I don't think that ethnic distribution specifics are necessary for the
emergence of "liberal democracies" but local ethnicities probably
played a role since travel after the decline of Rome was not great
enough for alot of mixing. "Institutional pluralism" and fighting
between groups for power is the main ingredient. The Roman empire
declined, the church took power, lords and kings emerged locally as a
force, kings councils arguing with kings, business interests get into
the mix, rights emerge as an issue, the populace learns of ways to get
rights, and then liberal or constitutional liberalism in the form of
social contracts can emerge.

The political theory of pluralism holds that political power in
society does not lie with the electorate but is distributed between a
wide number of groups. These groups may be Trade Unions, interest
groups, business organisations, and any of a multitude of formal and
informal coalitions.

http://en.wikipedia.org/wiki/Pluralism_%%28political_theory%%29
http://en.wikipedia.org/wiki/Pluralism_%%28political_philosophy%%29

Political power (imperium in Latin) is a type of power held by a
person or group in a society. There are many ways to hold such power.
Officially, political power is held by the holders of the sovereignty.
Political powers are not limited to heads of states, however, and the
extent to which a person or group holds such power is related to the
amount of societal influence they can wield, formally or informally.
In many cases this influence is not contained within a single state
and it refers to international power.

Political scientists have frequently defined power as "the ability to
influence the behaviour of others" with or without resistance.

...without following a principle of containing and balancing
legislative, executive and judiciary powers, there is no freedom and
no protection against abuse of power. Separation of power must be in
such grade, that any of the branches can operate without excessive
limitations from the others; but interdependecy between them must also
be in such grade, that one single branch cannot rule out the other's
decisions...

...A similar concept, termed Division of Power, also consists of
differentiated legislative, executive and judiciary powers. However,
while Separation of Power prohibits one branch from interfering with
another, Division of Power permits such interference...

http://en.wikipedia.org/wiki/Political_power

- The Geography of Freedom

The church gained power in the West for a simple reason: after the
decline of the Roman Empire, it never again faced a single emperor of
Europe. Instead, the Catholic Church was able to play one European
prince against another, becoming the vital "swing vote" in the power
struggles of the day. Had one monarch emerged across the continent, he
could have crushed the church's independence, turning it into a
handmaiden of state power. That is what happened to the Greek Orthodox
Church and later the Russian Orthodox Church (and, for that matter, to
most religions around the world). But no ruler ever conquered all of
Europe, or even the greater part of it. Over the millennia only a few
tried-Charlemagne, Charles V, Napoleon, Kaiser Wilhelm, and Hitler.
All were thwarted, most fairly quickly.

What explains this? Probably mountains and rivers. Europe is riven
with barriers that divide its highlands into river valleys bordered by
mountain ranges. Its rivers flow into sheltered, navigable bays along
the long, indented Mediterranean coastline-all of which means that
small regions could subsist, indeed thrive, on their own. Hence
Europe's long history of many independent countries. They are hard to
conquer, easy to cultivate, and their rivers and seas provide ready
trade routes. Asia, by contrast, is full of vast flatlands-the steppes
in Russia, the plains in China-through which armies could march
unhindered. Not surprisingly, these areas were ruled for millennia by
centralized *empires.

*Africa is particularly unlucky in its geography. Despite being the
second-largest continent in the world it has the shortest coastline,
much of which is too shallow to develop ports. So it has historically
had little trade. Its rivers are not navigable, because they are
either too shallow or, where deep, scarred by rapids and waterfalls
(dramatic scenery makes for disasterous commerce in this case). Add to
this tropical heat and accompanying disease and one has a sad
structural explanation for Africa's underdevelopment.

Europe's topography made possible the rise of communities of varying
sizes-city-states, duchies, republics, nations, and empires. In 1500
Europe had within it more than 500 states, many no larger than a city.
This variety had two wondrous effects. First, it allowed for
diversity. People, ideas, art, and even technologies that were
unwelcome or unnoticed in one area would often thrive in another.
Second, diversity fueled constant competition between states,
producing innovation and efficiency in political organization,
military technology, and economic policy. Successful practices were
copied; losing ways were cast aside. Europe's spectacular economic and
political success-what the economic historian Eric Jones has termed
"the European miracle"-might well be the result of its odd geography.

- Lords and Kings

Geography and history combined to help shape Europe's political
structure. The crumbling of the Roman Empire and the backwardness of
the German tribes that destroyed it resulted in decentralized
authority across the continent; no ruler had the administrative
capacity to rule a far-flung kingdom comprising so many independent
tribes. By contrast, in their heyday, Ming and Manchu China, Mughal
India, and the Ottoman Empire controlled vast lands and diverse
peoples. But in Europe local landlords and chieftains governed their
territories and developed close ties with their tenants. This became
the distinctive feature of European feudalism-that its great
landowning classes were independent.

From the Middle Ages until the seventeenth century, European
sovereigns were distant creatures who ruled their kingdoms mostly in
name. The king of France, for example, was considered only a duke in
Brittany and had limited authority in that region for hundreds of
years. In practice if monarchs wanted to do anything-start a war,
build a fort-they had to borrow and. bargain for money and troops from
local chieftains, who became earls, viscounts, and dukes in the
process.

Thus Europe's landed elite became an aristocracy with power, money,
and legitimacy-a far cry from the groveling and dependent courtier-
nobles in other parts of the world. This near-equal relationship
between lords and kings deeply influenced the course of liberty. As
Guido de Ruggiero, the great historian of liberalism, wrote, "Without
the effective resistance of particular privileged classes, the
monarchy would have created nothing but a people of slaves." In fact
monarchs did just that in much of the rest of the world. In Europe, on
the other hand, as the Middle Ages progressed, the aristocracy
demanded that kings guarantee them certain rights that even the crown
could not violate. They also established representative bodies-
parliaments, estates general, diets-to give permanent voice to their
claims. In these medieval bargains lie the foundations of what we
today call "the rule of law." Building on Roman traditions, these
rights were secured and strengthened by the power of the nobility.
Like the clash between church and state, the conflict between the
aristocracy and the monarchy is the second great power struggle of
European history that helped provide, again unintentionally, the raw
materials of freedom.

The English aristocracy was the most independent in Europe. Lords
lived on their estates, governing and protecting their tenants. In
return, they extracted taxes, which kept them both powerful and rich.
It was, in one scholar's phrase, "a working aristocracy": it
maintained its position not through elaborate courtly rituals but by
taking part in politics and government at all levels. England's kings,
who consolidated their power earlier than did most of their
counterparts on the continent, recognized that (heir rule depended on
co-opting the aristocracy-or at least some part of it. When monarchs
pushed their luck they triggered a baronial backlash. Henry II,
crowned king in 1154, extended his rule across the country, sending
judges to distant places to enforce royal decrees. He sought to unify
the country and create a common, imperial law. To do this he had to
strip the medieval aristocracy of its powers and special privileges.
His plan worked but only up to a point. Soon the nobility rose up in
arms- literally-and after forty years of conflict, Henry's son, King
John, was forced to sign a truce in 1215 in a field near Windsor
Castle. That document, Magna Carta, was regarded at the time as a
charter of baronial privilege, detailing the rights of feudal lords.
It also had provisions guaranteeing the freedom of the church and
local autonomy for towns. It came out (in vague terms) against the
oppression of any of the king's subjects. Over time the document was
interpreted more broadly by English judges, turning it into a quasi
constitution that enshrined certain individual rights. But even in its
day, Magna Carta was significant, being the first written limitation
on royal authority in Europe. As such, the historian Paul Johnson
noted, it is "justly classified as the first of the English Statutes
of the Realm,* from which English, and thus American, liberties can be
said to flow."

*The collection of English laws that make up its "unwritten
constitution."

The Future of Freedom - Illiberal Democracy at Home & Abroad
http://www.amazon.com/exec/obidos/ASIN/0393047644/
http://www.fareedzakaria.com/