How do the concepts of fertility, mortality, and, migration interact and affect how populations…

How do the concepts of fertility, mortality, and, migration interact and affect how populations either stay or migrate from their point of origin? With that in mind, what issues seem to be the most impactful when populations begin to move from one place to another? With that in mind, how does how do the issues stated affect social change overall? Do people have the ability to affect great change in society? Why or why not?
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