CBSE Class 10 Science 2017 · Paper 31(B)/C · Question 15

Homologous organs are organs that have the same basic structure and origin but perform different functions, for example the forelimbs of a frog, a lizard, a bird, a whale and a human (upper-arm bone, forearm bones, wrist bones and digits in all). Analogous organs are organs that have a different basic structure and origin but look alike and perform similar functions, for example the wings of a bird and of a butterfly (or, in NCERT's example, the wings of a bat and a bird). Evidence of evolution: homologous organs show that different organisms have descended from a common ancestor, the same basic plan having been modified for different uses (divergent evolution). Analogous organs show that unrelated organisms adapting to a similar function evolve similar-looking organs (convergent evolution); they also show that similar function alone does not mean a close relationship, so evolutionary relationships are traced through homologous organs.