In Darwin’s travels to distant lands from 1831 to 1836 on the Beagle, the different climates and environmental conditions he encountered made him aware of the weakness of the existing theory of ‘special creation’, where god was assumed to have created creatures best suited for their environment. Darwin saw for himself that very similar climates could produce hugely different kinds of species, and that the nature of these species seemed to be more influenced by the species in nearby areas than by anything else. This seemed to him to suggest that new species arose from the modifications of the old.

The discovery that the Earth was much older than had been previously thought, and the evidence for which was in the geology book by Charles Lyell that he had read on the boat, told him that it may be possible for these changes to occur gradually by very small steps provided that there was enough time for the changes to accumulate.

But why should species change at all? Why shouldn’t they stay the same forever? Or if they changed, why wouldn’t they change randomly instead of seeming to have a direction towards increasing complexity?

What Darwin still lacked was a mechanism that drove the change in organisms. The idea for this came in September 1838 when, after his return from his voyage and he was thinking about all the evidence he had gathered, he read Thomas Malthus’s Essay on the Principle of Population in which that political economist argued that the only thing that kept the population of anything (humans, other animals, plants) from experiencing runaway exponential growth was the limitation of essential resources (such as food and suitable habitats), and deprivations such as cruel climates, predators, and the like. (David Quammen, The Reluctant Mr. Darwin (2006), p. 42.)

Darwin knew that the size of plant and bird and animal populations in nature were fairly stable and he reasoned that the factors identified by Malthus might act differentially on members of the population, being more likely to remove the ones less suited and thus increasing the proportions of those more suited to the conditions. This kind of selection pressure, he felt, must be the driver of evolutionary change. Here at last was the mechanism that he had been seeking.

For the next twenty years, he carefully studied this process, starting with the breeding practices of pigeon owners and moving on to many others species. He even spent eight years studying barnacles. While breeders had the ability to artificially control the selection process, Darwin had the insight that the forces at work in nature might produce the same effect in the wild, hence his term ‘natural selection’.

Darwin eventually arrived at the basic tenets of evolution by natural selection. (The Advancement of Science, Philip Kitcher, 1993, p. 19. I have mentioned these before but reproduce them here for completeness.)

1. The Principle of Variation: At any stage in the history of a species, there will be variation among the members of the species: different organisms belonging to the species will have different properties.

In other words, children are never identical with their parents. Within each species there is considerable diversity in properties (the larger the population, the greater the diversity) and in support of this position, Darwin took great pains to point out how hard it was to distinguish between different varieties within the same species, and between species.

2. The Principle of the Struggle for Existence: At any stage in the history of a species, more organisms are born than can survive to reproduce.

If there is an abundance of food and other resources, the population of any species would multiply exponentially, as suggested by Malthus. The fact that it doesn’t is due to limitations in these necessary elements and this is what results in only some surviving and populations reaching more or less stable values.

3. The Principle of Variation in Fitness: At any stage in the history of a species, some of the variation among members of the species is variation with respect to properties that affect the ability to survive and reproduce; some organisms have characteristics that better dispose them to survive and reproduce.

The members of a species that are more likely to survive and pass on their properties to the next generation are those that have properties that give them some survival advantage in the environment in which they find themselves. It is important to note that only some of the properties need to be advantageous for the organism to have preferential survival. Other properties may also flourish not because they have an advantage but because they are somehow linked to advantageous properties and are thus carried along. Thus some properties may simply be byproducts of selection for other properties.

4. The Strong Principle of Inheritance: Heritability is the norm; most properties of an organism are inherited by its descendents.

Most properties that we have (five fingers, four limbs, one heart, etc.) are inherited from our ancestors.

All these four things were not controversial and were not hard to accept even for religious people. What gave Darwin’s theory its uniqueness and created controversy was that from these four principles, he inferred the crucial fifth. It was this extrapolation that is the key to Darwin’s theory of natural selection.

5. The Principle of Natural Selection: Typically, the history of a species will show the modification of that species in the direction of those characteristics which better dispose their bearers to survive and reproduce; properties which dispose their bearers to survive and reproduce are likely to become more prevalent in successive generations of the species.

So natural selection will favor those organisms that, by chance mutation, have properties that give them better chances for survival, and thus these characteristics will appear in the next generation in greater abundance. And from this he inferred that as these changes accumulate, eventually new species emerge.

But it was one thing to have a theory that satisfied him. It was quite another to convince others that it was the explanation for the diversity of life. There were many obstacles he had to overcome, not the least of which was the scale of time he was asking people to envisage was much longer than they were used to, the size of the mutations that underlay the process were so small as to be mostly invisible, and there was no agreement at that time on the whole question of how characteristics were inherited and how variations occurred in species.

It was to try and meet these objections that Darwin spent the rest of his life accumulating vast amounts of evidence from all over the world. Darwin, great scientist that he was, knew that just having a good idea wasn’t enough in science, however beautiful the idea was. You had to have evidence to support it.

Next in the series: How probability ideas can lead us astray

POST SCRIPT: How can we miss you if you won’t go away?

I was looking forward to British Prime Minister Tony Blair leaving office today. I found his preening pieties, his obsequious behavior toward Bush, and his self-righteous attitude irritating in the extreme and was looking forward to not having to see that on public display. But now comes the alarming news that Bush is thinking of making him some kind of special envoy to the Middle East, so we will be forced to endure even more of his grating presence.

Maybe Bush likes having his ‘pet poodle’ (which is actually an insult to a fine and dignified breed of dogs) around but as long-time Middle East correspondent Robert Fisk points out in the British newspaper The Independent, those who think that Blair, whom he describes as “this vain, deceitful man, this proven liar, a trumped-up lawyer who has the blood of thousands of Arab men, women and children on his hands,” has any credibility at all in the Middle East are woefully mistaken.