The notion of race that derived from Iberian rule in America evolved from an ideology to the kind of scientific racism that became prominent in the eighteenth and nineteenth centuries.
When the Spanish first began to conquer the Americas in the fifteenth century, the Castilian word raza—"race"—referred to a group of human beings, plants, or animals that had shared traits or a shared genealogy.
However, from the very outset of the Spanish colonial project, it became clear that what we would now call racist beliefs in relation to the indigenous population were firmly entrenched. Spanish colonialists regarded the people of the Americas as racially and culturally inferior, uncultivated pagans whose religious practices they found deeply offensive.
The harsh, exploitative treatment to which the Spanish notoriously subjected indigenous Americans was largely motivated and sustained by such preconceived racist attitudes, which, over time, hardened into a fixed ideology.
Spanish racism was by no means new. Racist attitudes towards the Muslim Moors, who had once conquered large swathes of the Iberian peninsula, had long been in existence. To some extent, Spanish colonialists took such attitudes with them to the Americas.
It was only from the eighteenth century onwards, however, that the notion of race evolved into scientific racism, a set of racist beliefs masquerading as science. What had once been ideology was given a legitimacy it did not deserve by being cloaked in the value-free language of scientific experiment and observation. As a result, the unthinking prejudice of bygone eras was considerably reinforced and strengthened, not just in Spain, but right across the Western world.