What is the "historical development of the concepts of social change and modernization"?
Social change is a vague term. There are four sub-sets of change reflecting on social change as a whole: population change, cultural change, relational change and catastrophic changes. Social change is often confused with cultural change.
Population change consists of several different topics. A population can change via heritage, volume or geography. For example, the "baby boomer" phenomenon is an example of population change via volume. There are now many more people of the same age with the same general life experiences which can greatly influence norms in society.
A population can also change when the heritage of a population changes, that is, when the balance of heritages changes. This is not to say ethnicity changes, although it can. This is also referred to as replacement population change because one heritage will replace another without adding or decreasing the population volume.
Geographical population change occurs when a population moves from one area to another. The volume and heritage do not change in significant numbers, but the way the population adapts to the new environment will affect the social order.
What historical events have caused mass changes in populations?
Cultural change occurs when some effort inside society introduces a new idea, action or belief which is adopted by a large percent of the population. Richard Dawkins refers to this as memes. Different cultures can exist in the same population. For example, the "counter culture" revolution of the 60s and 70s. There is also on the rise the "social media" generation. Cultural change is simply a different way a portion (or all) of the population views the world.
Relational change occurs usually as a result of cultural change. The relationships within society change because of culture shift or technology shift. Blacksmiths were cornerstones of villages for centuries because of their skills in building needed implements. However, with the advancement of technology our relationship to them have changed and they are not seen as valuable in today's society.
Catastrophic change occurs when there is a disturbance which affects the entire population. It usually obliterates the history of society and forces a new society to form. The plague which reduced a huge portion of Europe's population or the holocaust during WWII demonstrate how catastrophic changes force the population to form new societal norms.