In a single-gene, traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies. It also causes evolutionary, but not all the time. Evolution does not require natural selection.
Natural selection affected elephants by making them cool down. Elephants don't have the glands that keep them cool. But by natural selection, they are able to live in 80 degree weather.
Polygenic traits in animals include weaning weight, weight, hair color and length. Behavioral characteristics of animals are often controlled by multiple gene loci as well, although the environment, in the form of parental care, often influences behavior as well. On polygenic traits, natural selection can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.
The evolution of the peppered moth is an evolutionary instance of colour variation in the moth population as a consequence of the Industrial Revolution. This caused natural selection to cause an increase in the number of dark-coloured moths. Due to industrial pollution and a reciprocal decrease in the population in a clean environment, its has cause the moths to become more darker colors than lighter colors.
In a habitat there are red bugs and green bugs. The birds prefer the taste of the red bugs, so soon there are many green bugs and few red bugs. Natural selection causes the green bugs reproduce and make more green bugs and eventually there are no more red bugs.