Biology · Rung 4 of 6

Classifying life

Classifying is not grouping by resemblance but by descent, and those two part company often. A shark and a dolphin share a shape because swimming fast has only one good solution, not because they are relatives: that is called convergence. A whale is a mammal and a penguin is a bird, though one lives in water and the other does not fly. The tree is built from what cannot be copied: DNA and skeletal detail. In a tree, the higher a fork the older it is, and the common ancestor of two species is the lowest fork covering both; pointing at the topmost one is the classic mistake, because that is everybody's ancestor. And the ranks nest like Russian dolls: species, genus, family, order, class, kingdom and domain.

Where this sits

Comes after
The cell

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What is a whale?

    • a fish
    • a plant
    • a reptile
    • a mammal

    Why It is a mammal, because it has hair, lungs and milk; living in water does not make it a fish. Classifying is not grouping by resemblance but by descent, and those two part company often: living in water, flying or being torpedo-shaped are trades, and several unrelated groups invented them separately.

    easier

  2. What is a penguin?

    • a bird
    • a plant
    • a fish
    • a reptile

    Why It is a bird, because it has feathers and lays hard-shelled eggs, even though it does not fly. Classifying is not grouping by resemblance but by descent, and those two part company often: living in water, flying or being torpedo-shaped are trades, and several unrelated groups invented them separately.

    in between

  3. A shark and a dolphin have the same shape and are not relatives. Why do they look alike?

    • Because they copy each other while living together
    • Because they descend from an ancestor with that shape
    • Because water changes their shape over time
    • Because they solved the same problem separately

    Why It is called convergence: swimming fast in water has only one good solution, and shark and dolphin found it down separate paths a hundred million years apart. That is why resemblance is the worst guide for classifying, and the tree is built from what cannot be copied: DNA and skeletal detail.

    harder