During sexual reproduction, meiosis generates genetic variation in offspring because the process randomly shuffles genes across chromosomes and then randomly separates half of those chromosomes into each gamete. The two gametes then randomly fuse to form a new organism. Genetic variation stands as one of the key factors in evolutionary fitness and biological diversity. Reproductive cells undergoing meiosis make this possible, as the process has these specialized sex cells split and multiple after copulation.
TL;DR (Too Long; Didn't Read)
Creating new organisms requires the process of meiosis, the process by which a fertilized egg cell splits into multiple cells. Genetic variation in sexual reproduction only occurs because meiosis randomly shuffles the genes of the two organisms mating.
Genetic Variation and Its Importance
Genetic variation in a population of organisms means that different organisms have different strengths and weaknesses. This acts as important facet of a species' ability to survive and increase its population because if new predators show up or food resources become scarce, many organisms will die. However, because of genetic variation some will survive because they can do things such as run faster or eat different foods. Those who survive will reproduce and repopulate the community. In terms of having toughness against harsh circumstances that threaten to kill off a population, genetic variation increases the chances that some members of a population will survive.
Chromosomes Crossing Over
The first way that meiosis generates genetic diversity occurs when homologous chromosomes exchange parts by crossing over. Early on in meiosis, during prophase I, homologous chromosomes pair up. Homologous chromosomes have similar genes with other homologous chromosomes: one chromosome came from the mother and one came from the father. During meiosis, they look for each other and stick together length-wise. During this time, they exchange parts of their arms with each other, like combing two deck of cards, shuffling, and then equally separating the two decks. The results in paired homologous chromosomes that now have regions of DNA that were formerly on the other chromosome.
Independent Assortment of Chromosomes
The second way that meiosis generates genetic diversity is that each individual chromosome goes into one of four different gametes: a sperm or an egg cell. Meiosis in a normal human cell that has 46 chromosomes produces four gametes that each have 23 chromosomes. This can occur because each of the 46 chromosomes had been copied (46 x 2 = 92) before meiosis split that one cell into four ( 92/4 = 23). Not only did meiosis shuffle the homologous chromosomes through the cross-over event described above, it then splits the two pairs (2 x 2 = 4) of homologous chromosomes that "crossed over” into four separate chromosomes. Each of these chromosomes goes into a separate gamete cell.
Gamete Fusion and Sex Cells
The third way that meiosis generates genetic variation happens after meiosis occurs. In sexually reproducing organisms, such as humans, a sperm from the male must fertilize the egg from the female. Human males produce many sperm, each with 23 chromosomes that have been shuffled, that have a unique combination of genes compared to the many other sperm. The egg also has this shuffled genetic diversity. So when one unique sperm fuses with one unique egg, a cell with 46 chromosome forms. This cell has a combination of genes that is unique compared to the mother and father that produced the sperm and egg.