Flexi Says: Mammals are complex species evolved from the simpler one through evolution. Genetic variation is crucial if evolution is to occur and that is possible with sexual reproduction only. Without genetic variation within populations, there can be no differential fitness by genotype, and natural selection cannot occur. These factors are the "forces of evolution" which include mutation, gene flow, genetic drift, and natural selection. Sexual reproduction provides infinite possibilities of genetic variation to continue the vigour for survival. For this reason complex organisms are adapted for sexual reproduction only. Asexual reproduction is extremely rare in mammals.