1. How Populations Evolve
Chapter 13
Aultman Winter 2015

2. Populations Change Over Time
a group of individuals of the same species, living in the same place at the same time
the smallest biological unit that can evolve
Individual variation within a population
Genotype  Phenotype
Monogenic, polygenic traits
Sources of variation
Mutation
Sexual recombination

3. Selection Pressure Selection Pressure
aka Evolutionary pressure
Forces external to an individual limit its ability (propensity) to survive and produce viable, fertile offspring
Examples: drought/flood; predation; disease; antibiotics
Intensity of selective pressure can vary
Severity
Timing in individual’s or population’s life cycle
Affects rate of evolutionary adaptation
Evolutionary time scales
Denominated in generations
For humans, millennia; for bacteria, days or weeks

4. Natural Selection Darwin’s theory
Overproduction and competition among
individuals that vary in their phenotypes/genotypes
leads to unequal reproductive success.
Fitness: the ability to both survive and reproduce
Relative fitness: a measure of the proportional contribution of an individual to the next generation.
In successive generations the proportion of individuals with a particular trait changes

5. Natural Selection in Action
Evolutionary adaptation
Results from chance variation and sorting among the variants
Creates a shift in the average phenotypes or genotypes of a population
Outcomes of natural selection
Directional selection: shifts the overall makeup of a population by selecting in favor of one extreme phenotype.
Disruptive selection: lead to a balance between two or more contrasting phenotypic forms in a population.
Stabilizing selection favors intermediate phenotypes, occurs in relatively stable environments, and is the most common

6. Evolutionary Trees Common ancestors form the trunk
Each fork is the last common ancestor to all the branches extending from that fork.
Tips of millions of twigs represent the species living today.

7. Evolutionary Genetics
Populations are units of evolution
a group of individuals of the same species, living in the same place at the same time
the smallest biological unit that can evolve
The total collection of alleles in a population at any one time is the gene pool
Contrast with genome: the set of all genetic information in an individual
If differences between alleles of a given gene affect Darwinian fitness, then the frequencies of the alleles will change across generations; the alleles with higher fitness become more common

8. Analyzing Gene Pools
Alleles in a gene pool occur in certain frequencies.
Alleles can be symbolized by
p for the relative frequency of the dominant allele in the population,
q for the frequency of the recessive allele in the population, and
p + q = 1.
If we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.
Genotype frequencies can be calculated from allele frequencies (if the gene pool is stable = not evolving).
The Hardy-Weinberg formula
used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles
p2 + 2pq + q2 = 1

9. Population Genetics Individual variation abounds in all species.
Not all variation in a population is heritable
Phenotypes may arise from single (monogenic) or multiple (polygenic) genes
Bottlenecks: the reduction in number of variant alleles that occurs when the number of individuals in the population decreases
Crop pest populations expand and contract each year
An opportunity for extreme selection
Founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population; specific type of bottleneck
Genetic Drift: change in gene pool due to random mutations

10. Key Terms evolution evolutionary adaptation evolutionary tree
bottleneck effect
directional selection
founder effect
generation time
disruptive selection
fossils
gene flow
genetic drift
population
natural selection
gene pool
microevolution
selection pressure
Hardy-Weinberg equilibrium
Polygenic
Relative fitness
Sexual selection
Sexual dimorphism
Stabilizing selection
Vestigial structure

11. Malaria & Sickle Cell Trait
Population
Generation time
Selection pressure
Gene
Allele(s)
Allele frequency
Outcome
Figure 13.31, page 265

12. Endangered Species Figure 13.25, p 261
Species whose numbers are declining or dangerously low
What is the change in population variation?
What is the external stress?
What is the danger to the population?

13. Nosocomial Infection Population Generation time Selection pressure
An infection acquired by a patient during a hospital visit or one developing among hospital staff
1.7 million hospital-associated infections; 99,000 deaths each year
Antibiotic resistance is common
Example: methicillin resistant Staphylococcus aureus. Has a gene for β-lactamase, which breaks down the drug
Population
Generation time
Selection pressure
Gene
Allele(s)
Allele frequency
Outcome

14. Insecticide Resistance: Mosquitoes & Crop Pests
Mosquitoes that transmit malaria can be controlled with insecticides like DDT and pyrethrum, which bind to the sodium channel proteins at the neuromuscular junction. The insecticides paralyze (Knock down) the mosquitoes. Resistant (KDR) mosquitoes have a mutant sodium channel protein which doesn’t bind the insecticide. It is a recessive gene. The frequency of the kdr gene is increasing in African malaria vectors but this process can sometimes be reversed when the insecticide is withdrawn. The kdr trait is also found in pod borers, where it appeared more rapidly and has persisted.
Population
Generation time
Selection pressure
Gene
Allele(s)
Allele frequency
Outcome
Figure 13.15, page 253