If the population size is three, the two mutants have to be connected. If the population size is four or five, the two mutants can be separated by at most one wild-type individual. If the population size is six, the two mutants can be of distance zero, one and two, i.e., three types. In other words, six is the minimum population size of a circle, which gives rise to three distances between two mutants. Thus we adopt the population size six as an illustration model.</p
