Does natural selection affect humans? How and why

Yes it does affect humans.

Explanation:

Natural selection acts on the phenotype, but evolution is a change in the frequency of alleles in a population over time, a change in genotype. So natural selection acts on phenotype, but it is the connection to genotype that makes it the mechanism of evolution. Many traits vary among different members of a population of humans and animals, from body size to hair color, and those differences are often linked to differences in genes. Natural selection occurs when some of those traits help some individuals survive and reproduce more than others. In scientific terms, these hazards are referred to as selection pressures. They put pressure on us to adapt in order to survive the environment we are in and reproduce. It is selection pressure that drives natural selection "survival of the fittest" and it is how we evolved into the species we are today.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.