Crossing over and recombination are important because

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Crossing over and recombination are important because they dramatically increase the genetic variability of gametes produced by meiosis. The random assortment of homologous chromosomes during meiosis varies the combo of chromosomes present in gametes; crossing over varies the combo of alleles within each chromosome. How Does Fertilization Affect Genetic Variation? Crossing over and the random mixing of maternal and paternal chromosomes ensure that each gamete is genetically unique. Even if two gametes produced by the same individual fuse to form a diploid offspring, (called self-fertilization) the offspring are very likely to be genetically different form the parent. Self-fertilization is rare or nonexistent in many sexually reproducing species, however. Instead, gametes from different individuals combine to form offspring. This is called outcrossing. Outcrossing increases
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the genetic diversity of offspring because it combines chromosomes from different individuals, which are likely to contain different alleles. Why Does Meiosis Exist? The Paradox of Sex Bacteria and archaea normally undergo only asexual reproduction. Asexual reproduction is much more efficient than sexual reproduction because no males are produced. The Purifying Selection Hypothesis If a gene is damaged or changed in a way that causes it to function poorly, it will be inherited by all of that individual's offspring when asexual reproduction occurs. An allele that functions poorly and lowers the fitness of an individual is said to be deleterious. Asexual individuals are doomed to transmitting all of their deleterious alleles to all of their offspring. Sexual individuals are likely to have offspring that lack deleterious alleles present in the parent. Natural selection against deleterious alleles is called purifying selection. Over time, purifying selection should steadily reduce the numerical advantage of asexual reproduction. The Changing-Environment Hypothesis Offspring that are genetic clones of their parents are unlikely to thrive if the environment changes. If a new strain of disease-causing agent evolves, then all of the asexually produced offspring are likely to be susceptible to that new strain. But if the offspring are genetically carried, then it is likely that at least some offspring will have combinations of alleles that enable them to fight off the new disease and produce offspring of their own. If the changing-environment hypothesis for the advantage of sex Is correct, then the frequency of sexually reproducing individuals should be high in habitats where parasites are common, and low in habitats where parasites are rare. Sexual reproduction is helpful for two reasons: 1. Offspring are not doomed to inherit harmful alleles.
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