switching the alleles of chromatids results in the production of four different types of haploid gametes instead of the two different types that would have resulted if cross-over had not occurred. Thus, cross-over and meiosis II increase the total number of genetic combinations possible.
Further, while asexual reproduction does not give rise to genetic variations as all daughter cells are produced by mitosis, sexual reproduction does give rise to genetic variation via the different combinations of genetic materials that result during meiosis and fertilization. Without sexual reproduction, offspring are genetically identical to the parents and to each other: Such a group of individuals is called a clone. Sexual reproduction augments variation in a population by recombining chromosomes and the genes they bear. Humans with twenty-three types of chromosomes can produce 223 different chromosomal combinations (even without cross-over occurring) through meiosis and can produce (223)2 chromosomal combinations through fertilization. Sexual reproduction makes possible this genetic recombination and increases variations among individuals. The greater variety of descendants, the greater the chance that some will be well-adapted to changing environment conditions and survive to further propagate the species. Thus, genetic variability is the raw material on which natural selection acts, promoting the survival of well-adapted individuals and the extinction of ill-adapted individuals.
