A very interesting and widely known example of an evolutionary process is provided by the population changes of light and dark colored moths during England's industrial revolution. Basically, there was this peppered moth species who existed in light and dark colored variants (an example of polymorphism). Before the industrial revolution, most of the population of these moths was light colored, with the dark colored variant almost unseen. But with the industrial air pollution darkening the ecosystem, this trend progressively inverted, with dark colored moths ending up largely outnumbering the light colored ones. A change in the ecosystem changed the relative fitness of both variants, with natural selection promoting the survival of the now fittest dark colored variant.
You can read more about this here: https://en.wikipedia.org/wiki/Peppered_moth_evolution .