Question

Why do we build trees starting with the number of shared derived characters rather than the number of shared ancestral characters?

Answer 1

1. A shared derived character helps to group organisms into clades , Consider an example amphibians, yuetles, lizards, snakes, crocodiles, birds and mammals, they all historically have four limbs, most of modern snake does not have limbs, it is a shared derived character which is inherited from a common ancestor.
2. Shared derived character indicate relative recentness of common ancestry whereas shared ancestral characters do not.
3. Shared derived character is shared by the ancestral species and a single group, because of this it is considered as the only reliable guide to the inferring phylogeny, wereas ancestral character is shared with the species ancestral to more than one group and it contain more than one group, so it contain different groups to classify together because of this reason it is difficult to understand.
4. Shared derived character helps to figure out evolutionary relationship amoung a group of species that are shared by some species but not present in others, if the species share more derived character we can tell that they are closely related to each other.