When mutation occurs at several locations in the evolution from one phenotypic state to another, there are several possible orders in which the substitutions can occur. All signifies a different pathway by the genetic space that evolution could obtain.
For an allele to evolve six different mutations, 720 possible evolutionary pathways are required. To develop seven different mutations 5,040 evolutionary pathways and to evolve ten different mutations 3,628,800 paths are needed.
The order in which mutation takes place is of unfavourable importance in deciding the path of evolution and whether evolution by natural selection will or will not really attains the most advantageous state. As the order of event of mutation is random, several advantageous phenotypes may not at all be completed even if the individual mutations appear.
The major feature in verifying the evolutionary path a population may follow the randomness of the mutational process. When the first genetic variation is worn out by the selective and random fixation of alleles, another difference occurring from the mutation can be the basis of more evolutionary change. The experimental analysis of evolutionary pathways is extremely time consuming and costly.