One question when dealing with evolution is: why is sex necessary for reproduction? Why must so many creatures obtain a mate?The following material is flagged Yellow Level. It contains material that is disputed by some experts but accepted by others. Caution is advised when deciding whether you personally choose to believe it.
Suppose that we have a species that does not require mates. In this species, genes are passed on directly to offspring, with no interference. As a result, any mutations are unable to spread through the population except by displacing all other lines.
Now, suppose there is a mutation in that species that allows it to replace parts of its offspring's genetic code with parts of that of another member of the species. This mutation cannot become universal, unless the rest of the population is wiped out. So what possible advantage could it have?
Simple. Remember that evolution works because mutations sometimes make survival more likely. So, an individual whose offspring are able to have mutations from other individuals has offspring better suited to the conditions in which those mutations are advantageous. In other words, while evolution may be a matter of "survival of the fittest", it is important to remember that adaptability is fitness.
So why would sex become necessary? Consider that the one (inherent) advantage of sex is that it allows for the spread of mutations rapidly through a population, with purely detrimental mutations being eliminated or outcompeted quickly. In an environment that changes rapidly (and thus requires quick mutations from generation to generation in order to survive), it is far easier to copy someone else's mutations than to mutate rapidly enough to survive. So, there is a selective pressure favoring sexual reproduction. If the environment changes quickly enough, organisms that use asexual reproduction die off, eventually leaving only those organisms that are incapable of "deciding" to reproduce sexually.
(Note that this logic does not apply to all situations, and as such sexual reproduction is not universal. Some bacteria only reproduce asexually, other bacteria reproduce asexually but can exchange genetic information, and for that matter some species of lizard retain sexual instincts despite not exchanging genetic information.)
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