The correct answer to this open question is the following.
The causes for uneven distribution of population in the world are birth rate, death rate, and migration.
The birth rate is the number of newborns in one specific country annually. The death rate is the number of deceased people in one specific country annually. And immigration is the people that decide to leave their native countries to emigrate to another country -commonly, a developed country- trying to find better living conditions and work for their families. Some of these immigrants leave due to political or religious persecution, poverty, hunger, political instability , and more.
these three factors considerably affect the population in countries.
In the case of underdeveloping countries, the most important factor is the birth rate. That is why international organizations like the United Nations develop programs to invite the governments of these countries to establish natality control to stop the rate of newborns in these underdeveloping countries.