How selection works
Within any species there is variation caused ultimately by mutation and, in sexually reproducing organisms, by the reshuffling of alleles. Organisms produce more offspring than the environment can support, so there is competition for resources such as food, space and mates.
Key idea
Natural selection = variation + overproduction + competition. Individuals whose alleles give useful adaptations are more likely to survive, reproduce and pass on those alleles, so the useful alleles become more common over generations.
Selection pressures and resistance
A selection pressure is a factor (a predator, disease, climate or a drug) that reduces survival of poorly adapted individuals. A clear modern example is the evolution of antibiotic resistance in bacteria.
Worked example
In a bacterial population a chance mutation makes one cell resistant to an antibiotic. When the antibiotic is used it kills non-resistant cells but the resistant cell survives and reproduces rapidly. Its offspring inherit the resistance allele, so within a few generations most of the population is resistant.
Artificial selection
In artificial selection (selective breeding) humans choose the parents. Individuals with a desired feature - high milk yield, large grain, disease resistance - are bred together over many generations, increasing the frequency of the chosen alleles.
Remember
- Mutation is the original source of new alleles.
- Natural selection: the environment selects.
- Artificial selection: humans select.
- Both change allele frequency over generations.
Over very long periods the steady accumulation of these changes can produce new, better-adapted species - this is evolution. The spread of antibiotic resistance is a warning: overusing antibiotics increases the selection pressure and speeds up the appearance of resistant strains, which is why doctors are careful about prescribing them.