A species of fly lives in a canyon. Some of the flies prefer the side that gets sunlight during the day, while others prefer the shade. Over time, they develop different mutations and start to mate with those that prefer the same conditions. What would this be due to

habitat preference

Explanation:

One of the driving factors for ecological speciation that does not allow interbreeding between the members of a species is habitat preference. Some members of a species have a different preference for habitat than others in terms of habitat preference. They reside and do not interbreed in various environments. These two populations of a species live in diverse environments acquire genetic differences over generations and interbreed only with members of their own populations.