What might be the evoluntionary significance of the swicth from asexual to sexual reproduction when the environment changes?

Answer 1

Environmental changes almost demand variation in organisms so that the best fitted to the changing environment survive and reproduce in a greater number. Asexual reproduction produces clones with little to no genetic variation. Good enough for a static environment, but in a challenging environment with no variation that could be more beneficial than cloning the asexual species could find itself selected against.