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SEX REFERS TO the division between many biological species into male and female forms. These forms differ in their chromosomal composition and in those physical aspects that relate to reproduction. Sexual division is common among higher creatures, but is by no means the only arrangement. Many asexual species exist and do not show any disadvantage in evolutionary terms. However, the division, which is billions of years old, seems most likely to have been caused by evolution to increase genetic variation and, hence, survivability of the young. Asexual creatures reproduce through division into daughter creatures that are genetically identical to their mother; on the other hand, male and female partners mating provide a much wider possibility for variation.

Subsequently, evolution has led to changes in the ...

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