What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor. Justify your answer.

1. Homologous Structures: Homologous structures are organs or body parts in different species that share a similar basic structural design and developmental origin, but may perform different functions in their respective environments.

1. Example: The forelimbs of a human, a bird, and a frog. They all share a similar basic bone design, but they are modified to perform different functions (grasping, flying, and propping/leaping respectively).

1. Is a common ancestor necessary?: Yes, it is necessary.

Justification: According to the principles of heredity, "inheritance from the previous generation provides both a common basic body design, and subtle changes in it, for the next generation." The similarity in the fundamental skeletal or structural design of homologous organs exists because they have been inherited from a shared common ancestor. Over succeeding generations, environmental selection and the accumulation of variations lead to subtle modifications in these structures to suit different functional needs, but the underlying basic design remains conserved, proving their common ancestral origin.