Easily. The simple answer is that the evolved creature having come from the same family as a creature below him..can simply have sex with the creature below him. Of course..one could argue most mutations don't have the ability to reproduce..well that is "most" not all. So..not only does the evolved creature reproduce with the lower creature..it spreads its traits to the next generation. Now..if you're saying the very first creature to require sexual reproduction, it of course could not reproduce without a second partner...unless it also had a means of asexual reproduction which many things did and can still do, such as lower organisms that can not only reproduce asexually but also by ordinary means. Now..if this is the case then perhaps the very first organism to reproduce sexually didn't..maybe it did it asexually..and then reproduced with itself. Of course..this sounds ridiculous..but of course some frogs change sexes. And say..ok this whatever evolved from what..a split off of what? An asexual creature..it had to..which means it retains some characteristics of an asexual whatever.