IGCSE Biology 0610: Classification Systems and Evolutionary Relationships

A humpback whale lives in the ocean, has a streamlined body shape and swims using a tail fin, features it shares with many species of fish. Early classification systems grouped whales together with fish because of these shared external features.

(a) State one feature of a humpback whale, other than its body shape or swimming ability, that instead supports classifying it in the same group as mammals such as dogs and humans. [1]

(b) Explain why grouping the whale with fish, based only on its body shape and swimming ability, can produce a misleading classification, and state what modern classification systems aim to reflect instead. [2]

(c) State the type of evidence, beyond visible physical features, that modern biologists can compare between a whale and other mammals to confirm how closely related they are. [1]

Part (a): A genuine mammalian feature

Setting aside body shape and swimming ability, a humpback whale shares several features with other mammals: it feeds its young on milk produced by the mother, it breathes air using lungs rather than extracting oxygen from water using gills, and it gives birth to live young rather than laying eggs. Any one of these is a feature that places the whale with mammals such as dogs and humans, not with fish.

Part (b): Why body shape alone can mislead, and what classification aims to reflect

A streamlined body and the ability to swim using a tail fin are useful adaptations for moving through water, and they can evolve independently in different groups of organisms that come to live in a similar watery environment, even if those groups are not closely related. This means that grouping organisms by body shape and swimming ability alone can produce a misleading classification, placing together organisms, like the whale and fish, that resemble each other because of a shared environment rather than because of a shared recent ancestor.

Modern classification systems instead aim to reflect evolutionary relationships: organisms are grouped together according to how recently they share a common ancestor, so that more closely related organisms are placed closer together in the classification, regardless of superficial resemblances caused by adapting to similar surroundings.

Part (c): Confirming relatedness using DNA

Beyond visible physical features, biologists can compare the sequences of bases in DNA between a whale and other mammals. Organisms that share a more recent common ancestor have DNA base sequences that are more similar to one another, so this comparison provides evidence of how closely related the whale is to other mammals, supporting its classification with mammals rather than with fish.

Final answers

(a) Any genuine mammalian feature, e.g. feeding young on milk.
(b) Body shape can evolve independently through adapting to a similar environment, so it does not always show true relatedness; classification systems instead aim to reflect evolutionary relationships (recency of common ancestor).
(c) Comparing the sequences of bases in DNA.