What primarily causes genetic variation in sexually reproducing organisms?

Genetic variation in sexually reproducing organisms is primarily caused by meiosis. Meiosis is a specialized type of cell division that occurs in the formation of gametes (sperm and egg cells). During meiosis, processes such as crossing over and independent assortment play crucial roles in reshuffling genetic material.

Crossing over occurs when homologous chromosomes exchange segments of DNA during prophase I of meiosis. This results in new combinations of alleles on each chromosome, contributing significantly to genetic diversity. Independent assortment refers to the random distribution of maternal and paternal chromosomes into gametes during metaphase I and anaphase I, further promoting variation among offspring.

While fertilization does contribute to genetic variation by combining genetic material from two parents, it is the processes in meiosis that create the unique gametes that come together during fertilization. Genetic drift and mutation are also factors in genetic variation but are not the primary mechanisms in sexually reproducing organisms. Genetic drift is a mechanism of evolution that occurs due to random changes in allele frequencies within a population, while mutations introduce new genetic material into a population, but they occur less frequently compared to the variation produced by meiosis.