1. 16-2 Evolution as Genetic Change
Outline 16-2: Evolution as Genetic Change
16-2 Evolution as Genetic Change
Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises
Copyright Pearson Prentice Hall

2. 16-2 Evolution as Genetic Change
I. Variation & Gene Pools
A. A gene pool consists of all the genes, of all possible alleles, that are present in a population of a species.
1. The relative frequency of an allele is the relative proportion of an allele compared to the total number of all alleles.
a. It is often expressed as a percentage.
B. In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Copyright Pearson Prentice Hall

3. 16-2 Evolution as Genetic Change
II. Natural Selection & Gene Pools
A. Evolution never acts directly on genes. Why?
1. It is entire organisms that either live or die
a. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool.
b. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.
2. Thus, only populations can evolve, not individuals
Copyright Pearson Prentice Hall

4. Natural Selection on Single-Gene Traits
III. Examples of Natural Selection at Work
A. A lizard population:
1. They are normally brown but have mutations that produce red and black forms.
2. Suppose red lizards are more visible to predators.
a. Less likely to survive & reproduce so allele for red color will decrease in population
3. Black lizards may warm up faster on cold days. This may give them energy to avoid predators.
a. More likely to survive & reproduce so allele for black color will increase in population
Copyright Pearson Prentice Hall

5. Natural Selection on Single-Gene Traits
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors.
Copyright Pearson Prentice Hall

6. Natural Selection on Single-Gene Traits
B. Results of Natural Selection over Time
1. Natural selection can affect the distribution of phenotypes in one of three ways:
a. Directional Selection
When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.
Birds with larger beaks survive better & beak size increases over time
Copyright Pearson Prentice Hall

7. Natural Selection on Polygenic Traits
b. Stabilizing Selection 
When individuals near the center of the curve have higher fitness than individuals at either end of the curve.
Human babies born of an average mass are more likely to survive than either smaller or larger babies
Copyright Pearson Prentice Hall

8. Natural Selection on Polygenic Traits
c. Disruptive Selection 
When individuals at both ends of the curve have higher fitness than individuals near the middle.
If the pressure of natural selection is strong enough and long enough, the curve will split, creating two distinct phenotypes.
If average seeds become scarce, the population might split into two groups: one of small beaks & one of large beaks
Copyright Pearson Prentice Hall

9. Copyright Pearson Prentice Hall
16-2
Copyright Pearson Prentice Hall

10. Copyright Pearson Prentice Hall
16-2 1.
Which of the following patterns of natural selection on polygenic traits favors both extremes of a bell curve?
stabilizing selection
disruptive selection
directional selection
genetic drift
Copyright Pearson Prentice Hall

11. Copyright Pearson Prentice Hall
16-2
Which of the following patterns of natural selection on polygenic traits favors one end of the bell shaped curve instead of the middle or other end?
stabilizing selection
disruptive selection
directional selection
genetic drift 2.
Copyright Pearson Prentice Hall

12. Copyright Pearson Prentice Hall
16-2 3.
Which of the following patterns of natural selection on polygenic traits favors the middle of the bell shaped curve rather than either end?
stabilizing selection
disruptive selection
directional selection
genetic drift
Copyright Pearson Prentice Hall