned with the forces that shape the world in which we live.