Sexual reproduction is one of the two basic manners in which organisms reproduce, along with asexual reproduction. In asexual reproduction, the organisms are sexless, and they reproduce using methods like fission, budding or spore formation. For example, strawberries reproduce by vegetative propogation, or sending out runners [source: PBS]. Asexual reproduction doesn't require direct interaction with another organism to produce offspring.
Sexual reproduction, on the other hand, takes two: It requires a male sex and a female sex, each of whom contributes a gamete (sex cell). The female sex cell is the egg, and the male sex cell is the sperm [source: Buckley]. The egg and sperm unite to create a zygote, which contains the genetic information from both parents. A complex organism's cells have their DNA arranged into pairs of chromosomes. Humans, for example, have 23 pairs. When sexual organisms reproduce, the male and the female each contribute a copy of each chromosome for the offspring. Which copy happens to be chosen for the new chromosome is random and leads to much greater genetic diversification of the organism's population.
In addition to genetic diversity, there are a couple of theories that some subscribe to that explain why sexual reproduction is more advantageous than asexual reproduction. For example, some believe that sex rids a species of harmful genetic mutations over time, while others believe that sex fights disease by allowing a species to create new genetic defenses against parasites [source: PBS].
Interestingly, the complex asexual organisms that exist today appear to have descended from sexually reproducing organisms at some point during their evolutionary history [source: The Journal of Evolutionary Philosophy].The human chromosomes hold the DNA of the human genome. Each parent contributes 23 chromosomes. (2001 HowStuffWorks)
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