Sexual reproduction causes more viable variations due to the following reasons:
Error in copying of DNA, which are not highly significant.
Random segregation of paternal and maternal chromosome at the time of gamete formation.
Exchange of genetic material between homologous chromosomes during formation of gametes.
Accumulation of variations occurred due to sexual reproduction over generation after generation and selection by nature created wide diversity.
In case of asexual reproduction, only the very small changes due to inaccuracies in DNA copying pass on the progeny. Thus, offsprings of asexual reproduction are more or less genetically similar to their parents. So, it can be concluded that evolution in sexually reproducing organisms proceeds at a faster pace than in asexually reproducing organisms.