What Is Culture?
Before discussing the core part of your question, let us determine what a culture is.
www.dictionary.com defines culture as: The sum of attitudes, customs, and beliefs that distinguishes one group of people from another.
Basically, culture is the collective uniqueness and the identity of a group of people living in an area. It is the overall lifestyle of a set of people including things like religion(s), language(s), traditions, food, gender roles and even building style, music and superstitions.
When/How Does A New Culture Develop?
There are many factors and processes which determine when, why and how a culture would change, merge, be influenced or fade away completely. Here are some of these:
Geographical And Climatic Requirements
People living in a tropical, plains area have a different lifestyle than people living in cold, mountainous area. The building style would be different, the food would be different and if the people are not technically very advanced, they would gradually form new folklores and a set of superstitions. Also, if the region of the mountainous people is hard-to-reach, they might also encourage addition of foreign genes to their gene pool to avoid long term inbreeding.
If plains people migrate to mountainous region or mountainous people migrate to plains, their culture would gradually change.
Gradual Foreign Influence
This includes trade relations and political alliances with other peoples. Foreign knowledge gradually adds to the understanding of the world. The availability of new spices and food items influences and adds to the food types. Some foreign words also find their way into the local languages. Sometimes a whole new language can be developed (e.g. Urdu). Dressing options widen and new dress codes find their way in.
This foreign influence is two-way as both nations involved in the relationship are gradually affected by the culture of the other and the most beneficial aspects of the other culture are adopted.
Conquest And Defeat
Unlike long term, gradual influence associated with economic and political relationships, the cultural influence brought about with militaristic conquest and defeat is abrupt. If the victorious party decides to reside in the conquered country and call it home, they are assimilated into the local culture. Their linguistic impact is visible and vivid and sometimes they also affect the local architectural style too, but on the whole it is the invading party which adopts the local culture.
On the other hand, if the masters decide to rule the country by forming a government of their favored local people and return to their borders after conquering, it is the conquered land which is culturally raped. Since the conquerors do not settle in the conquered land, they do not adopt the local culture. Instead, it is their way of life (culture) which influences the local lifestyle.
Technological Progress / Regress
The culture of a region is also affected by the people's technological evolution. With the advance of time, new inventions are made and the cultural values modify with them. For example, since the invention of motor cars, all the set of mannerism associated with horse carriage has vanished and instead a new set of mannerism has formed around this new invention.
Generally, as new inventions and discoveries raise the lifestyle of people, they tend to form new cultural norms aimed at an easy lifestyle.
A large part of any region's culture is directly formed in the light of the prevailing religion. For example, in Muslim countries, women cover up their faces and heads from non-family men. If/when the prevailing set of religious beliefs gets hit (e.g. when religious myths are debunked by technological advancements or when conquerors set up large scale missionary campaigns), the set of cultural values formed around religious commandments gets wiped out. Similarly, a new set of mannerism is formed around the commandments of the new religion.
How/Why A Race Would Split Culturally?
I suggest different course of cultural influence for each part.
A group of this race lives to the south-west of the central region of the continent. Their region is plagiarised by raiders and plunderers originating beyond their border. These people set out to defeat those people. They gather a large army and chase the raider gangs a long way (a thousand miles or so) into foreign lands. They are finally able to crush the raiders militarily but find themselves lacking too much in resources to return. They settle in these new lands which abound with natural resources. They are culturally absorbed by the wild raiders. A good real life analogy are the Kalash people of extreme north Pakistan. They are said to be the descendants of Alexander who decided to reside in India instead of returning to Greece.
Then there is a group which lives to the north of the continent. They are farming people. When floods starts destroying their crops year after year, they gradually seek higher and higher grounds and finally have to cross a steep mountain range which geographically cuts them from the rest of the continent. In this new land, they find peaceful hunter-gatherer people who welcome them and help them settle (as the resources are enough for both people). With time, these people adopt to the new geographic conditions and constant interaction with the new people also affects their language and dress code. The two cultures gradually merge to form one.
Then there can be a group of the race who are merchants and traders by profession. Perpetual trade-interactions with other cultures leaves its impact, slowly alienating them from other groups of their own race.
The group living in the central part of their land are only affected in time by their progress in technology as religious myths are debunked and new inventions gradually erode old traditions and new ones are formed. New words are also introduced with technological advancement.
You can also introduce a group which is slowly alienated from the rest of their kind because they have to make military and political alliances to help survive against some powerful enemy in the region. Their alliance is not welcome by the other groups and these people slowly absorb the cultural impacts of their allies, slowly changing into a form which is no longer relatable to the original people.