Most organisms do not reproduce sexually, so why is sexual reproduction advantageous for some organisms?

Higher organisms have resorted to sexual reproduction in spite of itscomplexity. Why?

Some organisms are capable of asexual or sexual reproduction. Under favorable conditions, reproduction proceeds asexually. When conditions become more stressful reproduction switches to a sexual mode. Why?   Question options:   Sexual reproduction is simple and more rapid allowing larger numbers of offspring to be produced.   Sexual reproduction requires two separate individuals, who can mutually provide nutrient support during stress.   Sexual reproduction produces individuals with new combinations of recombined chromosomes increasing diversity.   Asexual reproduction requires more energy.

Sexual reproduction involves crossing-over to mix up genes. Is this a good thing or a bad thing? In the discussion this week explain one pro and one con to crossing-over. Overall, what is your opinion of crossing-over? Can evolution occur without it? (Min. 50 words)