1. Evolution of Populations
Biology Chapter 17
Evolution of Populations

2. I. Genes and Variation [17.1]
A. Upon what does natural selection act?
Natural Selection acts upon an entire organism
B. Organisms with higher fitness –
Leave more offspring
C. Vocabulary Check
1. GENE POOL – All genes (including all alleles) present in a population
2. ALLELE FREQUENCY – # times an allele appears in gene pool compared to (divide) total # of alleles

3. Genes and Variation [17.1]
D. Evolution in “Genetic Terms”:
Change in frequency of alleles in a population over time
E. Sources of Genetic Variation
1. MUTATION –
changes in DNA
2. GENETIC RECOMBINATION IN SEXUAL REPRODUCTION –
Crossing over and joining haploid gametes
3. LATERAL GENE TRANSFER –
often seen in bacteria (think transformation)

4. II. Evolution as Genetic Change in Populations [17.2]
A. How might pesticide resistant insects come about?
B. Natural Selection on Single-Gene Traits
1. How does natural selection affect single-gene traits? Natural Selection acts on individuals, causes changes in allele frequency, phenotype frequency

5. II. Evolution as Genetic Change in Populations [17.2]
C. Natural Selection on Polygenic Traits
1. How does natural selection affect polygenic traits?
N. S. affects relative fitness of phenotypes – causes selection in one of 3 types

6. II. Evolution as Genetic Change in Populations
C. Natural Selection on Polygenic Traits
2. DIRECTIONAL SELECTION –
Selection where there is a shift toward a particular (different) phenotype
a. Movement of distribution away from original
(1 direction)
3. STABILIZING SELECTION – Selection where individuals near center of distribution curve have highest fitness
a. Distribution curve becomes tighter
4. DISRUPTIVE SELECTION – Selection where the pressure pushes the distribution away (2 directions)from original distribution
a. Creates 2 distinct phenotypes

7. II. Evolution as Genetic Change in Populations
D. GENETIC DRIFT -
 Change in allele frequency due to certain chance occurrences (random changes)
1. BOTTLENECK EFFECT –
 Change in allele frequency due to dramatic reduction in population
a. Example:

8. II. Evolution as Genetic Change in Populations
D. GENETIC DRIFT -   
2. FOUNDER EFFECT –
 Change in allele frequency due to a few individuals colonizing a new habitat
a. Example:

9. II. Evolution as Genetic Change in Populations
E. HARDY – WEINBERG PRINCIPLE –
 Allele frequencies in populations remain constant (no evolution)
unless one or more of the listed factors change
1. Conditions which can disturb genetic equilibrium –
IF CONDITION IS ALTERED, EQUILIBRIUM WILL CHANGE
a. Population must be large
b. No mutations
c. Must be random mating
d. No immigration/emigration
e. No natural selection

10. III. Speciation [17.3]
A. SPECIES –
Population of individuals who can interbreed and produce fertile offspring
B. SPECIATION –
Formation of new species
(dependent on genetic change and isolation)

11. III. Speciation [17.3] C. REPRODUCTIVE ISOLATION –
Situation where a population is split and the groups can no longer interbreed
1. Mechanisms of Isolation
a. BEHAVIORAL ISOLATION –
Changes in courtship or other behaviors which cause two groups to become separate
b. GEOGRAPHIC ISOLATION –
Populations become separated by an impassable barrier
c. TEMPORAL ISOLATION –
Situation where populations are separated by differences in breeding times

12. IV. Molecular Evolution [17.4]
A. MOLECULAR CLOCK – 
Use of known rates of mutation (in DNA, RNA, proteins) to develop an index of speciation (species separation)
1. What is the basis for the time index called the “clock”?
Mutation rates (in DNA) can be determined –
more differences in sequences mean more distant common ancestors
fewer differences mean more recent common ancestor a.
Mutations which happen prior to speciation don’t show as differences;
Only those mutations which occur after speciation show as differences