BIOLOGY 2E

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Chapter 19 THE EVOLUTION OF POPULATIONS

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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Mesa Shumacher/Santa Fe Institute

COLLEGE PHYSICS

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Chapter # Chapter Title

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POPULATION GENETICS

• Microevolution = evolutionary change in a population over time
• Population genetics is the study of what changes allele frequencies in populations through time
• Allele frequency = frequency of a single allele in the gene pool of a population
• The sum of all the alleles in a population
• Example: allele frequency for cystic fibrosis in Caucasians in the United States is 1.8%
• Population genetics became formally incorporated into the Theory of Evolution in the 1940s after the Modern Synthesis
• Evolutionary processes affect a population’s genetic makeup which results in the gradual change of populations and species

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HARDY-WEINBERG PRINCIPLE OF EQUILIBRIUM

• Allele frequencies can be used to determine the genotype frequencies
• Genotype frequencies of a population = its genetic structure
• The Hardy-Weinberg principle gives a mathematical baseline of a non-evolving population to which an evolving population can be compared
• If a population is in equilibrium, the gene pool and genetic structure will never change
• If allele or genotype frequencies deviate from the expected values, the population is evolving
• Can then infer what evolutionary forces might be present

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HARDY-WEINBERG PRINCIPLE OF EQUILIBRIUM

• A population’s allele and genotype frequencies are inherently stable unless some kind of evolutionary force is acting upon the population
• Assumptions:
1. No mutations
2. No migration or emigration (gene flow)
3. No natural selection
4. An infinite population size
5. Random mating
• Do real populations meet these assumptions?

HARDY-WEINBERG EQUATIONS

p + q = 1

Alleles must be either dominant or recessive. The dominant allele is p while the recessive allele is q. Since no other option is possible, the % of p alleles and % of q alleles must add up to 100%.

p² + 2pq + q² = 1

Individuals can have only one of three genotypes. Homozygous dominant individuals are p² . Heterozygotes have one dominant and one recessive allele and are therefore 2pq. Homozygous recessive individuals are q². Since no other genotype is possible, the % of the three genotypes added together must equal 100%.

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If you know one of these terms, you can find the other terms through algebra.

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EXAMPLE 1

Incomplete dominance in a population of wildflowers

• 320 red flowers (C^RC^R)
• 160 pink flowers (C^RC^W)
• 20 white flowers (C^WC^W)

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2. Find the allele frequencies in the gene pool. Use allele frequency formula.
3. Use the allele frequencies to find the genotype frequencies. p² + 2pq + q² = 1
4. Can you think of another way to find the genotype frequencies?

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EXAMPLE 2

A greenhouse contains 455 pea plants with yellow seeds and 45 pea plants with green seeds. Yellow is dominant over recessive.

1. What are the yellow and green allele frequencies?
2. What are the genotype frequencies?

GROUP DISCUSSION

Five male birds and five female birds (all unrelated to each other) settle on an uninhabited island. All 10 of the original birds had large beaks, and 2 were heterozygous. Large beakvks is dominant to small beaks. Which of these is closest to the allele frequency in the founding population?

A) 0.1 a, 0.9 A

B) 0.2 a, 0.8 A

C) 0.5 a, 0.5 A

D) 0.8 a, 0.2 A

E) 0.4 a, 0.6 A

GROUP DISCUSSION

If one assumes that Hardy-Weinberg equilibrium applies to the population of birds, about how many will have small beaks when the island’s population reaches 10,000?

A) 100

B) 400

C) 800

D) 1,000

E) 10,000

GROUP DISCUSSION

In peas, a gene controls flower color such that R = purple and r = white. In an isolated pea patch, there are 36 purple-flowering plants and 64 white-flowering plants. Assuming Hardy-Weinberg equilibrium, what is the value of q for this population?

A) 0.36

B) 0.64

C) 0.75

D) 0.80

POPULATION VARIATION

• Genetic variance = the diversity of alleles and genotypes within a population 
• High genetic variation associated with high population variation
• Population variation = the various phenotypes present in a population
• Evolutionary forces can only act on heritable variation
• Heritability = the fraction of population variation caused by genetic differences among individuals
• The greater the heritability of a population’s variation, the more susceptible it is to evolutionary forces

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EVOLUTIONARY FORCES THAT CAUSE CHANGE IN POPULATIONS

1. Natural selection
2. Genetic drift
3. Gene flow
4. Mutation
5. Nonrandom mating
6. Environmental variances

EVOLUTIONARY FORCES THAT CAUSE CHANGE IN POPULATIONS

• Natural selection
• Detrimental alleles are eliminated and beneficial alleles increase in frequency
• Selection pressure = driving selective force acting on a population
• Depends on the current environmental conditions

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EVOLUTIONARY FORCES THAT CAUSE CHANGE IN POPULATIONS

• Genetic drift
• By chance, some individuals will have more offspring than others
• Right place at the right time (a receptive female walks by)
• Wrong place at the wrong time (a predator walks by)
• Allele frequencies within a population change randomly with no advantage to the population
• Susceptibility depends on population’s size

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https://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_24

RAPID GENETIC DRIFT

• Bottleneck
• A chance event or catastrophe that drastically reduces the genetic variability within a population
• Founder Effect
• Random changes in allele frequencies (usually loss of alleles) due to a colonization event

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Tsaneda

https://commons.wikimedia.org

EVOLUTIONARY FORCES THAT CAUSE CHANGE IN POPULATIONS

Andrew Z. Colvin

https://commons.wikimedia.org

EVOLUTIONARY FORCES THAT CAUSE CHANGE IN POPULATIONS

• Mutation
• Changes to an organism’s DNA
• Most common way to introduce novel genotypic and phenotypic variance.
• Some are unfavorable or harmful and are quickly eliminated from the population by natural selection.
• Some are beneficial and will spread
• Some do not do anything and can linger, unaffected by natural selection

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EVOLUTIONARY FORCES THAT CAUSE CHANGE IN POPULATIONS

• Nonrandom mating
1. Mate choice
• Natural selection picks traits that lead to more matings for an individual
• Assortative mating = individual’s preference to mate with partners who look the same
2. Physical location
• In large populations spread over vast geographic distances, not all individuals will have equal access to one another

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EVOLUTIONARY FORCES THAT CAUSE CHANGE IN POPULATIONS

• Environmental variance
• Human skin color
• Temperature-dependent sex determination in reptiles
• Geographical variation in environmental conditions
• Can produce clines
• Phenotype of a species' populations varies gradually across an ecological gradient (ie. Bergmann’s Rule)

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Nmccarthy16

https://commons.wikimedia.org

GROUP DISCUSSION

Describe the different types of variation in a population.

Explain why only natural selection can act upon heritable variation.

Explain how each evolutionary force can influence a population's allele frequencies.

Describe the founder effect and the bottleneck effect.

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ADAPTIVE EVOLUTION

• Adaptive evolution = increase in the frequency of beneficial alleles in a population due to natural selection
• Natural selection selects for individuals that contribute beneficial alleles to the next generation 
• Evolutionary (Darwinian) fitness = an individual’s absolute contribution to the gene pool of the next generation
• BUT an individual's absolute fitness doesn’t count
• Relative fitness = the contribution of an individual relative to others in the population

REMEMBER, NATURAL SELECTION ACTS ON INDIVIDUALS TO AFFECT CHANGE IN A POPULATION. INDIVIDUALS DO NOT EVOLVE. POPULATIONS EVOLVE.

ADAPTIVE EVOLUTION

• Ways natural selection can affect population variation
• Stabilizing selection
• Directional selection
• Diversifying selection
• Frequency-dependent selection

FREQUENCY DEPENDENT SELECTION

• A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males, allowing it to sneak copulations.
• Populations of side-blotched lizards cycle in the distribution of these phenotypes.
• In one generation, orange might predominate, and then yellow males will begin to rise in frequency. Once yellow males are the most common, blue males will be selected. When blue males become common, orange males once again will be favored.

SEXUAL SELECTION

• Typically males have more variance in reproductive success than females
• Only a few males get most of the matings
• This generates a strong selection pressure among males to obtain matings, resulting in a physical appearance different from females
• = sexual dimorphism

Jeff Finn

www.flickr.com

SEXUAL SELECTION

• Sexual dimorphism is associated with the amount of parental care given to the offspring
• In species whose sex-role are reversed, females have bigger body size and elaborate traits

Factumquintus

https://commons.wikimedia.org

SEXUAL SELECTION

• Sexual selection = the selection pressures on males and females to obtain matings
• Selects for secondary sexual characteristics that often do not benefit the individual’s survival and may even be detrimental

• Handicap principle: only the most fit male can afford the risk associated with significant adornments
• Good genes hypothesis: the most impressive male traits indicate genetic superiority which will then pass on to their offspring
• Choosy females produce fewer, healthier offspring rather than many, weaker offspring

NO PERFECT ORGANISM!!!

• Natural selection…
• Can only select from existing variation in the population
• It cannot create variation
• Is limited by existing structures
• Works at the individual, not allele, level
• Any given individual may carry some beneficial and some unfavorable alleles
• Good alleles can be lost if individuals who carry them also have several overwhelmingly bad alleles and vice versa

NO PERFECT ORGANISM!!!

• Not all evolution is adaptive
• Genetic drift and gene flow can introduce deleterious alleles
• Adaptations are compromises
• Evolution has no purpose
• Simply the sum of the various forces described in this chapter

GROUP DISCUSSION

• Explain the different ways natural selection can shape populations.
• Explain the difference between natural selection and sexual selection.

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