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First off, the question you are asking is extremely vague because you are assuming all Islamic sects (Sunni and Shia, including branches within such as Ismai'ilism, Twelvers, Zaydiyyah, Shafi'i, Hanafi, etc.) are the same. This question is asking something similar to the evolution of Christianity (Catholicism, Eastern Orthodoxy, Protestantism, etc.) since its inception; you will not get a response that answers the entire history of Christianity. Not even a book on this question can cover the depth of what you are expecting, much less for a response for this subreddit. But let's narrow down your question to make this answerable by looking at Wahhabism and the Saudi government in Saudi Arabia.
The name Saudi Arabia, literally Arabia of the al-Saud, was adopted in 1932 by the dynasty founder King Abdulaziz bin Saud. Saudi Arabia occupied one of the harshest land in the world but with two major compensations. The Arabian peninsula changed over the centuries as the geographic and weather shifts gave Saudi Arabia the largest oil reserves in the world today. Second, Prophet Mohammad began preaching Islam in the small oasis town of Mecca, where he was born in the seventh century A.D. The religion spread quickly and was centered around the Prophet's birthplace. Saudi Arabia, by virtue of its guardianship of Mecca and the Prophet's burial place in Medina, is the spiritual home of Islam and one-fifth of the world's population.
The ethics of Islam are not all that different from the ethics of Christianity: love your neighbor, keep your word, help the poor. But Islam is everywhere and its ubiquity is inescapable and increasingly comprehensive. New mosques are in every street, the mention of God and his prophet in every sentence, even the King's budget speech... Islam governs every detail of the believer's life. It tells him how to wash, how to urinate, how much estate to leave to his step-daughter.^1
Sir James Craig, the former British ambassador to Saudi Arabia and Syria, wrote in his valedictory address about Saudi Arabia. Islam, literally "submission to God," permeates Saudi life, where the dominant Saudi brand of Islam is Wahhabism. The peninsula's religious direction changed in 1703 with the birth of Muhammad al-Wahhab. Young al-Wahhab went under intensive religious training, memorizing the Koran by the age of seven and learned under renowned clerics in Nejdi, al-Wahhab's birthplace. Al-Wahhab grew dismayed by what he saw: Islamic law was only laxly applied; people worshipped stones, statues and trees, believing them to harbor the souls of holy people. Around 1720, al-Wahhab began his revivalist message, criticizing Nejdi sheikhs for not enforcing the true Sharia (Islamic law). He preached against the use of tobacco and said that dancing, music, and all outward signs of ostentatiousness hindered true believers from contemplating God.^2 In addition, al-Wahhab also said it was not enough for a true believer to purify his own life; he also had to reform those around him, this being the jihad or the holy war. He argued that any object or action not found in the Koran or Hadith (deeds and sayings of the Prophet) was forbidden - although some exceptions such as guns were quietly made. Al-Wahhab's movement might never have succeeded except that he found an important ally in a local chieftain, Muhammad bin Saud, and together they succeeded in unifying the Arabian peninsula.
It is tempting to portray Saudi Arabia as a religiously unified state, a Wahhabi monolith; but nothing could be further from the truth. Among Wahhabis, different interpretations exist. In Hejaz, where the more liberal Hanafi and Shafii legal schools of Islamic thought hold sway, there was strong support for liberalization along Islamic guidelines. Hejazi "progressives" supported a more open society in which women would be allowed to drive and work without restrictions and where judiciary would be freed from Islamic religious courts and laws. In contrast, Nejdi fundamentalists, following the more puritanical Hanbali legal school, favored the creation of an orthodox and conservative Islamic state. Their ideal state would enforce Islamic law and curtail or abolish reforms such as education for women, television, or Western-style banking system. These fundamentalists blamed Saudi Arabia's problems on "secularism" and further hinted that the country has been corrupted by un-Islamic influences.
These religious differences were supplemented by a regional divide in Saudi Arabia. The conquest of the Hejaz by the forces of Adbulaziz in 1925 brought the Hejazis into immediate conflict with the Central Arabian Nejdis. The Nejdis, whose homeland had never been colonized, considered themselves ethnically purer than Hejazis, whom they called "driftwood" because their pilgrim forefathers "washed up" on the Red Sea shores. Many Nejdis also viewed the Hejazis as little better than heretics, because their form of Islam had been diluted by contact with foreigners. The Hejazis, possessing modern technology such as telephones and cars, and relatively advanced legal and banking systems, viewed the Nejdis as ignorant and uncultured. That rivalry was worsened by al-Saud, who initially staffed the Saudi bureaucracy with Hejazis by virtue of their better education, worldliness and commercial experience.
The death of the late King Faisal bin Adbulaziz in 1975 signaled the end of Hejazi domination in the Saudi civil service. Faisal's successor, first the late King Khalid bin Abdulaziz (r. 1975-1982) and later Fahd bin Abdulaziz (r. 1982-2005) have been more inclined to favor Nejdis. Nejidis during this time made up a majority of the ministers, deputy ministers and top government officials. The Nejdization in many Saudi ministries have pressured Hejazis to leave. Ironically, the appointment of thousands of Nejdis, who are more sympathetic to Kingdom's fundamentalists and their conservative view of Islam, has confronted the ruling elite with the problem that now large sections of the bureaucracy are staffed by people opposed to their modernization policies. Of course, it would be a disservice to conclude with talking only about these two factions, the Nejdis and the Hejazi, (the equivalent of lumping all Americans as either Democrat or Republican while ignoring Libertarians, the Green Party and those who don't vote >:), while other minorities such as the Shiites in Saudi Arabia does impact Saudi policy but that is another story for another day.
Endnotes:
Sir James Craig. "Valedictory Number 2: The Saudi Arabians." Sir James's memo was leaked in November 1987, causing a minor diplomatic flap.
David Holden and Richard Johns. "The House of Saud." Pg. 21-22.
Sources:
Abir, Mordechai. "Saudi Arabia in the Oil Era, Regime and Elites, Conflict and Collaboration." Croom Helm Ltd. London. 1988.
Abir, Mordechai. "Saudi Arabia, Government, Society and the Gulf Crisis." Routledge. London. 1993.
Holden, David, and Richard Johns. "The House of Saud: the Rise and Rule of the Most Powerful Dynasty in the Arab World." New York: Holt, Rinehart and Winston, 1982.
Vassiliev, Alexei. "The History of Saudi Arabia." New York University Press. 2000.
Wilson, Peter W. and Graham, Douglas F. "Saudi Arabia, the Coming Storm." M. E. Sharpe, Inc. 1994.
Edit: Grammar and stuff.