Perceived racial difference from Muslims outweighed actual biological difference

Evidence

But what can be asserted as absolutely certain is that by the end of the sixteenth century a great number of Christian Spaniards were firmly convinced that there were indeed great differences between themselves, descendants of the populations native to the peninsula, and these “alien” intruders. — L. P. Harvey, Muslims in Spain, 1500 to 1614, 83

Categorization

• XREF: Connects to the rhetorical construction of out-group difference despite biological similarity — parallels other episodes of invented ethnic boundaries such as Irish racialization under English rule.
• QUESTION: How did the belief in irreducible biological difference get propagated and maintained, and did any contemporaries challenge it?