Question

S. M. Willston has formulated a principle according to which

• A. Organisms have a tendency towards increase in size during their evolution.
• B. During evolution of a lineage, serially homologous parts tend to reduce in number but get more and more differentiated.
• C. During development various parts of the body of an organism exhibit differential rate.
• D. None of the above.

Answer 1

Answer: (B)

During evolution of a lineage, serially homologous parts tend to reduce in number but get more and more differentiated.

According to Cope’s rule, evolution tends to increase the body size in a lineage of the population over the geological time. For example, evolution of horse includes increase in height that transformed Eohippus (the Dawn horse of Eocene epoch; 28 cm) into Mesohippus (the Intermediate horse of Oligocene; 60 cm), Merychippus (the Ruminant horse of Miocene; 100 cm), Pliohippus (the Pliocene horse, 108 cm) and finally into Equus; 150 cm high the recent horse. This makes option A incorrect.

According to Williston’s law, evolution tends to reduce the number of serially homologous parts as well as make them highly differentiated and functionally advanced structures. For example, trilobites had three-lobed bodies and undifferentiated appendages. During the evolution of arthropods, body segments tended to fuse causing the reduction in their number and appendages became more specialized for special functions. This makes option B correct.

The tendency of living organisms to exhibit differential growth rates in various body parts is known as allometry; a concept first proposed by Otto Snell. This makes option C incorrect.

Therefore, the correct answer is option B.