After decades of establishing concepts such as fitness, adaptation, and reproductive success on top of one another, a paradox quaked evolutionary theory in the early seventies (cf. Williams 1975: preface). Why should sex have evolved in the first place? Nature did not obey theory any more! Asexually reproducing organisms pass on 100% of their genetic information to their offspring, while sexually reproducing organisms inherit only 50%. is the two-fold advantage for asexual reproduction, and other obvious of sexual reproduction are apparent although harder to quantify. To find a sexual partner, mate, and reproduce successfully costs time and energy. And so does the often necessary courtship, when the genders need to overcome conflicting interests. So, while there should not have been a lot of sex in theory, it obviously was the dominant way of life among animals and plants. George C. Williams (1975: V) opened his seminal book on Sex and evolution with the proclamation of the crisis: This book is written in the conviction that the prevalence of sexual reproduction in higher plants and animals is inconsistent with current evolutionary theory. [... ] Many well informed readers may disagree with much of my reasoning, but I hope at least to convince them that there is a kind of crisis at hand in evolutionary biology. Since then, much research has been done and many models proffered to answer the question: Why is there sex? But instead of getting one straight answer more and more questions cropped up. Discriminating two different kinds of questions will be important in the following: 1) How much of a genome is reproduced and what advantage does this convey? 2) In which way is reproduction achieved and what advantage does this convey? Comparing reproductive units, like genes or organisms, while answering the first question leads to the paradox: 100% inheritance (asexual reproduction) will be of two-fold advantage compared to 50% (sexual reproduction), although sexuality is the prevalent mode of reproduction among animals and plants. The answers to the second question are manifold and complex, often involving models. As a general result, sexual reproduction has advantages under circumstances like intense sibling competition; an onslaught of diseases combined with vulnerability to parasites and other enemies; accumulation of detrimental mutations; or a saturated and heterogeneous environment (Stearns 1987). Asking in which way and under which circumstances reproduction is achieved, these models now have famous nicknames like Tangled Bank hypothesis, Red Queen hypothesis, or Miller's Ratchet. But while sexual reproduction will be favoured under these circumstances, nobody was able to cast this into a simple, quantitative advantage exceeding a two-fold ratio. And, like a jack-in-the-box, the original question jumped up again with ever new twists, whenever the paradox seemed to be put to rest: Why should sexual reproduction exist? How can its short-term disadvantage be overcome by its long-term advantage? Why do ancient asexual species, such as bdelloid rotifers and darwinulid ostracods persist, if asexuality was an evolutionary dead-end in the long run? If the benefits of sexuality could be obtained through facultative or temporary sex, why are most animal species obligatory and continually sexual? Why are asexual organelles within sexual cells, such as mitochondria and chloroplasts, not suffering from piling up mutation loads, parasitic DNA or intraclonal competition? Or why are they not taking over the sexual cells?
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