Why is sexual reproduction considered a hallmark of evoluntionary advancement?
Sexual reproduction is the process by which genetic information is taken from two different individuals and combined. It is important to note that the combination of different sexes is not necessary in all cases. Organisms like bacteria or protists do not necessarily have male and female individuals, yet they are capable of sexual reproduction because all that matters is that genetic information is being obtained from two different individuals.
The reason that sexual reproduction is considered a hallmark of evolution is because sexual reproduction creates new, never-before-seen genetic combinations. Even before the genetic information from two individuals is combined, new genetic combinations can already be created through crossing over, which occurs during meiosis. Crossing over occurs when random sections of chromosomes swap places with homologous pairs. All of this genetic mixing creates new genotypes, and those varying genotypes result in phenotypic differences among individuals within a population. Some of those changes are not beneficial to the survival of the individual. That organism is likely killed before having a chance to reproduce, thus removing that "errant" genetic code from the gene pool.
Some of the genetic changes are beneficial to the survival of the individual. Such genetic changes result in an adaptation that allows the organism a better chance at survival. That organism is more "fit" to survive, and that is the reason for the phrase "survival of the fittest." Organisms that are surviving are likely reproducing and passing on their genetic adaptations. After enough time has passed, an entire species might have that adaptation. Thus a given species may appear different than it did thousands of years prior. All species evolve through nature's selection of those individuals with the adaptation to survive. Without the recombination of genetic material from sexual reproduction, such adaptations are unlikely to arise.
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