Abstract

Within the philosophy of biology, Michael Scriven’s twins has become a well-known thought experiment, due to its being the impetus for various lines of discussion concerning the contentious definition of evolutionary fitness, the distinction between natural selection and genetic drift, and evolutionary environments. As one version of the story goes: two twins who, ex hypothesi, are genotypically and phenotypically the same are located side by side on a mountain. An unfortunate event ensues whereby a lightning strike kills one of the twins, whilst the other twin survives. For added disparity, the surviving twin goes on to proliferate extensively by bearing a large number of offspring. Importantly, it is usually inferred, from the stipulations of the thought experiment, that the twins also have the same evolutionary fitness. The inference is important because the premise that the twins have the same fitness paves way for a number of influential views; most notably, it has led to the propensity interpretation of evolutionary fitness.