Sexual Reproduction In organisms that reproduce sexually most of the genetic variation in a population results from unique combina tion alleles each individual receives its parents. Of course at nucleotide level all differences among these have originated past mutations. reproduction then shuffles existing and deals them random to produce genotypes. Three mechanisms contribute this shuffling: crossing over independent assortment chromosomes fer tilization (see Concept 13.4). During meiosis homologous one inherited parent trade some their by over. The resulting recombinant chro mosomes are distributed into gametes. Then because myriad possible mating combinations exist pop ulation fertilization typically brings together gametes with different backgrounds. Together three mecha nisms ensure sexual rearranges fresh generation providing much makes evolution possible. Mastering Biology Animation: Origins Genetic Variation CONCEPT CHECK 23.1 1. Explain why within is pre requisite for evolution. 2. mutations occur population do only small fraction become widespread? 3. MAKE CONNECTIONS If stopped reproducing sexually but still reproduced asexually how would be affected over time? (See 13.4.) For suggested answers see Appendix A