The first stage can be considered the presence of a single source population.
At the second stage, the geographical barrier leads to the emergence of two populations. This can be a physical or geographical barrier – a river, a canal, a quarry, etc. The presence of a barrier impedes the free crossing of individuals, which means gene exchange.
In the third stage, two populations become genetically distinct. Over time, these differences become so significant that some mechanisms of reproductive isolation are included.
At the fourth stage, reproductive isolation persists after the barrier disappears.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.