For natural selection to work, it has to occur along with a bunch of other things. Historians and biologists who have analysed Darwin’s work, for example Ernst Mayr, have identified fivetheories which Darwin outlined in On the Origin of Species, and which work together to bring about evolution.

Darwin’s five theories were:

1. Evolution: species come and go through time, while they exist they change.
2. Common descent: organisms are descended from one, or several common ancestors and have diversified from this original stock
3. Species multiply: the diversification of life involves populations of one species diverging until they become two separate species; this has probably occurred billions of times on earth!
4. Gradualism: evolutionary change occurs through incremental small changes within populations; new species are not created suddenly.
5. Natural selection: evolutionary change occurs through variation between individuals; some variants give the individual an extra survival probability.

Darwin considered all these theories as parts of one grand idea; they all occur together. Scientists however took a while to see this; they weren’t accepted as a package until the modern synthesis of the 1930/40s. Before then scientists would favour some ideas but propose alternatives to fill in the gaps, natural selection was one of the least popular, to find out why click here. Eventually, as more evidence accumulated and these different ideas were tested it became clear that Darwin was right all along!

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?

Darwin himself wrote ‘an unverified hypothesis is of little or no value’. To verify his ‘hypothesis’ Darwin collected a vast number of facts from a wide range of fields. He assembled reports from other naturalists, as well as from his own work and observation, to support his five facts. His greatest challenge perhaps, was to convince people that species really are variable and that this variation is suitable for natural selection to act. Darwin chose to demonstrate this using artificial selection and production of various breeds of domestic animals and plants as an analogy for natural selection. You can read more about how he did this by clicking here.

Darwin added to his bulk of evidence throughout his lifetime, for example with studies on humans. Since 1859 the scientific community has been busy testing his theories, and alternatives, to see what best holds up. The wealth and diversity of evidence is now vast and includes evidence from the DNA record, fossil record, and from case studies section.

Written by Stephen Montgomery

A nice article on how to understand natural selection can be found here.

Almost Like a Whale
by Steve Jones, Doubleday: 1999

The Autobiography of Charles Darwin
by Charles Darwin (Edited by Francis Darwin), The Thinker's Library: 1929

Darwin
by John van Wyhe, Andre Deutsch: 2009

Darwin
by Adrian Desmond & James Moore, Penguin: 1991

Darwin: Discovering the Tree of Life
by Niles Eldredge, WW Norton & Co.: 2005

On the Origin of Species
by Charles Darwin, 1859 (any reprint - 2nd edition preferable)

Evolution
by Carl Zimmer, Arrow: 2003

Evolution
by Mark Ridley, Wiley Blackwell: 2003

Evolution
by Nick Barton, Derek Briggs & Jonathan Eisen, Cold Spring Harbour: 2007

Evolution: What the Fossils Say and Why it Matters
by Donald Prothero, Columbia University Press: 2007

One Long Argument
by Ernst Mayr, Allen Lane: 1991

What Evolution Is
by Ernst Mayr, Phoenix: 2002

Why Evolution is True
by Jerry Coyne, OUP: 2009