Variation within a species is important as it may lead to selection. Most mutations are not very helpful, but a few are beneficial. This is also a source of variation. Humans have learned to select characteristics in plants and animals to promote artificial selection. Humans are a species.
Humans reproduce only by finding other humans as mates.
In the case of different but close species e.g. apes and humans, there may be a distant common ancestor. However in the case of apes and humans, they still differ as species and actually possess different numbers of chromosomes in the nuclei of their body cells so making reproduction impossible.