Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually?

Sexual reproduction is a process in which there is fusion of gametes produced by male and female individuals of a species. It causes more viable variations because:
Because maternal and paternal chromosomes are randomly segregated during the creation of gametes, this leads to more viable variants. While gametes are being formed, genetic material is exchanged between homologous chromosomes. It results in genetically unique gamete which later fuses with another unique gamete and produces a zygote that is not genetically identical to its parents.

And since only one parent is involved in asexual reproduction, variance is substantially lower. Sometimes, very small mutations could be generated due to error in DNA replication but they are still uncommon and often unnoticeable. So we can assume that asexual reproduction produces offspring who are genetically relatively similar to their parents. 

Therefore, it can be said that evolution in species that reproduce sexually advances more quickly than it does in organisms that reproduce asexually.