Is infertility part of Darwin's natural selection?
Any genetic change that reduces the lifespan of an organism, such that it can not reach adulthood and reproduce does relate to natural selection and evolution. Infertility in a few individuals of a species simply means that those individuals may not have offspring and pass on their genes. However, if this detrimental genetic change is widespread enough in the species, it could mean that the species as a whole may lose their ability to reproduce in numbers large enough to survive and they might face extinction.
For example, a few years ago there was some evidence that human males may be losing parts of their uniquely male "y" chromosomes and if so, this could impact human fertility and survival in the long-run. However, more recent work in 2014, suggests that this damage may have halted. http://phys.org/news/2014-11-sex-genes-chromosome-future-men.html