Why anatomical and molecular features often fit a similar nested pattern. in addition descibe a orocess that can cause this not to be the case

Anatomical and molecular features often fit a similar pattern due to Homology. Homology is the existence of shared ancestry between two structures or genes in a different species. 

A process that can cause this not to be the case is Convergent Evolution.
Convergent Evolution occurs when two unrelated species independently evolve similar traits to cope with evolutionary changes. 

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