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Name: ______________________________________ Date: ___________________ Period: _______________
Types of Natural Selection
DIRECTIONS: Which type of selection does each of the following scenarios describe? Draw and label
the graph (X and Y axes) that would represent the type of selection. Draw the original curve and
then effect of natural selection on the population (use 2 different colors for original and after
natural selection).
1. In a population of wooly-beasts, individuals with long snouts tend to survive better on an island
that has burrowing termites. Over time, the wooly-beast population consists of individuals with
extremely long snouts.
2. Birds with average beak size had a higher survival rates than birds with smaller or larger beaks.
3. Early breeders were interested in dog with the greatest speed. They carefully selected from a
group of hounds those who ran the fastest (a.k.a. artificial selection). From their offspring, the
greyhound breeders again selected those dogs that ran the fastest. By continuing this selection for
those dogs that ran faster than most of the hound dog population, they gradually produced a dog
that could run up to 40mph.
4. Light colored moths were favored before the pollution of the Industrial Revolution. Soot
darkened the bark of trees and favored colored moths. Soon dark colored moths were more fit to
the new polluted forest.
5. Look at the Siberian Husky, a dog bred for working in the snow. The Siberian Husky is a medium
dog, males weighing 35-60lbs. These dogs have strong pectoral and leg muscles, allowing it to
move through dense snow. The Siberian Husky is well designed for working in the snow. If the
Siberian Husky had heavier muscles, it would sink deeper into the snow, so they would move
slower or would sink and get stuck in the snow. Yet if the Siberian Husky had lighter muscles, it
would not be strong enough to pull sleds and equipment, so the dog would have little value as a
working dog.
6. A species of pocket mice were moved into an area that had dark rocks (from lava flow) and lightcolored sandy areas. There were mice with dark, medium, and light fur. The mice with dark fur
could hide from the predators when on the dark rocks, and the mice with the light fur would be
able to hide from predators when on the light-colored sandy areas. The medium colored mice
were easily seen in both environments and thus could not survive.
7. A population of arboreal rodents lives in one species of pine tree. These pine trees have very
evenly spaced branches of uniform thickness. Rodents that are too small cannot reach between
the branches to move around in the tree. Rodents that are too big and heavy break through the
branches and fall out of the trees. The rodent population contains adults of very uniform size.
DIRECTIONS (8-11): Answer the question following each scenario.
8. A group of early giraffes lives in a lush forest with many trees and shrubs of different heights to
feed on. These giraffes have necks of various lengths, with most giraffes having a medium length
neck. Then a blight passed through the area, killing off all of the low growing plants, leaving only
tall trees whose leaves are far off the ground. Giraffes with shorter necks now have a hard time
getting food, while those with longer necks can feed on the high-growing leaves. After several
generations, the giraffes in this area all have longer necks than those before the blight. What type
of selection has occurred in this giraffe population?
9. A population of finches lives on an island with an abundant food supply. These finches have highly
variable beak lengths, allowing them to feed on different types of food. Then several other species
of birds move into the area, making food supplies scarce. The only types of food left for the finches
are small seeds and nectar from long tubular flowers. Finches with small beaks can easily grasp
and crack the seeds. Finches with long beaks can reach inside the long flowers to reach the nectar.
However, some finches have beaks that are too large to deal with the small seeds, but too small to
reach inside the flowers. After several generations, there are two populations of finches: those
with short beaks and those with long beaks. What kind of selection has acted on these finches?
10. A population of arboreal rodents lives in one species of pine tree. These pine trees have very
evenly spaced branches of uniform thickness. Rodents that are too small cannot reach between
the branches to move around in the tree. Rodents that are too big and heavy break through the
branches and fall out of the trees. The rodent population contains adults of very uniform size.
11. What type of distribution would you expect for a trait on which there is no selection pressure? On
which there is stabilizing selection pressure? Disruptive selection pressure? Directional selection
pressure?
12. Create your own scenario, using an animal or plant you are familiar with, and explain what type of
selection it is. You must tell me what the selection pressure would be and why.