Only mosquitoes of the genus Anopheles transmit the Plasmodium parasites that cause human malaria, and within that genus a minority of species are important vectors. Anopheles gambiae was long treated as a single, unusually effective one.
Crossing experiments and later molecular markers showed instead a group of sibling species, morphologically near-identical but reproductively separated, including forms tolerant of salt water and forms restricted to fresh. Anopheles arabiensis and Anopheles coluzzii, once treated as members or forms of gambiae, are now recognised as species in their own right. They differ in resting behaviour indoors or outdoors, in the strength of their preference for human blood, and consequently in how much malaria they move.
That matters for control. Insecticide-treated nets and indoor residual spraying act most strongly on mosquitoes that bite and rest indoors at night, so a shift in the species mix towards outdoor, earlier-biting members changes how much protection the same intervention delivers.