Let's take a look at Spanish!
Spanish on the peninsula is broken up into 2 major (secondary) dialects, or two norms.
Norm 1: part of the northern old dialects of Spain, centered around Burgos, the original name was castellano after the region, castilia. This expanded to the south through the reconquista and repupulation of the conquered lands, but is not the norm. Other dialects influence on this one in the process. With the conquest of Toledo we have a new center (wasn't it even the capital at one time?) where dialects come together, the old Burgos dialect is dominant and becomes the language to write literature and anything relatated to judicial purposes in (in other parts of Europe this is still Latin).
Norm 2: Conquest of Andalucia/Sevilla. Another new center where dialects converge, but we already have a written norm (Toledo, later Valladolid and Madrid). This one will be heavily exported into Latin America and is distinguished by some pronounciation and grammar differences.
voila, the making of a dialect. But the Spanish way to a (minor) dialect is quite unique due to it's history. However, it has been said that Spanish now consists of about 7 major dialects all over the world.
P.S: I do not believe that a Texan English can be considered a dialect of English English.Well, maybe a tertiary dialect.
P.P.S: on secondary and trtiary dialects:
- primary dialect: this would be for example any romance language that evolved into a new dialect by being superceded by vulgar Latin. In it's time, before being recognized and normatized as a language Castellano and Catalan would be examples of this.
- secondary dialect: This is what happens when a primary dialect mixes with other primary dialects. The Spanish of Toledo and Sevilla would be good examples. Therefore all Spanish dialects are not able to form the following third dialect:
- tertiary dialect: primary dialect meets the standardized language and this creates a mixture between the two. Most common occurence is when a speaker of the primary dialect (my best example: Bavarian) tries to talk in the standardized language (High German) but falls back every now and then due to whatever reasons -> new dialect, "Light Bavarian".