Take one of the main ideologies and examine how it has changed from its classical form. What practical differences do these changes make? What do you see as the main weaknesses of the contemporary form of this ideology?

You may be referring to ideologies such as capitalism and communism. Capitalism has changed significantly since its conception in the late 1700s with Adam Smith's The Wealth of Nations. For example, while Smith imagined a world of perfect competition, economies of scale created monopolies during the Industrial Revolution. In this situation, larger companies had an advantage in the market over smaller companies, and they often pushed smaller companies out of business. In order to facilitate fairer competition, President Teddy Roosevelt used the power of the federal government and courts to regulate monopolies.
In addition, capitalism in its purest form leads to economic inequality. This inequality has effects on society and is in many ways contrary to the principles of democracy. Therefore, at times in American history, the government has stepped in to curb the excesses of capitalism and create more economic and social equality through policies such as graduated income taxes (in which people who have higher incomes pay a greater percentage of their income in taxes).
While governmental policies have restored some degree of fairness to capitalism, there are still weaknesses in the system. Global capitalism has created even greater forms of inequality, and businesses do not always have to pay for the costs of producing their products. For example, companies' production processes may result in pollution, but companies do not always have to assume these costs. These are problems with the contemporary form of capitalism.