17.2.1—Natural selection mechanism

Natural selection is a population process: inherited variation exists, environmental selection pressures affect survival and reproductive success unequally, and advantageous heritable alleles can become more frequent over generations.

• A population produces more offspring than can usually survive because resources and other conditions are limited.
• Individuals with different heritable phenotypes experience a selection pressure differently; those with a favourable phenotype are more likely to survive to reproduce.
• Their offspring inherit some of the associated alleles, so the frequency of advantageous alleles increases across generations while other alleles may decrease.

Fitness means reproductive success in a particular environment, not strength or perfection. A change in the environment can alter which phenotype is favoured, and selection does not guarantee that every well-adapted individual survives.

Individuals do not change their alleles because they need to adapt. Selection acts on existing phenotypic variation; the measurable evolutionary change is a shift in allele frequencies in a population over generations. This card does not classify selection types, use antibiotic resistance, or apply Hardy–Weinberg equations.