I sent this to Daniel Mendez my cousin and his answer I thought would be of interest to many who's lines come from there
I was told there were originally 10 Espanol families who agreed to marry only Espanol and I remember you saying you go back to 1 ancestor 17 times from there.
Daniel MéndezdeTorres Camino Yes! The gene pool or rather the nucleus are the following: Mendoza (by far the most influential, produced several branches: Gómez-Hurtado de Mendoza, Gutiérrez de Mendoza, Jiménez de Mendoza, Sánchez de Mendoza, etc), González de Hermosillo, González Rubio, Macías Valades, Fernández de Rueda, Hernández Gamiño, Reynoso-Rentería, Gómez de Portugal, González de Rubalcaba, Alcalá y Hurtado de Mendoza, Vásquez de Lara, Ruiz de Esparza and Padilla Davila
The other 100 or so families gradually settled in the area and began to marry into the nucleus. The nucleus were fond of the peninsulars and quickly married their daughters to them. Often times they came in service of the king, held a social position, land was granted, etc. Most if not all people from Los Altos, Aguascalientes, western Guanajuato and Jalisco can trace repeated times to these families. My mom for example descends from 9 children of Lope Ruiz, Lord of Esparza, some 11 times. Thats intense!
original families from los
There was never an agreement though. The marrying of españoles with other españoles was due to being able to maintain a higher status in society by marrying other people considered to be españoles. Since marrying into a lower class would cause them to be reduced in class they tried to avoid that. This happened all over Mexico and not just in Los Altos. When peninsulares with daughters moved to the region that allowed the Alteños to marry them maintain a higher ratio of Spanish ancestry. There would be no need to have an agreement to do something that the rest of society in Latin America expected a person to do.
Armando