What does speciation mean?

Speciation is the process in which new species arise from exisiting ones. A species is defined as organisms that can reproduce together to produce fertile offspring. This process starts with the isolation of two ancestral populations of a species by a geographical barrier and genetic variation affected each population differently due to presence of different selection pressures (environmental conditions) that allowed them to be better adapted to survive and reproduce. This lead to inheritance of favourable alleles until eventually the two populations were so different to each other that they couldn't produce fertile offspring - hence meaning they are two seperate species.