Chapter 19, Problem 14STP

Summary Introduction

To analyze : 

The tempo of the evolution of horses to be gradual or punctuated.

Introduction :

According to the theory of gradualism, the evolution proceeds slowly, consistently, constantly, and gradually. The variations acquired over a short period are hard to notice. Those traits that help an organism to thrive and increase the fitness of organisms are selected. The organisms that are unable to acquire changes die. It leads to the change in population gradually, over a long period. 

Whereas, the theory of punctuated equilibrium suggests that speciation or divergence is caused due to sudden abrupt genetic changes. The variations come in spurts and result in a huge alteration in genetics. Mutations often lead to such sudden changes. The species of a population changes rapidly over a few generations.

Explanation of Solution

The given figure indicates that the evolution of horses supports the theory of gradualism. The evolution of the modern horse began about 60 million years ago in the Eocene epoch. The first fossil is named  Eohippus (Hyracotherium). Gradually some of the evolutionary trends took place over a period such as an increase in size, elongation of neck and head, lengthening of the limbs, increase in the size and complexity of the brain, development of sense organs, etc. 

Orohippus  was a little higher than  Eohippus. Miohippus  is evolved in the Oligocene epoch and is considered as forest dwellers.  Megahippus  descended from  Miohippus  in the early Miocene epoch.  Merychippu  is known as a ruminant horse and evolved in the Miocene epoch around 25 mya.  Pliohippus  is known as Pliocene horse and it is evolved in the Pliocene epoch about 10 mya.  Eqqus  is the modern-day horse and it is evolved in the Pleistocene epoch about 1 mya. 

Conclusion

Thus, we can conclude that the modern-day horse has evolved from the earliest known fossil named  Hyracotherium  by acquiring slow and gradual changes that took place over a long period. These changes are consistent and resulted in evolution slowly and gradually.