Reproductive isolation by mutations and changes in gene pools. Definition of a new species. Types of barriers that can physically separate populations. Most speciation initiated by barriers. Barriers may result in environments that produce different selective pressures.
Allopatric populations can no longer occupy the same range due to adaptations to climate, food, etc.
Sympatric populations can demonstrate habitat or niche isolation. Seasonal variations in fertility cycles or migratory patterns. Territoriality may lead to dispersal and establishment of peripheral populations. Cellular processes resulting in polyploidy. More commonly a speciation factor in plants.