Sexual reproduction increases genetic diversity of the resulting offspring. The greatest source of genetic diversity in sexual reproduction is the contribution of chromosomes from two different parents. Crossing-over also helps with genetic diversity by switching pieces of each homologous pair during prophase I. This moves genes from one chromosome to another. Chromosomes are randomly inherited by the offspring because of independent assortment, which is the random distribution of the pairs of genes on different chromosomes to the gametes during meiosis. This is a phenomenon discovered by Gregor Mendel, an Austrian monk who studied the inheritance of traits in pea plants. His research led to the determination that chromosomes are distributed randomly into gametes. It is complete chance as to which half of the homologous pair goes to each pole during anaphase I. Most species have more than two chromosomes. Humans have 23 pairs. This means there are more than eight million possible combinations that can be produced by independent assortment in humans. Add those to the reorganization that happens with crossing-over, and there is an enormous number of possibilities.
In contrast, asexual reproduction introduces very little genetic diversity. Consider a bacterial cell. This single-celled organism has a very simple circle of genetic material inside of it. When the cell becomes too large (the main reason for mitosis to occur), it duplicates that genetic material and then pinches off into two new cells. Each cell is genetically identical to the original parent cell. This means that an entire population of bacteria is vulnerable to being wiped out by a single environmental change, such as an antibiotic. Asexually reproducing organisms must rely on mutations in order to diversify their genes.
Random fertilization also leads to genetic diversity in sexually reproducing organisms. Each sperm has the chromosomes that were randomly assigned to it during meiosis. Remember that each sperm is different from all of the rest because of this independent assortment. Each one has a different set of genes that could possibly meet the genes of the egg to create a new offspring. The egg, which has also undergone independent assortment, has a random set of genes as well. It is unknown which sperm will fertilize the egg, so the traits of the resulting offspring are completely random.