A Level Biology Practice Exam

What characterizes a homologous structure?

1. Structures with different functions

2. Structures that look different but have the same underlying structure

3. Identical structures in different species

4. Structures developed exclusively for one function

The correct answer is: Structures that look different but have the same underlying structure

Homologous structures are characterized by their underlying anatomical similarities despite potentially differing functions. This concept is integral to the theory of evolution, where structures in different species share a common ancestor. The presence of similar bone structures in the limbs of mammals, for example, illustrates how these diverse species have evolved to adapt to different environments and functions, while retaining a common structural blueprint. In contrast, structures with different functions might not share any underlying similarities and may be classified as analogous structures. Identical structures in different species would suggest a lack of variation or evolutionary divergence, which does not apply to homologous structures. Structures developed exclusively for one function indicate specialized adaptation and might not demonstrate the commonalities of homologous structures. Thus, the defining characteristic of homologous structures lies in their similar anatomical framework, regardless of their specific roles in an organism.