Answer:
People typically employ several arguments in their efforts to explain the prevalence of sexual reproduction. One such argument is that organisms engage in sex because it is pleasurable. However, from an evolutionary perspective, this explanation arrived only moments ago. The first eukaryotes to engage in sex were single-celled protists that appeared approximately 2 billion years ago, over 1.3 billion years before development of the first animals with neurons capable of assessing pleasure. These bacteria (as well as their modern counterparts) engaged in genetic exchange via processes such as conjugation, transformation, and transduction, all of which fall under the umbrella of parasexuality. Surely, pleasure was not in a bacterium's realm of experience.
A second, more serious argument is that sex generates variable offspring upon which natural selection can act. This is one of the oldest explanations for sexual reproduction, tracing back to the work of German biologist August Weismann in the late 1800s. Although this explanation may very well account for why sexual reproduction is so commonplace, the explanation is far more subtle than many people realize for two reasons. First, sex does not always increase the variability among offspring. Second, producing more variable offspring is not necessarily favorable. In the next two sections, we describe these flaws in Weismann's explanation for sex, so that we can better understand the processes that help and those that hinder the evolution of sex.
