Answer: Sexual reproduction
Sexual reproduction is the primary process which increase genetic variability. A new allele combination is increased with every sexual reproduction from three primary processes. These processes are crossing over during prophase 1 of meiosis, Independent assortment of homologous chromosomes and random fusion of sperm and egg (fertilization). Increasing genetic variation and the environmental effects on that variation form the basis for natural selection. It states that those individuals who are best adapted to live given geographical area will survive and reproduce.