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Crossing over occurs during meiosis I. During crossing over, homologous pairs of chromosomes exchange sections of DNA that contain the same genes. It is important to note that crossing over does not have to occur during each generation, nor does it always take place at the same point of exchange. Over time, however, crossing over leads to a greater variety of genes in a population and contributes to a diversity of characteristics and increased fitness of the population. This is reflected in the ability of the population to adapt to changes in the environment.