evolution of sex
...that the evolution of sex is a major puzzle of evolutionary biology, due to the so-called two-fold cost of sex?
...that the <b id="mwAVk"><a class="mw-redirect" href="./Evolution_of_sex" id="mwAVo" rel="mw:WikiLink" title="Evolution of sex">evolution of sex</a></b> is a major puzzle of <a href="./Evolutionary_biology" id="mwAVs" rel="mw:WikiLink" title="Evolutionary biology">evolutionary biology</a>, due to the so-called <i id="mwAVw">two-fold cost of sex</i>?