Why have sex?
Sex is expensive, dangerous and results in only half of ones genetic material being passed to the next generation. Many species survive and reproduce successful without it. So why do we and many other species have sex? What is the point?
There are advantages and disadvantages for both asexual reproduction and sexual reproduction. Asexual reproduction is the type of reproduction that only involves one parent, and offspring are genetically identical to this parent. Asexual reproduction is quick and easy. There is no need to find a mate, so there is minimal waste of time, energy and resources and there is no loss or dilution of ones genes from generation to generation.
Sexual reproduction on the other hand requires finding a mate. This is both an expensive and dangerous process. It is expensive in the sense that there is a cost of time, energy and resources. It is dangerous because often in the process of trying to attract a mate, many species attract a less welcome form of attention (for example predators) and this kind of attention can very often lead to their death. However, of course there must be some benefits of sexual reproduction or else no species would bother. The answer is variation! Variation within a population comes about through sexual reproduction, because each individual gets a unique set of genes both from their mother and their father. This means that every individual has a slightly different genetic make-up and so has slightly different advantages and disadvantages over other individuals in different environmental situations. Variation is incredibly important because without variation there is no possibility for adaptation.
If every habitat on the earth were perfectly stable then there would be no need for sexual reproduction, asexual would work just fine. But in reality there is so much environmental variation in the world that this variation needs to be matched by variation amongst individuals. Without individual variation, organisms would never have been able to adapt to fill the many different habitats on earth.