Effect of Natural Selection on Phenotypes

Natural selection does not directly act on genes. Instead, it acts on phenotypes, the physical traits an organism shows.

Imagine two rabbits in the snow: one white, one brown. A fox can easily spot the brown rabbit over the white. As a result, more white rabbits survive and reproduce, passing on their genes.

Some genotypes, like homozygous dominant AA and heterozygous Aa, can have the same phenotype. Since natural selection acts on traits, not genetic sequences, it treats them alike.

Take sickle cell disease: Individuals with homozygous recessive aa develop the illness, but heterozygous Aa individuals stay healthy and resist malaria. This advantage keeps the recessive allele in the population.

Similarly, lethal recessive alleles can persist in carriers without affecting them, though homozygous recessive aa offspring may inherit fatal traits.

All these genetic variations in a population make up its gene pool, providing the raw material for natural selection to act on.

This genetic variation leads to more diversity, which improves a species' chances of adapting and surviving in changing environments.