Name_____________________
Pd_____
Date_______________
Biologists identify 3 basic patterns in which natural selection occurs.
 Directional selection occurs when environmental factors cause a trait to become less common
in favor of a different trait.
 Diversifying selection (also called disruptive selection) occurs when environmental factors
select for individuals with extreme traits but not for those with moderate traits.
 Stabilizing selection occurs when environmental factors select for a moderate trait, with little
diversity within the population.
Examine the diagrams above to answer the following questions:
1. What is being represented on the top graph? Describe the original population of mice. ________
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2. What happened to the mouse population in graph (a)? _________________________________
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3. What happened to the mouse population in graph (b)? _________________________________
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4. What happened to the mouse population in graph (c )?_________________________________
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5. Is this an example of directional selection, diversifying selection, or stabilizing selection?
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6. How can you tell? _______________________________________________________________
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7. What environmental factors are driving selection among these birds?_____________________
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8. Is this an example of directional selection, diversifying selection, or stabilizing selection? ______
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9. How can you tell?________________________________________________________________
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10. What environmental factors are driving selection among the gall-flies?_____________________
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11. Is this an example of directional selection, diversifying selection, or stabilizing selection? ______
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12. How can you tell?________________________________________________________________
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13. What environmental factors are driving selection among these birds?______________________
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Human babies usually weigh about 6-10 pounds at birth. Babies that are smaller than 6 pounds may be
underdeveloped and unable to survive out of the womb. Babies that are bigger than 10 pounds may
have difficulty exiting the birth canal, which can lead to complications during the birthing process,
possibly even death.
14. Is this an example of directional selection, diversifying selection, or stabilizing selection? ______
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15. How can you tell?________________________________________________________________
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16. What environmental factors are driving selection among these babies?_____________________
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17. Draw a graph in the space below that shows how size at birth is distributed across the human
population. You do not need to include numbers on your Y-axis, but you do need to label it.
There is a population of beautiful cacti that is found on a desert highway in Arizona. Humans that drive
by will often dig up a cactus and bring it home as a houseplant. People tend to skip over the cacti with
too many spines. They also tend to skip over the cacti with too few spines because they look unhealthy
and produce fewer flowers.
18. Is this an example of directional selection, diversifying selection, or stabilizing selection? ______
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19. How can you tell?________________________________________________________________
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20. What environmental factors are driving selection among these cacti?______________________
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21. Draw a graph in the space below that shows how number of spines is distributed across the cacti
population. You do not need to include numbers on your Y-axis, but you do need to label it.