In every group of individuals of the same species, there is a tendency of genetically similar individuals to group together. They will form sub-groups based on this similarity, and eventually this sub-group will split off (or drive away the other members of the parent group) and form another species. Until then, this group will be the dominant gene in the group-the others will have to obey the rules laid out by them.
This is how new species are formed. The dominant group is a precursor to new species.
An example with humans. You will see people of similar characteristics form groups. For example, tall people prefer to hang out with tall people, and marry tall people, and so on. This is a precursor to forming new species.
In case of conflicts, the dominant genes of let's say tall people will drive away all the rest to new places, or there will be war, and the others will be wiped out.
This also explains the formation of new towns, countries, and why we would form new languages (it is the way of speaking of the dominant group with others copy, and eventually leads to a new language).
It is easy to see this for individuals in species-a dominant individual will act the same way. A dominant individual in a group will try to impose his will, his genes, on the group; will subjugate or even kill the others.
These conclusions are easy to apply for all animals, and plants. Darwin pointed out that the competition within individuals of a species is more severe than the competition between species (because they share the same food habits, same geographical area) and this here I want to explain how that competition really works, that some individuals or group begin to dominate, and eliminate the others.