In order for sexual reproduction to occur, an organism must make gametes, also known as sex cells. Meiosis, a reduction division, accomplishes that. It reduces the diploid chromosome number to the haploid amount. This means that sex cells have half the chromsomes as the diploid body cells do. Crossing over allows for more variation in the gametes. Homologous chromosomes form tetrads(groups of four) and exchange similar parts between them. This is like shuffling a deck of cards, resulting in the wide variety of gametes an organism produces. Sometimes, a gene may mutate(change). Mutations may occur spontaneously, or due to a mutagenic agent-X rays, U.V. rays, chemicals, or environmental agents. A mutation in a gene produces a protein that is altered. It may be functional, partially functional or non-functional. If a mutation confers an advantage on an organism, if it survives and reproduces, more organisms will inherit this mutation. Over time, the species may inherit so many mutations, that it can no longer reproduce with the ancestral population and is considered a new species. Mutation is a key reason as to why evolution occurs.