CDS (II) 2020 General Knowledge · Question 94 of 120

Correct — B, different origin and common function. That pair of words is the definition of an analogous structure: same job, separately arrived at. A bird's wing is a flight surface made of feathers carried on a much-reduced hand; a bat's wing is a sheet of skin, the patagium, stretched between four enormously elongated fingers. The two are built from different tissue on a different skeletal plan and appeared independently in two lineages that were already separate before either could fly, so they were not inherited from a common winged ancestor. What they share is the task — generating lift — and similar physics pushed similar shapes out of very different starting material. That is convergent evolution, and the structures it produces are analogous.

• (a)common origin and common function — Common origin plus common function describes a homologous pair that has kept its ancestral job — the forelimbs of a horse and a dog, for instance. Bird and bat wings do share the function, but not the developmental origin.
• (c)common origin and different function — This is the definition of homologous structures in their classic textbook form: the human hand, the whale's flipper and the bat's wing, all the same bones of the tetrapod forelimb put to different uses. It is the exact opposite of what analogous means.
• (d)different origin and different function — Different on both counts makes the two structures simply unrelated, with nothing to compare. Bird and bat wings plainly do share a function, so this cannot be right.

Comparative anatomy sorts similar structures into two kinds. Homologous structures share a common ancestral origin and the same underlying plan even when the uses have diverged — they are evidence of divergent evolution from a shared ancestor. Analogous structures share only a function, having been built independently from different material — they are evidence of convergent evolution under similar selection pressure. The bat is the standard example of both at once: its wing is homologous with the human arm and analogous with the bird's wing.

The safest way through this item is to strip the comparison to its two axes and fill them separately. Origin: feathers on a shortened hand against a skin membrane on four long fingers — different. Function: flight in both — same. That gives option B without needing to remember which word means which. If you would rather work from the terms, the memory hook is that homo means same, and what is the same in a homologous pair is the ancestry, not the job. A third example rounds the idea off: the wing of an insect shares the function too but is not even made of limb bone, being an outgrowth of the body wall — analogy stretched further still.

• Analogous structures have different evolutionary origins but the same function, and are produced by convergent evolution.
• Homologous structures have the same evolutionary origin and basic plan but often different functions, and are produced by divergent evolution.
• A bird's wing carries feathers on a reduced hand; a bat's wing is a skin membrane stretched over four greatly elongated fingers.
• The bat's wing is homologous with the human arm and the whale's flipper — the same forelimb bones — while being analogous with the bird's wing.
• Other standard analogous pairs are the eye of an octopus and of a vertebrate, and the streamlined body of a shark, an ichthyosaur and a dolphin.

• Swapping the two definitions — homology is sameness of ancestry, not of job.
• Forgetting that one organ can be homologous with one structure and analogous with another; the bat's wing is both.
• Treating similarity of appearance as proof of relatedness when convergence can produce it without any shared ancestry.

As a definition item on analogous against homologous, or as a pair-identification item asking which of four given pairs of organs is analogous.

No directly related past PYQ was found.

• practice — not a real PYQ

  The forelimbs of a human being, a whale and a bat are best described as

  • (a)analogous organs
  • (b)homologous organs
  • (c)vestigial organs
  • (d)unrelated organs

  Answer(b) homologous organs — the same set of forelimb bones inherited from a common ancestor, put to grasping, swimming and flying respectively.

• practice — not a real PYQ

  Which one of the following pairs of plant structures is analogous?

  • (a)Thorn of Bougainvillea and tendril of Cucurbita
  • (b)Potato tuber and sweet potato
  • (c)Leaf of a pea and leaf of a mango
  • (d)Root of a banyan and root of a carrot

  Answer(b) Potato tuber and sweet potato — both store food, but the potato is a modified stem and the sweet potato a modified root, so the function is shared and the origin is not.