sentences:  From a mathematical standpoint, an unused gene (selectively neutral) has a steadily decreasing probability of retaining potential functionality over time.The time scale of this process varies greatly in different phylogenies; in mammals and birds, there is a reasonable probability of remaining in the genome in a potentially functional state for around 6 million years.When two species are similar in a particular character, evolution is defined as parallel if the ancestors were also similar, and convergent if they were not.