Speciation - the process of one species splitting into two - involves the evolution of reproductive isolating barriers such as hybrid sterility or hybrid inviability between previously interbreeding populations. Even in his seminal masterpiece "On The Origin of Species", Darwin could find no satisfactory solution to the apparent paradox of why natural selection would tolerate the onset of genetic barriers such as hybrid sterility and inviability and, therefore, termed this problem the "mystery of mysteries." This "mystery of mysteries" is one of the most important and unanswered questions in biological research over the past 150 years. How do two species evolve from one? Here I address how molecular arms races involving selfish genes can drive the evolution of new species.