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Sexual reproduction increases genetic variation through two linked processes: meiosis and fertilization. 1. Meiosis — reshuffling parental genes - Homologous recombination (crossing over): During meiosis, pairs of matching chromosomes (one from each parent) exchange segments. This creates chromosomes that are new combinations of the two grandparents’ DNA, so gametes (sperm or eggs) carry unique mixes of alleles. - Independent assortment: Chromosome pairs line up and separate randomly into gametes. For organisms with many chromosome pairs, this produces an enormous number of possible chromosome combinations. For example, with n chromosome pairs, there are about 2^n different combinations from independent assortment alone. - Mutation in germ cells: Meiosis and DNA replication can introduce new mutations, adding novel genetic variants (usually rare but evolutionarily important). 2. Fertilization — mixing genomes from two parents - When two gametes fuse at fertilization, alleles from two different individuals combine in the offspring. This produces new genotypes (combinations of alleles) that were not present in either parent alone. - Sexual mixing breaks up linked combinations of alleles (linkage disequilibrium) and can assemble beneficial alleles from different lineages into the same individual. Why this variation matters - Adaptation: Populations with greater genetic diversity are more likely to contain individuals suited to survive novel environmental challenges (climate change, new predators, changing food sources). Natural selection can act on this variation. - Disease resistance: Genetic diversity reduces the risk that a single pathogen or parasite will exploit the entire population. Different genotypes may confer resistance to different threats, preventing catastrophic losses. - Long-term evolvability: Recombination can separate beneficial mutations from harmful genetic backgrounds, allowing advantageous traits to spread more effectively. References: - Mayr, E. (1963). Animal Species and Evolution. - Otto, S. P. (2009). The evolutionary enigma of sex. American Naturalist. - Parker, G. A., Baker, R. R., & Smith, V. G. F. (1972). The origin and evolution of gamete dimorphism and the male–female phenomenon.