Natural Selection & Horse Evolution
In this activity you will examine the evolutionary trends of Equus, or horse. Equus is the only surviving
genus in a once diverse family of horses. Over time, the habitat of the horse gradually changed, shifting
from forest to plains. As the habitat changed so did the available food and shelter. Some of the horses
had variations that allowed them to adapt to these changes in the environment and survive. An adaptation
is an inherited characteristic that increases an organism’s chance of survival. To adapt, the genetic
variation must already exist. Adapting is not a matter of choice but of chance that an organism has a
beneficial genetic variation that will allow them to survive in changing environments.
Horses originally inhabited a woodland environment. It was advantageous to be small so they could hide
and slip through the foliage easily. As the environment changed and the forests became thinner,
eventually disappearing in many areas, horses that were larger were better able to get away and defend
themselves.
1. Describe the effect that the change in the environment had on the horse ancestor. ______________
________________________________________________________________________________
2. As the shift from forest to plains continued, what variation allowed some horses to survive better than
others? _________________________________________________________________________
3. Study the diagrams and names of the four horses shown below. Rank the horses from most ancient (1)
to most recent (4).
a) _____ Merychippus
b) ____ Hyracotherium
c) ____ Equus
d) ____ Mesohippus
Over many generations, as the woodlands continued to disappear, horses changed: Their new
environment contained grasslands. The grasses were much tougher and harder to chew than the soft
forest leaves. Horses with a genetic variation of larger flatter molars could chew the tough grasses much
easier.
4. What difficulty in survival did the advent of grasses have on the horses? ______________________
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5. As the horses gradually adapted to become grazing animals, what happened to their teeth? _______
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6. Study the diagrams below that show the changes in the grinding surface of the horses molar teeth.
Place the name of the correct genus under the appropriate tooth. Use the genus names from the
figures in Part 1 above. (Merychippus, Hyracotherium, Equus, and Mesohippus)
a) _________________
b) _________________
c) _________________
d) _________________
Shelter and escape from predators presented a new problem as the forest dwindled. Some horses had
longer stronger legs and could run faster on the plains. The result was a loss of toes with a single toe
becoming longer and sturdier, eventually developing a hoof from the toenail.
7. Describe the problem of shelter and escape that the horses faced with the environmental change. __
________________________________________________________________________________
8. Describe how a horse with a fewer number of toes, one becoming longer and sturdier (eventually
becoming with a single hoofed toe) was an advantage on the plains. _________________________
________________________________________________________________________________
9. Study the diagrams below that show the feet and legbones of the horses. Write the genus name of the
horse in the appropriate space.
a) _________________
b) _________________
c) _________________
d) _________________
10. Describe the influence of the environmental change to grassland on horse evolution. _____________
________________________________________________________________________________
________________________________________________________________________________
APPLY THE FIVE CONCEPTS OF DARWIN’S THEORY TO THE EVOLUTION OF THE HORSE:
1. ________________________________________________________________________________
________________________________________________________________________________
2. ________________________________________________________________________________
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3. ________________________________________________________________________________
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4. ________________________________________________________________________________
________________________________________________________________________________
5. ________________________________________________________________________________
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