Natural selection

Variation, heredity, and who leaves more offspring.

The idea

Darwin’s mechanism needs three ingredients: individuals in a population vary (thanks to mutation and the reshuffling of meiosis), some of that variation is heritable, and in a given environment individuals with certain heritable traits survive and reproduce more than others. Fitness means reproductive success — how many offspring carry your alleles — not strength or size. Over generations the alleles behind the favored traits become more common, and that change in allele frequencies is evolution; depending on which phenotypes are favored, selection is called directional, stabilizing, or disruptive. Individuals do not evolve — an animal cannot change its own genes to suit its needs — populations do, because selection changes which individuals leave descendants.

Work through an example

The peppered moth is the classic case. The dark (melanic) form was first recorded near Manchester in 1848; as soot blackened tree trunks and killed pale lichens, pale moths became easy prey for birds, and by 1895 about 98% of Manchester’s moths were dark. After the Clean Air Act of 1956 the trees lightened and the pale form rebounded. Antibiotic resistance runs the same way on a faster clock: in a culture of 10⁹ bacteria, a resistance mutation carried by 1 cell in 10⁸ means about 10 resistant cells are already present; add the antibiotic, and those 10 are the only ones that keep dividing every 20–30 minutes.

What to watch for

Bacteria do not "become resistant because they need to." Resistance mutations arise at random before the drug appears, and the antibiotic merely selects among existing variants — Joshua and Esther Lederberg showed this in 1952 by replica plating, finding streptomycin-resistant colonies on plates that had never met streptomycin. "Survival of the fittest" therefore does not mean the strongest survive; it means the individuals that reproduce most pass on the most alleles.