This diagram illustrates how sex might create novel genotypes more rapidly. Two advantageous allelesA and B occur at random. The two alleles are recombined rapidly in a sexual population (top), but in an asexual population (bottom) the two alleles must independently arise because of clonal interference.
Clonal interference is a phenomenon in the population genetics of organisms with significant linkage disequilibrium, especially asexually reproducing organisms. It occurs when two (or more) different beneficial mutations arise independently in different individuals. Prior to, or in the absence of genetic recombination, the mutations cannot join to form an organism carrying both of them, but they compete against each other. This typically leads to the loss of one of them, confirming that the fate of mutations fitness are effected for other variations in the population.
^Marianne Imhof and Christian Schlötterer: Fitness effects of advantageous mutations in evolving Escherichia coli populations,Proc Natl Acad Sci U S A. 2001 January 30; 98(3): 1113–1117
^Lang, G. I.; Rice, D. P.; Hickman, M. J.; Sodergren, E.; Weinstock, G. M.; Botstein, D.; Desai, M. M. (2013). "Pervasive genetic hitchhiking and clonal interference in forty evolving yeast populations". Nature500 (7464): 571. doi:10.1038/nature12344. PMID23873039.edit