Please note the Thracian peltast is a true skirmisher with more but still very limited melee capability than regular skirmishers. The Iphicratian peltast is an outright melee unit, more like a ligher and more mobile hoplite. They share the name merely due to the type of shield they share, namely a pelta, which is smaller than an aspis that covered almost the whole body. As such, there is no evolution per se and in my opinion it is preferrable to handle the two separately.