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. Natural Selection on Single -Gene Traits
Is the following sentence true or false?
Natural selection on single-gene traits cannot lead to changes in allele frequencies.
FALSE

3. Evolution as Genetic Change
Answer on Whiteboard: If a trait made an organism less likely to survive and reproduce, what would happen to the allele for that trait?
Fewer copies of the allele would pass to future generations, and the allele could even disappear from the gene pool completely.

4. 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

5. 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

6. 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

7. 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

8. Natural Selection on Polygenic Traits
On Boards: List the three ways that natural selection can affect the distributions of phenotypes.
Directional selection
Disruptive selection
Stabilizing selection

9. 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

10. 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

11. 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

12. Evolution as Genetic Change
On whiteboard: An increase in the average size of beaks in Galápagos finches is an example of ____ selection.
directional

13. Natural Selection FALSE
Is the following sentence true or false?
The weight of human infants at birth is under the influence of disruptive selection.
FALSE
Stabilizing!

14. Evolution as Genetic Change
What is genetic drift?
Random changes in allele frequencies

15. Copyright Pearson Prentice Hall
Genetic Drift
Genetic drift may occur when a small group of individuals colonizes a new habitat.
Individuals may carry alleles in different relative frequencies than did the larger population from which they came.
Copyright Pearson Prentice Hall

16. Evolution as Genetic Change
Why is large population size important in maintaining genetic equilibrium?
Genetic drift has less effect on large populations.

17. Evolution Versus Genetic Equilibrium
What does the Hardy-Weinberg principle state?
Allele frequencies in populations will remain constant unless one or more factors cause them to change.

18. Evolution as Genetic Change
The situation in which allele frequencies remain constant is called ______.
genetic equilibrium

19. Evolution Versus Genetic Equilibrium
The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.
When allele frequencies remain constant it is called genetic equilibrium.
Copyright Pearson Prentice Hall

20. Evolution Versus Genetic Equilibrium
Five conditions are required to maintain genetic equilibrium from generation to generation:
there must be random mating,
the population must be very large,
there can be no movement into or out of the population,
there can be no mutations, and
there can be no natural selection.
Copyright Pearson Prentice Hall

21. Evolution Versus Genetic Equilibrium
Random Mating
Random mating ensures that each individual has an equal chance of passing on its alleles to offspring.
In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits.
Copyright Pearson Prentice Hall

22. Evolution Versus Genetic Equilibrium
Large Population
Genetic drift has less effect on large populations than on small ones.
Allele frequencies of large populations are less likely to be changed through the process of genetic drift.
Copyright Pearson Prentice Hall

23. Evolution Versus Genetic Equilibrium
No Movement Into or Out of the Population 
Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population.
The population's gene pool must be kept together and kept separate from the gene pools of other populations.
Copyright Pearson Prentice Hall

24. Evolution Versus Genetic Equilibrium
No Mutations
If genes mutate, new alleles may be introduced into the population, and allele frequencies will change.
Copyright Pearson Prentice Hall

25. Evolution Versus Genetic Equilibrium
No Natural Selection
All genotypes in the population must have equal probabilities of survival and reproduction.
No phenotype can have a selective advantage over another.
There can be no natural selection operating on the population.
Copyright Pearson Prentice Hall