Natural Selection is when organisms who are better adapted to an environment tend to survive and reproduce. This process takes place over four stages: overpopulation, variation, selection, and adaptation. Here, I am using mountain goats as an example.
First, the mountain goat population overpopulates. Every population produces more offspring than can survive to maturity.
Through generations of breeding, the population becomes more diverse. This happens because there is a mixture of alleles after generations of offspring. There is about the same number of black goats as there is tan goats.
The black goats blend in with the environment a lot better than the tan goats. Therefore, the tan goats are much easier to spot by predators such as snow leopards and slowly die off due to this predation.
Because the tan goats have slowly died off, the black goats are the ones reproducing. This allows the black fur gene to be passed down to future generations. This results in the black gene becoming more common.
Eventually, the whole population could have the black fur trait because the tan goats aren't able to survive, and the black trait is continuing to be passed down to each generation. This exemplifies a structural adaptation.