How Populations Evolve

Chapter 13Aultman Winter 2015

Populations Change Over Time

• Population: • 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

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

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

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

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.

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

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

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

Key Termsevolution evolutionary adaptation evolutionary tree

bottleneck effect directional selection founder effect

generation time disruptive selection fossils

founder effect 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

Malaria & Sickle Cell Trait

• Population• Generation time• Selection pressure• Gene• Allele(s)• Allele frequency• Outcome• Figure 13.31, page 265

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?

Nosocomial Infection

• Population• Generation time• Selection pressure• Gene• Allele(s)• Allele frequency• Outcome

• 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• Figure 13.15, page 253

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.