If ibericus/structor hybrid females become sterile workers, but pure ibericus females become queens, that increases the share of ibericus genes in the next generation. So as soon as a mutation arose that had this effect, it outcompeted all other lineages, but also made ibericus dependant on hybridization with structor males to produce workers for the colony. The adaptation that is the focus of the article works around this drawback by letting ibericus colonies produce their own supply of structor males.

So there are roughly three evolutionary phases:

1. Hybridization is common, but largely inconsequential.

2. Hybrid queen elimination.

3. Male cloning.