Natural selection was Darwin’s most novel and revolutionary idea, but in truth (like all the best ideas) it is very simple. Despite its simplicity, since the publication of the theory right up until today, it has widely been misunderstood. Ernst Mayr, in his book One Long Argument (1991) provides a useful way of breaking down the process into just five facts and three inferences, or conclusions, drawn from the five facts; they can be linked in a flow diagram:
Figure: modified from One Long Argument by Ernst Mayr (1991)
The first inference is drawn from three facts which Darwin observed in the natural world around him. He saw that organisms produce more offspring than is required to replace themselves, so population sizes should increase rapidly (think about the number of frogspawn laid each year, or how many eggs a spider lays). That’s fact one: a fancy word for this over-reproduction is ‘super fecundity’. However Darwin saw for himself, and confirmed his observation with others, that population numbers tend to stay at about the same level (you don’t see a doubling of the number of frogs or mice in your garden each year do you?): that’s fact two. What accounts for this disparity? Darwin found the answer with another fact: resources, such as food, water or places to sleep or mate, are limited. A major influence on Darwin observing this fact was his reading the work of Thomas Malthus who published a paper stating that the human population was increasing at a rapid pace and would soon run out of food, water and space. These are three simple facts which Darwin put together to draw a simple conclusion: individuals compete with each other for scarce resources.
Next, Darwin made two other observations about individuals. First he had come to the conclusion through his work on theH.M.S. Beagle, when he was working on barnacles and later pigeons, that individuals are unique and that individuals vary in almost every aspect: that’s fact four, and you only need to take a cursory glance round a group of people to see that it is true! Finally fact five: Darwin had taken to breeding pigeons to investigate variability further. He performed many crosses between different breeds of fancy pigeons to look at whether their offspring had the same variations. He also collected lots of observations from various animal and plant breeders to help him draw out the conclusion that these individual differences are heritable: they are passed on from parent to offspring.
The next two inferences demonstrate Darwin’s genius. Darwin could see that if individuals must compete, and if they are all unique, some individuals will have variations which give them a survival boost so they will have more opportunity to reproduce and leave a greater number of offspring. These offspring will inherit the variations which made their parents successful, so they too will have an advantage. Over time these successful variantions will spread through the population – the population will change: that is evolution! Simple, isn’t it?