Ch 22 Population and Evolutionary Genetics

Question 1

Evolution

Answer 1

• Evolution: consequence of changes in genetic material through mutation and changes in allele frequencies in populations over time
• Union of population genetics with theory of natural selection generated new view of evolutionary process
  (neo-Darwinism).
• Speciation: formation of new species caused by mutation, migration, and drift

Question 2

Micro and Macroevolution

Answer 2

Microevolution
– Evolutionary changes within populations of species Macroevolution
–Evolutionary events leading to emergence of new species and other taxonomic groups

Question 3

Genetic Variation Is Present In Most Populations and Species

- populations

Answer 3

Population
–Group of individuals belonging to same species
–Live in same geographic area
–Actually or can potentially interbreed

Question 4

Population Gene Pool

Answer 4

• All alleles present in population
• Genetic information carried by members of population
• Most populations contain high degree of heterozygosity.

Question 5

Variations in Nucleotide Sequence

Answer 5

Most direct way to estimate genetic variation in a population
- Compare nucleotide sequences of genes carried by individuals in population.

Question 6

The Hardy–Weinberg Law Describes Allele Frequencies and Genotype Frequencies in Populations

Answer 6

Hardy–Weinberg law

1. Describes what happens to allele and genotype frequencies in “ideal” populations
2. “Ideal” population
   - There is an equal rate of survival and reproduction (no selection).
   - No new alleles arise or are created by mutation.
   - No migration into or out of population occurs.
   - Population is infinitely large.
   - Random mating occurs.

Question 7

Hardy–Weinberg Model

Answer 7

Uses Mendelian principles of segregation and simple probability to explain relationship between allele and genotype frequencies in population

ex: Single autosomal allele with two alleles: A, a
Frequency of A= 0.7 and a= 0.3. (Note: A+a= 1)
AA= (0.7) × (0.7) = 0.49 means the AA genotype will occur 49 percent of the time.
Aa= (0.7) × (0.3) (2) = 0.42 or 42 percent will be heterozygous.
aa= (0.3) × (0.3) = 0.09 or 9 percent will be recessive.

Question 8

Hardy-Weinberg Assumptions

Answer 8

• Allele frequencies in our population do not change from one generation to the next.
• After one generation of random mating, genotype frequencies for two alleles are calculated as:
  p2+2pq+ q2=1
  p equals frequency of allele A.
  q is frequency of allele a (Figure 22-4)

Question 9

Hardy–Weinberg law—additional consequences

Answer 9

– Dominant traits do not necessarily increase from one generation to next.
– By knowing the frequency of one genotype, the frequencies of other genotypes can be calculated.
- If given p, can solve for q (and vice-versa)

Question 10

The Hardy–Weinberg Law Can Be Applied to Human Populations

Answer 10

– Analysis of susceptibility to HIV-1 infection
– Based on CCR5 gene
Encodes protein CCR5—receptor for strains of HIV-1
Allele exists in population that has a deletion of portion of gene (Δ32)
– Homozygous individuals resistant to HIV-1 infection
– Heterozygotes susceptible to infection but progress more slowly to AIDS

Question 11

Testing for Hardy–Weinberg Equilibrium in a Population

Answer 11

Table 22.2: Two methods for computing frequencies of alleles in population surveyed for CCR5 genotypes
1) Counting alleles
2) From genotype frequencies
Genotype frequencies are predicted to fit p2+2pq+q2= 1 relationship.

Question 12

Hardy-Weinberg with Multiple Alleles

Answer 12

Frequencies for multiple alleles
–Calculated by adding additional variables to Hardy–Weinberg equation
–Example: situation involving three alleles
p+q+r= 1
Frequencies of genotypes given by
(p+q + r)2=p2+q2+r2+ 2pq+ 2pr+ 2qr=1

Ex: of genotype freq calculations for multiple alleles
–ABO blood type

Question 13

Natural Selection Is a Major Force Driving Allele Frequency Change

Answer 13

Natural selection
– Major force driving allele freq change
– Chief mechanism for transforming populations
– Principal force that shifts allele freq within large populations

Question 14

Wallace—Darwin concept of natural selection

Answer 14

1. Individuals exhibit variations in phenotype.
2. Variations are heritable (passed on).
3. Organisms tend to reproduce in exponential fashion.
   - More offspring are produced than can survive.
4. Some phenotypes are more successful at survival and reproduce at higher rates.

Question 15

Wallace-Darwin Natural Selection (cont.)

Answer 15

As a consequence of natural selection, populations and species change.
– Phenotypes that confer improved ability to survive and reproduce become more common.
– Phenotypes that confer poor prospects for survival and reproduction may disappear.

Question 16

Fitness and Selection

Answer 16

Hardy–Weinberg analysis allows fitness (w) to be examined for each genotype.
–Fitness: individual’s genetic contribution to future generations
–Homozygous recessive individual who dies before producing offspring: w = 0
Frequency of recessive allele will decrease in each generation. (Figure22-7)

Question 17

Types of Selection

Answer 17

Selection for traits classified as
–Directional selection
–Stabilizing selection
–Disruptive selection

Question 18

Directional Selection

Answer 18

• Phenotypes at one end of spectrum become selected for or against.
• Usually as a result of changes in environment
• Example: Beak size in finches during dry years increased due to strong selection.

Question 19

Stabilizing selection

Answer 19

– Intermediate types are favored.
– Both extreme phenotypes are selected against.
– Reduces population variance over time
– Example: human birth weight study over an 11-year period (Figure22-10)

Question 20

Disruptive Selection

Answer 20

Both phenotypic extremes are selected for.
–Intermediates are selected against.
–Results in population with increasingly bimodal distribution for trait
–Example: applied selection for low and high bristle number in Drosophila population

Question 21

Mutation Creates New Alleles in a Gene Pool

Answer 21

Mutation
–Within a population, the gene pool is reshuffled each generation.
–Only process that creates new alleles in gene pool
–Most mutations are recessive.
–Indirect methods using probability and statistics or large-scale screening programs are often employed to estimate mutation rates.

Question 22

Mutation rates

Answer 22

• Number of new mutant alleles per given number of gametes

- If mutation rate is known, the extent of change to allele frequency from one generation to next can be estimated.

Question 23

Migration and Gene Flow Can Alter Allele Frequencies

Answer 23

Migration
- Occurs when individuals move between populations
Species divides into populations that are separated geographically.
- Allele frequencies in new populations may differ over time. (Figure22-12)

Question 24

Genetic Drift Causes Random Changes in Allele Frequency in Small Populations
- genetic drift

Answer 24

–Significant random fluctuations in allele frequencies in small populations
–Possible by chance alone
–Degree of fluctuation increases as population size decreases.
–Can also occur as result of
1. Founder effect
2. Genetic bottleneck

Question 25

Founder effect

Answer 25

Genetic drift can also arise through founder effect.

• Occurs when population originates from small number of individuals
• Gene pool may not reflect larger population from which founders are drawn.

Question 26

Genetic bottleneck

Answer 26

– Genetic drift can also occur as a result of genetic bottleneck.
–Develops when large population undergoes drastic but temporary reduction in numbers
–Populations may recover, but with greatly reduced genetic diversity.

Question 27

Example of founder effect in human populations

Answer 27

–High freq of oculocutaneous albinism (OCA) in Navajo
OCA frequency
1/1500–1/2000 in Navajo
1/36,000 in whites
1/10,000 in African-Americans

Question 28

Nonrandom Mating

Answer 28

Nonrandom mating
–Can change frequencies of genotypes in population
–Subsequent selection for or against certain genotypes can affect overall allele frequencies.
–But nonrandom mating itself does not directly change allele frequencies.

Question 29

Forms of nonrandom mating

Answer 29

1. Positive assortive mating: Similar genotypes are more likely to mate than dissimilar ones.
2. Negative assortive mating: Dissimilar genotypes are more likely to mate than similar ones.
3. Inbreeding: Mating individuals are closely related.

Question 30

Interbreeding

Answer 30

• For a given allele, inbreeding increases the proportion of homozygotes in the population.
• Completely inbred population theoretically will consist only of homozygotes.

Question 31

Speciation Occurs Via Reproductive Isolation

Answer 31

Species
– Group of actually or potentially interbreeding organisms reproductively isolated in nature from all other such groups
–In sexually reproducing organisms, speciation typically divides single species into two or more separate species.

Question 32

Macroevolution

Answer 32

– Genetic changes that result in reproductive isolation between or among populations
– Leads to formation of new species

Question 33

Reproductive Isolation Mechanisms

Answer 33

–Biological barriers that prevent or reduce interbreeding between populations 
–Mechanisms may be
1. Ecological
2. Behavioral
3. Seasonal
4. Mechanical
5. Physiological

Question 34

Changes Leading to Speciation

Answer 34

Geographic changes can lead to speciation.
–Example: ancestors of snapping shrimp (Figure22-16)
–Prior to formation of Isthmus of Panama, members were of single species.
 When the isthmus closed, each of seven ancestral species was divided into two separate populations.

Question 35

The Rate of Macroevolution and Speciation

Answer 35

Average time for speciation is 100,000–10 million years

Question 36

Phylogeny Can Be Used to Analyze Evolutionary History

Answer 36

Phylogeny (evolutionary history)

• Genetic differences among present-day species can be used to reconstruct their evolutionary histories (phylogenies).
• Phylogenetic trees: Figure22-19
• Branches represent lineages over time.
• Node represents a speciation event.

Question 37

Constructing Phylogenetic Trees

Answer 37

Constructing species-level phylogenetic tree using DNA sequences
– Acquire DNA sequences representing genome of interest from numerous different species
– Align sequences: many computer programs available
– DNA differences used to construct phylogenetic tree

Question 38

Complex origins of our genome

Answer 38

– Genetic diversity in mtDNA used to infer where Homosapiens originated
–H. sapiens originated in Africa from earlier species of Homo.

Question 39

Neanderthals (Homoneanderthalensis)

Answer 39

–Lived in Europe & western Asia ~ 300,000 years ago
–Disappeared about 40,000 years ago–Coexisted with anatomically modern humans (Homosapiens) for about 30,000 years in several regions

Question 40

Phylogenetic tree shows pattern and times of divergence of Neanderthals and our species.

Answer 40

Conclusions from this study
–Neanderthals are not direct ancestors of our species.
–Neanderthals and members of our species interbred.
–Neanderthals contributed to our genome.