Sibling species

Sibling species are species which look alike, but cannot breed with each other.
The classic case is "Drosophila pseudoobscura" and "D. persimilis". They are almost identical, but if flies from one species are put with flies from the other species, they do not interbreed. Another example was the European mosquito known as "Anopheles maculipennis". This turned out to be a cluster of six different species, some of which transmit malaria, and some do not.