Process of population occurs when allele frequency in a population may change due to fundamental forces of evolution.
For example natural selection, genetic drift, gene flow and mutation. This process takes place when new traits transfers genes between populations for example between species or in migration.
Speciation of population to separate species will take forms like sympatric and allopatric. In allopatric is where a physical barrier is been able to separate population why sympatric the population which is sharing the geographical location is formed by environment factors to separate.
If they are separated post zygotic or prezygotic between species.