For evolution by natural selection to occur individuals of a species must be variable. Slight differences in size, colour, behaviour, physiology and all the other components of living things must exist. Furthermore these variations must be evolutionarily meaningful; that means two things:
- they have to have some effect on survival and reproductive success
- these differences must be heritable; they have to be passed on from parent to offspring
The existence of variation may not seem like a major problem now; scientists have become very used to thinking about species in a way known as ‘population thinking’. Population thinking emphasizes the uniqueness of each individual within a population. For example when you see lions on a TV documentary it’s clear some are bigger than others, or better hunters, or better parents, some are more aggressive to other lions and some sleep more than others.
This may seem like an obvious way to view species, but Darwin was the first scientist to really see this; it was one of his greatest observations. Darwin’s peers, from the great Victorian naturalists to the beetle collectors and village parsons who dabbled in science, all agreed: species were species. They ascribed to a school of thought (to varying degrees) known as ‘essentialism’. The same logic that led Pythagoras to state that a triangle is always a triangle led Victorian scientists to consider a species as a fixed type or ‘natural kind’.
The predominance of essentialism posed a serious problem for Darwin. If essentialism was correct, if species are fixed, if a lion is and has always been a lion in its present state there can obviously be no evolution; a new species could only come to be if it was created in its modern form.
Not only did Darwin’s theory argue that species were not fixed and that they did evolve, but his mechanism for evolution, natural selection, was based on small differences between individuals of the same species. This posed three further problems: first of all, what differences? If species varied to an extent that they could change, why had no one noticed before? Secondly, natural selection depends on individuals of the same species competing and out doing each other. But to Victorian eyes the natural world was harmonious; they saw no struggle for existence. Finally, an animal or plant works as a unit; if you change one bit, won’t it affect all the others?
It would be no good for Darwin to present the bones of his theory and hope, in time, people would come to accept it, because they probably wouldn’t have. The vast majority of scientists just weren’t looking at the living world in the right way. Darwin himself would have to change this, he would have to provide the evidence and the argument to persuade the world that:
- individuals of a species vary (a lot)
- this variation affects the individual's survival and the number of offspring it has (even just a little)
- this variation is heritable and can be additive
- selection can act on particular traits independently from the rest of the organism
Luckily, there was ample proof available, and it was already familiar to most people. Darwin’s proof of concept lay in wheat, rice, cows, sheep, chickens and pigeons: the products of domestication of wild animals and plants by artificial selection.