Political structures are emergent in the sense that they tend to mirror and reinforce cultural and social organizations in society, and survive a winnowing process between various organizational forms, with the "best" ones surviving.
China, for example, had followed a Confucianist code for @ 2000 years, and today's Communist Party government would be recognizable to earlier Imperial governments in terms of general form (government by meritocracy and bureaucracy), if not in specific content. Western Democracies have followed a much more chaotic developmental path, and various events have changed the forms in many instances. France, for example is a much more centralized state due to the long period of civil wars ending when Louis XIV (The Sun King) was able to assert himself and centralize power in the court, suppressing any possible social or political organization which might challenge Royal power. England, on the other hand, went the other way with a relatively constrained monarchy after the end of the English Civil Wars and the supremacy of Parliament. Germany, being assembled from a collection of bickering States, adopted a much more Federal structure.
Other societies have different culture and histories, which define their political systems and development. Russia is far different from the European nations because of her different history and society (the influence of things like the Orthodox Church, invasions from both the East and West due to open land frontiers and the vast spaces of Russia all play a part).
A good author to read is Robert D Kaplan (http://robertdkaplan.com). Many of his books seem to be travel books on the surface, but he is a keen observer and his vision is "History is Geography", in the sense that areas and nations with defendable frontiers can maintain civilizations and cultures over extended periods of time, while more open areas are the cockpits of conflict as civilizational forces can "flow" across them and come in conflict with other civilizations. Areas broken by difficult terrain are essentially ungovernable, since authorities cannot project their writ or power across the terrain.
In terms of political structures evolving, England is a good place to start as well. England has essentially been an island fortress against Europe for over 1000 years, and English Kings and Parliaments have been developing systems to ensure the Treasury is capable of supplying enough "coin" to ensure the defense of the realm. (A good author to follow this evolution is Naill Ferguson, particularly "The Ascent of Money"). English structures of taxation, banking, money and so on were driven by the need to be able to raise more and more money to fight against European rivals and prevent anyone from being able to cross the channel. Monarchs as different as Henry V, Elizabeth I and George III wrestled with this, as well as Prime Ministers from Pitt to Churchill.
So for a fictional nation, look at the boundaries (can there be a stable state, or will it be constantly invaded?), the culture, the long term external challenges the nation faces (England's challenges are the same today as they were in the time of Henry V, just different actors across the channel these days), and perhaps any long term internal challenges (a nation like Japan with limited farmland and natural resources will evolve a different culture, society and institutions than a rich, wide open nation like America).