Oh what a tangled web we weave,
When first we practise to deceive!
When first we practise to deceive!
Sir Walter Scott
I came across this passage in Daniel Dennett's must read book, Darwin's Dangerous Idea
Darwin shows us that questions like "What is the difference between a variety and a species?" are like the question "What is the difference between a peninsula and an island?". Suppose you see an island half a mile offshore at high tide. If you can walk to it at low tide without getting your feet wet, is it still an island?. If you build a bridge to it, does it cease being an island?. What if you build a solid causeway? If you cut a canal across a peninsula (like the Cape Cod Canal), do you turn it into an island? What if a hurricane does the excavation work? This sort of enquiry is familiar to philosophers. It is the Socratic activity of definition-mongering or essence-hunting, looking for the "necessary and sufficient conditions" for being-an-X. Sometimes almost everyone can see the pointlessness of the quest - islands obviously don't have real essences, but only nominal essences at best. But at other times there can still seem to be a serious scientific question that needs answering.
More than a century after Darwin, there are still serious debates amongst biologists (and even more so amongst philosophers of biology) about how to define species. Shouldn't scientists define their terms? Yes, of course, but only up to a point. It turns out that there are different species concepts with different uses in biology - what works for palaeontologists is not much use to ecologists, for instance - and no clean way of uniting them or putting them in an order of importance that would crown one of them (the most important one) as the concept of species. So I am inclined to interpret the persisting debate as more a matter of vestigial Aristotelian tidiness that a useful disciplinary trait.
Dennet, D.; Darwin's Dangerous Idea; p. 95 (paperback edition)
And of course, this is undeniably true. I have previously blogged about the problem of trying to apply a taxonomic system designed for classifying co-existing species, to extinct species in Where Creationists Get Confused. For most species, it is obvious that this generation could interbreed with the previous generation, and the one before that, but if we could jump into a time machine and go back, say, a thousand generations taking some specimens along with us, it might not be so clear cut. We might well find that successful breeding did not occur, or only occurred infrequently.
Go back ten thousand generations and we might not even be able to identify the precise ancestors of our samples. And this is of course true for every generation, not just the present one. Every generation will always be able to interbreed with the previous one and its immediate descendent one, but somewhere along this line one species has gradually changed into another and it is impossible to say precisely where this occurred. (There are rare exceptions to this however, such as when a new species arises from a single occurrence of a mutation, such as I described in The Good Shepherd's Purse Is Bad News For Creationists or from a hybrid between two different species as the example of wheat I described in Evolution Gave Us This Day Our Daily Bread).
Normally, creationist 'scientists' would have seized on something like Daniel Dennett's statement with glee, waved it triumphantly in the faces of real scientists and written books about how it 'devastates' the theory of evolution and proves a god created every species as is. What good is a theory which purports to explain how species arose when scientists don't even know what a species is, eh?
So why haven't they?
They haven't done so because they are hooked on a definition of 'species' which is intentionally simplistic and naive - remember the audience they are writing for - and which requires there to be a clear-cut and large difference between species, so they can trot out the familiar old chestnut that evolution is possible within a species at something they call the 'micro level' to get around the overwhelming evidence that evolution is not only possible but can be observed, but not at the 'macro-level', i.e. to give rise to new species.
To concede that species are just not that distinct but may blur into one another, especially over time, often remaining in a state of stable but incomplete speciation for long periods of time, as we see with ring species, would destroy that important straw man argument which they have carefully cultured in the unfortunate minds of the people they mislead for a living.
It must be one of the few examples of creation 'scientist' crediting real scientists with an unjustified success, rather than looking for and trying to prise open the gaps in science or presenting disagreement and compromise as evidence of failure and unreliability.
So, creationists, if I'm wrong about this, what definition of 'species' are you using when you talk about 'macroevolution' being impossible, and what is your scientific basis for using it?