What is non-adaptive evolution?

According to the way most people think about evolution, species evolve because they are becoming more adapted to their environments (natural selection). This is adaptive evolution. Evolution, however, can also happen for reasons other than natural selection. For instance, sexual selection often has little to do with adaptation to the environment (in many species, the colors or loud calls of males attract females for mating but also attract predators; a sexual advantage often does not correspond to an environmental advantage). Sexual selection impacts evolution but has nothing to do with adapting to the environment.

The classic example of nonadaptive evolution is genetic drift, in which sheer chance leads to changes in the frequencies of alleles. Evolution occurs because the frequencies are changing, but the alleles are not changing for any reasons of adaptation; it is just random luck.