1. Natural Selection : Fitness = individual’s ability to…
a) survive long enough to reproduce
b) find a mate
c) produce live offspring
Natural Selection = survival of the fittest

2. For Natural Selection to work:
1) more offspring born than can live in area
2) genetic variation (not clones)
3) some alleles better than others

3. Artificial Selection
Humans select which individuals survive or reproduce

4. Sexual Selection Selection for traits that help get mates
Physical Traits
Behavioral Traits

5. Bower Birds

6. The Effects of Natural Selection
Natural Selection is a process
It changes the gene pool of a population
It results in a change in allele frequency

7. Alleles that improve fitness become more common
Alleles that decrease fitness become less common,…. may be eliminated from population

8. Some traits only have 2 alleles
Example:
Peppered Moth

9. Natural Selection of monogenic Traits
Can be shown by a line graph
Y-axis =
# of individuals
X-axis =
time

10. Natural Selection of Polygenic Traits
Shown by before and after histograms
Traits show a range of phenotypes

11. 3 Ways Natural Selection Can Go
1) It can select FOR one extreme
2) It can select For being average
3) It can select AGAINST being average

12. Directional Selection

13. 1. Directional Selection
Select for one extreme = directional
Graph moves in one direction over time

14. Directional Selection example
1976 All Birds
N = 751
Figure 1: Histogram of distribution of beak depth of medium ground finches (Geospiza fortis) on Daphne Major, before and after the drought of 1977 (Grant 1986). Reprinted by permission of Princeton University Press.
1978 Survivors
N = 90

16. 2. Stabilizing Selection
Selects for being average
Graph gets
narrower

17. Stabilizing selection example
Too few spines = eaten by Pecarries
Too many = more parasites that lay eggs at the base of spines

19. 3) Disruptive Selection
Selects for BOTH extremes
Graph gets low in center