I thought it would be helpful to use a specific example of natural selection for those having issues understanding how it works. Coat color in pocket mice is one of those examples. The paper I will be working from is found here:
First, let’s look at the environment where these mice are found. They are found in the southwest deserts of the US, and the vast majority of their range is made up of light brown dirt and rocks. However, there are isolated areas where recent volcanic eruptions have created areas with dark black basalt rock. Interestingly, when we look at the distribution of coat color in pocket mice it matches the environment they are found in:
The black mice are found in islands of black basalt rock, and brown mice are found in the sea of brown desert between those islands of volcanic rock. I don’t think it is a stretch to state that camouflage is playing a strong role in the distribution of coat color alleles.
The scientists in the paper also looked at interbreeding between the black and brown mice. Sure enough, there was free intermingling between the populations. However, no black mice are seen in the sea of brown desert spanning the two black volcanic rock islands. They also found that the allele for black fur is dominant, meaning that a mouse only needs one copy of the allele to have black fur.
So why don’t we see an even distribution of black and brown mice across the entire range of these mice? Why do we only see black mice associated with these black volcanic rock islands in the sea of brown desert? Why is there selection for or against specific alleles depending on the background color of the environment they find themselves in?