Sexual reproduction is able to generate genetic variation in offspring because the process of meiosis 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 is one of the keys to evolutionary fitness. This is made possible because reproductive cells undergo meiosis, which is the process in which one cell divides twice to produce four sperm or four eggs.
Genetic variation in a population of organisms means that different organisms have different strengths and weaknesses. This is important for the survival of 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 will survive.
The first way that meiosis generates genetic diversity is when homologous chromosomes exchange parts by crossing over. Early on in meiosis, during prophase I, homologous chromosomes pair up. Homologous chromosomes are ones that have similar genes: 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 result is that the paired homologous chromosomes now have regions of DNA that were formerly on the other chromosome.
The second way that meiosis generates genetic diversity is that each individual chromosome goes into one of four different gametes. A gamete is a sperm or an egg cell. Meiosis in a normal human cell that has 46 chromosomes produces four gametes that each has 23 chromosomes. This is possible 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 split the two pairs (2 x 2 = 4) of homologous chromosomes that did “crossing over” into four separate chromosomes. Each of these chromosomes goes into a separate gamete cell.
The third way that meiosis generates genetic variation happens after meiosis is done. 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 have 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.