Evolution of molecular structures has exhibited greater stability than morphological structures as morphology of organisms has changed continuously over time. Lets consider the example of human evolution, humans have achieved this morphology after many changes in their body structure. Changes in morphological structures can also be understood by taking examples of homologous and analogous organs. Molecules such as DNA, RNA, lipids have remained in their basic form right from the time of their formation thus we can say that evolution of molecular structures has exhibited greater stability than morphological traits exhibited by animals.
Evolution occurs gradually over a long period of time and this process generates variations and some traits appear which give a particular species certain advantages to survive