The union of male and female sex cells

Sexual reproduction occurs through the union of sex cells, or gametes. Gametes differ widely in males versus females for a given organism. For humans, male gametes are called spermatozoa while female gametes are called ova. Spermatozoa are also known as sperm and ova are also known as eggs.

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Gametes are reproductive cells or sex cells that unite during sexual reproduction to form a new cell called a zygote. Male gametes are called sperm and female gametes are ova (eggs).

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in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".