What is the advantage of sexual reproducers over asexual reproducers when exposed to an unstable environment?

In the human life cycle, germ cells undergo ___ in order to produce haploid gametes. The two haploid gametes then unite in a process called _____. When sperm and ovum unite, they form a zygote. The zygote undergoes _____ to begin the development of the human embryo which eventually becomes a baby.