Evolution is a change in the inherited characteristics of a population over time, through a process of natural selection. It describes populations, not individuals. A single animal does not evolve during its lifetime, but the alleles in a population can become more or less common from one generation to the next.
If a population is split, for example by a mountain range or a stretch of sea, the two groups face different environments. Natural selection favours different phenotypes in each group, and mutations arise separately. Over many generations the two populations can become so different that they can no longer interbreed to produce fertile offspring. When this happens they have become two separate species, so evolution may result in the formation of new species.
This process, repeated over a very long time, explains the huge number of different species on Earth. It also explains why all living things are related to different degrees, since they share common ancestors. If the environment changes and a species has no suited variants, it may become extinct.
