Natural selection was Darwin’s key insight, the idea that truly made him famous. The idea of evolution existed before he was born, but there was no credible mechanism. Darwin, and Alfred Russell Wallace, uncovered that mechanism.
Natural selection has been defined as “the non-random survival of randomly varying replicators.” Before I explain what that means, I want to explain how Darwin came across this insight. (Before I go further, I should note that, while natural selection is the primary force driving evolution, sexual selection and genetic drift also have an impact).
On the Origin of Species is filled with what we might term comparative biology. Especially about pigeons. Artificial selection is the process by which human breeders create different breeds of an animal, such as the dozens of species of dog we now have. In the mid-19th century, pigeon breeding was fairly popular. Darwin was fascinated by all of the species of pigeon and began to wonder where they all came from. Based on their physical similarities, he discerned that all the domestic pigeon breeds must have originated from a single species of wild pigeons. The reason they look different is that different lines have been artificially selected for different traits. In the Origin, artificial selection (why pigeons look different) acts as a key metaphor for natural selection (why all birds look different).
We can think of natural selection as artificial selection without conscious forethought. It is deterministic, in that its outcomes are predictable. In other words, natural selection is not random. What are random are mutations in DNA, the “replicators” in the above quote. Or maybe not.
For all intents and purposes, mutations can be thought of as random. Biologists understand what causes mutations. Biologists understand the frequency of mutation in the presence of each “mutagen” (any agent that causes mutations). The problem is that we can’t yet model an entire cell and its surrounding environment; it’s just too complicated for modern technology. It is thought that, once we can model the entire system, we will be able to predict mutations. But for now, we can think of them as random.
Darwin’s key insight in part stemmed from Malthus’ observation that population levels are stable, even though, in most species, females produce more than two offspring (which should lead to population growth). Malthus realized that populations are stable because there is a finite amount of food; some offspring must die simply because there are no resources for them.
However, Darwin realized that there is more at work than just food. Which offspring will survive to reproductive age is determined by a number of factors: random accidents, disease, predators – anything that will “select” certain individuals for death before they can reproduce.
Successful reproduction is the key to natural selection. Whoever has the largest number of successful offspring “wins” so to speak; the high number of successful offspring increase the proportion of the members of the next generation that carry a specific set of genes.
I’m sure everyone has heard the joke that you don’t need to outrun the bear, you just need to outrun the other hiker. The same principle applies. If you are a gazelle, you don’t need to outrun the lion, you just need to outrun the slowest gazelle. Thus, genes for being slow are selected against. Or, conversely, we can say that genes for being fast are selected for.
Who gets what genes and what mutations is purely random; not only are mutations random, but the genes you get from your father are a mix of both of his parents’ genes, and the same goes for your mother. The genes you get from your mother are not either from your maternal grandmother or grandfather; during the formation of egg cells, the DNA mixes. The same goes for sperm.
The key to which traits are successful is reproduction. Whoever has the most successful offspring has their genes live on; the lines without enough successful offspring die out.
Darwin had extrapolated that, if all pigeons were related, then all birds must be related. And if all birds are related, all animals must be related. And if all animals are related, why not all life? (His thinking was actually more complicated than this, involving a lot of comparative anatomy and developmental biology. His understanding of common ancestry didn’t come from just pigeons!) And if all life has descended from a single common ancestor (something we now know for absolute certain: the odds of this not being true are1 in 10 to the 2,680th power…or 1 divided by a 10 followed by 2,680 zeros), then why do we all look different? Darwin’s answer was natural selection.