D4.1 Natural selection

Why do organisms fit their environments so well? Before Charles Darwin and Alfred Russel Wallace, the match between a polar bear’s coat and the Arctic, or a moth’s wings and the bark it rests on, looked like deliberate design. Their insight was that such adaptation can arise from a simple, unguided process: where there is heritable variation and a struggle to survive, the variants that happen to reproduce best leave more offspring, and the population gradually changes. This is natural selection, the principal mechanism of evolution. For D4.1, you must be able to lay out its logical steps and recognise the evidence that the process is happening today.

The conditions for natural selection

Natural selection follows logically whenever a few conditions are met, and you should be able to state them in order:

• Variation: individuals in a population differ in their characteristics.
• Heritability: at least some of that variation is genetic and can be passed to offspring.
• Overproduction: organisms produce far more offspring than the environment can support, so there is competition for limited resources.
• Differential survival and reproduction: individuals whose characteristics make them better suited to the environment are more likely to survive and reproduce.

The result is that favourable alleles increase in frequency in the population over generations, while less favourable ones become rarer. Note carefully that selection acts on whole organisms (the phenotype), but its effect is measured as a change in allele frequency in the gene pool.

Where variation comes from

Natural selection can only act on variation that already exists; it does not create it. The ultimate source of new alleles is mutation — random changes in DNA. Mutations are not directed towards usefulness: most are neutral or harmful, but occasionally one produces a variant that happens to suit the environment.

In sexually reproducing organisms, this raw variation is then shuffled into new combinations by meiosis (through crossing over and the independent assortment of chromosomes) and by the random fusion of gametes at fertilisation. Together these processes generate the unique, varied individuals on which selection works.

Common exam trap: never write that an organism develops a trait because it needs it, or that individuals adapt during their lifetime to pass changes on. That is Lamarckism. Variation arises first, by chance; the environment then selects among existing variants.

Adaptation, fitness and sexual selection

An adaptation is a heritable feature that increases an organism’s chance of surviving and reproducing in its environment. In evolutionary terms, fitness is not strength or speed but reproductive success — the number of offspring an individual contributes to the next generation. A trait spreads because the individuals carrying it leave more offspring, not because it is better in any general sense.

The syllabus also highlights sexual selection, in which traits spread because they improve mating success rather than survival. The peacock’s tail is the classic example: it is costly and may even reduce survival, yet it persists because peahens prefer to mate with males that display it, so the alleles for elaborate tails are passed on. This shows that survival of the fittest ultimately means survival of those who reproduce most.

Evidence: selection happening now

Natural selection is not only a historical idea; it can be observed directly. Two examples favoured by the syllabus are:

• Antibiotic resistance in bacteria. A bacterial population already contains rare variants with mutations that confer resistance. When an antibiotic is applied, susceptible bacteria die but resistant ones survive and reproduce. Over repeated exposure, the resistant allele becomes common, producing resistant strains. The antibiotic does not cause the mutation — it selects for variants that already exist.
• Beak size in Galapagos finches. During drought years, only large, tough seeds remain, so finches with deeper, stronger beaks survive and breed better, and average beak depth in the population increases the following generation. When conditions reverse, selection can push the trait the other way.

Both cases display every condition in turn — variation, heritability, a selective pressure, and a measurable shift in allele frequency — making them ideal exam material.