From the beginning of T.E. Lawrence's Seven Pillars of Wisdom comes an account of how population and cultural forces drove human geography in early 20th-century Yemen. I have no idea how realistic this account is, but as an engineer I think it's kind of inspiring to see this kind of systems-minded analysis used to describe complex human behavior-- and so poetically, at that.
In Yemen the solution was different. There was no foreign trade, and no massed industries to accumulate population in unhealthy places. The towns were just market towns, as clean and simple as ordinary villages. Therefore the population slowly increased; the scale of living was brought down very low; and a congestion of numbers was generally felt. They could not emigrate overseas; for the Sudan was even worse country than Arabia, and the few tribes which did venture across were compelled to modify their manner of life and their Semitic culture profoundly, in order to exist. They could not move northward along the hills; for these were barred by the holy town of Mecca and its port Jidda: an alien belt, continually reinforced by strangers from India and Java and Bokhara and Africa, very strong in vitality, violently hostile to the Semitic consciousness, and maintained despite economics and geography and climate by the artificial factor of a world-religion. The congestion of Yemen, therefore, becoming extreme, found its only relief in the east, by forcing the weaker aggregations of its border down and down the slopes of the hills along the Widian, the half-waste district of the great water-bearing valleys of Bisha, Dawasir, Ranya and Taraba which ran out towards the deserts of Nejd. These weaker clans had continually to exchange good springs and fertile palms for poorer springs and scantier palms, till at last they reached an area where a proper agricultural life became impossible. They then began to eke out their precarious husbandry by breeding sheep and camels, and in time came to depend more and more on these herds for their living.
Finally, under a last impulse from the straining population behind them, the border people (now almost wholly pastoral) were flung out of the furthest crazy oasis into the untrodden wilderness as nomads. This process, to be watched to-day with individual families and tribes to whose marches an exact name and date might be put, must have been going on since the first day of full settlement of Yemen. The Widian below Mecca and Taif are crowded with the memories and place-names of half a hundred tribes which have gone from there, and may be found to-day in Nejd, in Jebel Shammar, in the Hamad, even on the frontiers of Syria and Mesopotamia. There was the source of migration, the factory of nomads, the springing of the gulf-stream of desert wanderers.