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

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The structures that are similar in related organisms because they were inherited from a common ancestor are Homologous structures. These may or may not have the same function as in the descendants.
Examples: forelimbs of reptiles, amphibians, birds and mammals.
Yes, homologous structures are inherited from a common ancestor.
For example,

  1. Tendril of a pea plant and phylloclade of Opuntia both the structures belong to the shoot system and are inherited from a common ancestor. Phylloclade of Opuntia is modified for photosynthesis and tendril is meant to provide support to the weak stem. Hence these are homologous.
  2. The mammals as well as birds, reptiles and amphibians all have four limbs. The basic structure of limbs is similar though it has been modified to perform different functions in various vertebrates. Thus, they have evolved from a common ancestor
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