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Lecture #2 – Evolution of Populations

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Key Concepts:

• The Modern Synthesis

• Populations and the Gene Pool

• The Hardy-Weinberg Equilibrium

• Micro-evolution

• Sources of Genetic Variation

• Natural Selection

• Preservation of Genetic Variation

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Images – species, population, community

Some preliminary definitions

• Species – individual organisms capable of mating and producing fertile offspring

• Population – a group of individuals of a single species

• Community – a group of individuals of different species

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The Modern Synthesisintegrates our knowledge about

evolution

• Darwin’s natural selection

• Mendel’s hereditary patterns

• Particulate transfer (chromosomes)

• Structure of the DNA molecule

All explain how the genetic structure of populations changes over time

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KEY POINT

Environmental factors act on the individual to control the genetic future of

the population

Individuals don’t evolve…..populations do

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Image – population of iris

Population = a +/- localized group of individuals of one species

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Critical Thinking

• How do we determine the boundaries of a population???

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Critical Thinking

• How do we determine the boundaries of a population???Boundaries are scale dependentSome sub-populations overlapSome are more isolatedWe can look at populations at many different

scales – micro to meta

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Recall basic genetic principles:

• In a diploid species (most are), every individual has two copies of every geneOne copy came from each parent

• Most genes have different versions = alleles

• Diploid individuals are either heterozygous or homozygous for each geneHeterozygous = AaHomozygous = AA or aa

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Recall basic genetic principles:

• The total number of alleles for any gene in a population is the number of individuals in the population x 2If the population has 10 individuals, there are

20 copies of the A gene – some “A” alleles and some “a” alleles

• All these alleles comprise the “gene pool”

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Hardy-Weinberg Theorem

• Gene pool = all alleles in a population

• All alleles have a frequency in the populationThere is a percentage of “A” and a

percentage of “a” that adds up to 100%

• Hardy-Weinberg Theorem demonstrates that allele frequencies don’t change through meiosis and fertilization alone

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Hardy-Weinberg Theorem

• A simple, mathematical model

• Shows that repeated random meiosis and fertilization events alone will not change the distribution of alleles in a populationEven over many generations

p2 + 2pq + q2 = 1

we will not focus on the math – you’ll work on this in lab

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Hardy-Weinberg Theorem

• Meiosis and fertilization randomly shuffle alleles, but they don't change proportionsLike repeatedly shuffling a deck of cardsThe laws of probability determine that the

proportion of alleles will not change from generation to generation

• This stable distribution of alleles is the Hardy-Weinberg equilibrium

Doesn’t happen in nature!!!

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Conditions for H-W Equilibrium:

• No natural selection

• Large population size

• Isolated population

• Random mating

• No mutation

Doesn’t happen in nature!!!The violation of each assumption acts as

an agent of microevolution

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The value of H-W???

• It provides a null hypothesis to compare to what actually happens in nature

• Allele frequencies DO change in nature

• BUT, they change only under the conditions of microevolutionIn nature, all the H-W assumptions are violated

• Result – populations DO evolve

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Critical Thinking

• What are the limitations of the Hardy-Weinberg theorem???

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Critical Thinking

• What are the limitations of the Hardy-Weinberg theorem???

• The H-W model considers just one trait at a time, and assumes that just one gene with 2 alleles (one completely dominant) controls that trait

• Recall your basic genetics – is this realistic???

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Critical Thinking• Reality is much more complex for most traits

in most organismsIncomplete dominance or codominanceMore than 2 alleles for many genesPleiotropy – one gene affects multiple traitsPolygenic traits – multiple genes affect one traitEpistasis – one gene affects expression of

another geneEnvironmental effects on phenotypic expression

• Reproductive success depends on the way all genes and traits interact

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Individuals Do Not Evolve

• Individuals vary, but populations evolve

• Natural selection pressures make an individual more or less likely to survive and reproduce

• But, it is the cumulative effects of selection on the genetic makeup of the whole population that results in changes to the species

The environment is a wall; natural selection is a gate

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The environment is the wall; natural selection is the gate

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***** *****

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Image – natural variation in flower color; same image for all these summary slides

Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection

• Genetic Drift

• Gene Flow

• Selective Mating

• Mutation

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Cartoon – beaver with chainsaw paws “natural selection does not grant organisms what they “need””

Natural Selection – the essence of Darwin’s theory

Mor

e on

thi

s la

ter…

. More on this later…

.

Differential reproductive success is the only way to account for the accumulation of

favorable traits in a population

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Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection

• Genetic Drift

• Gene Flow

• Selective Mating

• Mutation

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• Reproductive events are samples of the parent population

Genetic Drift – random changes in allele frequency from generation to generation

Larger pop = ~29% blue Smaller pop = 100% blue

Parent pop = 10% blue

Larger samples are more representative than smaller samples (probability theory)

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Genetic Drift – random changes in allele frequency from generation to generation

• More pronounced in smaller and/or more segregated populationsBottleneck effectFounder effect

Segregated pop = ~29% blue Segregated pop = 100% blue

Parent pop = 10% blue

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Diagram – bottlenecking

Bottlenecking = extreme genetic drift

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Critical Thinking

• What events could cause a bottleneck???

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Critical Thinking

• What events could cause a bottleneck??? Bottlenecks occur when there is an extreme and indiscriminate reduction in the reproducing populationDiseaseHerbivoryMalnutritionMajor disturbance (flood, fire)Human intervention

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Image – cheetah

Conservation implications – cheetahs are a bottlenecked species

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Maps – historic and current range of cheetahs

Extreme range reduction due to

habitat destruction and poaching

+Cheetahs were

naturally bottlenecked about 10,000 years

ago by the last major ice age (kinked tail)

The species is at risk of extinction

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Images – bottlenecked and now endangered species

Australian Flame Robin, California Condor, Mauritian Kestrel

…..and many more, all driven nearly to extinction…..

Some colorful results of a quick web search on “bottlenecked species”

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Founder Effect = extreme genetic drift

• Occurs when a single individual, or small group of individuals, breaks off from a larger population to colonize a new habitatIslandsOther side of mountainOther side of a river…

• This small group may not represent the allele distribution of the parent population

33Founder Effect

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Image – a founding population of seeds; possibly also the bird if it’s a gravid female

Long distance dispersal events can lead to the founder effect

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Critical Thinking

• What do you think follows long distance dispersal to a new ecosystem???

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Critical Thinking

• What do you think follows long distance dispersal to a new ecosystem???

• Adaptive radiation frequently leads to many new, closely related species as the organisms adapt to new habitat zones in their new home

FoundingPopulation

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2

3

4

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Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection

• Genetic Drift

• Gene Flow

• Selective Mating

• Mutation

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Gene Flow

• Mixes alleles between populationsImmigrationEmigration

• Most populations are NOT completely isolated

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Critical Thinking

• Will gene flow tend to increase or decrease speciation???

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Critical Thinking

• Will gene flow tend to increase or decrease speciation???

• Gene flow tends to preserve species by shuffling alleles between all sub-populations

43Gene Flow

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Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection

• Genetic Drift

• Gene Flow

• Selective Mating

• Mutation

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Image – peacock with mating display

Selective Breeding

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Critical Thinking

• Animal behaviors are obvious examples

• Can you think of others???

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Image – fungi spores

Critical Thinking

• Animal behaviors are obvious examples

• Can you think of others???

• Proximity is important even in species that do not have mating behaviorsMany plants and fungi are randomly fertilized

or pollinated…..but generally the exchange is between closer neighbors

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Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection

• Genetic Drift

• Gene Flow

• Selective Mating

• Mutation

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Diagram – mutations

Cartoon - jackalope

Mutations• Random, rare, but

regular events• The only source of

completely new traits

just for fun…..

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Evolution = random events

x“the gate”

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Review: Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection

• Genetic Drift

• Gene Flow

• Selective Mating

• Mutation

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Sources of Genetic Variation

• Natural selection acts on natural variation

• Where does this variation come from???MeiosisMutation

• Additional mechanisms help preserve variation (later)

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Diagram – meiosis I

Meiosis = key source of variation

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Diagram – meiosis II

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Diagram – results of meiosis with n=2

Random, Independent Assortment of Homologous Chromosomes

n = 2

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Probability theory reveals that for random, independent events:

• If each event has 2 possible outcomesIn this case, one side of the plate or the other

• The possible number of distribution combinations = 2n, where n = the number of eventsIn this case, the distribution event is the

distribution of chromosomes to the gametesn = the haploid number of chromosomes

• If n is 2, then combinations are 22 = 4

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Diagram – results of meiosis with n=2

Random, Independent Assortment of Homologous Chromosomes

n = 2

Four possible

distributions

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Probability theory reveals that for random, independent events:

• If each event has 2 possible outcomesIn this case, one side of the plate or the other

• The possible number of distribution combinations = 2n, where n = the number of eventsIn this case, distribution refers to the distribution

of chromosomes to the gametesn = the haploid number of chromosomes

• If n is 23, then combinations are 223 = 8.4 million!

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Probability is Multiplicative:

8.4 million x 8.4 million > 70 trillion!!!

That is the number of possible combinations of maternal and paternal chromosomes in the offspring of a randomly mating pair of

humans

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Diagram – recombinationRecombination increases the

potential variation to

infinity

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Critical Thinking

• Can meiosis produce totally new traits???

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Critical Thinking

• Can meiosis produce totally new traits???

• No – remember, normal meiosis just shuffles the alleles

• Only mutation can make entirely new alleles

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Natural Selection as a Mechanism of Evolutionary Adaptation

• Natural selection acts on the variation produced by meiosis and mutation

• Selection increases the “fitness” of a population in a given environment

• Fitness = ???

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Natural Selection as a Mechanism of Evolutionary Adaptation

• Natural selection acts on the variation produced by meiosis and mutation

• Selection increases the “fitness” of a population in a given environment

• Fitness = reproductive success NOT big, NOT smart, NOT strongThe production of successful offspring is the

key

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Natural selection has limits

• Individuals vary in fitnessNatural selection promotes the most fit

• Selection acts on the phenotype – the whole, complex organismResults from the combination of many different

genes for any organismThese genes are expressed in the whole,

complex environment

• Selection is always constrained by the whole, complex evolutionary history of the species

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Critical Thinking

• Can evolution respond to “needs”???

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Critical Thinking

• Can evolution respond to “needs”???

• NO!!!

• Evolution is a combination of random events + successful reproduction in a given environment

• The environment is the wall; natural selection is the gate!!!!If the phenotype “works”, the genotype

passes through the gate

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Diagram – patterns of natural selection

Patterns of Change by Natural Selection

• Directional Selection

• Diversifying Selection (AKA disruptive)

• Stabilizing Selection

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Diagram – patterns of natural selection

Remember, all populations exhibit a range of natural variation

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Diagram – directional selection

Directional Selection

• Phenotypes at one extreme of the range are most successfulColorPatternFormMetabolic processes

• The population shifts to favor the successful phenotype

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Diagram – diversifying selection

Diversifying Selection

• Multiple, but not all, phenotypes are successfulPatchy environmentsSub-populations migrate to new habitats

• The population begins to fragment and new species begin to diverge

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Diagram – stabilizing selection

Stabilizing Selection

• The intermediate phenotypes are most successfulHomogenous environmentsStable conditions

• The range of variation within the population is reduced

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Critical Thinking

• Which selection mode will most quickly lead to the development of diversity???

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Critical Thinking

• Which selection mode will most quickly lead to the development of diversity???

• Diversifying selection tends to produce multiple species, and the parent species may also persist

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Diagram – patterns of selectiondirectional diversifying

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Critical Thinking

• Can you think of a real-life example of an adaptive phenotype???

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Critical Thinking

• Can you think of a real-life example of an adaptive phenotype???

• Everything!Variation is randomSelection is adaptive

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Images – natural variation in flower color

Preservation of Natural Variation

• Diploidy

• Balanced Polymorphism

• Neutral Variation

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Diploidy – 2 alleles for every gene

• Recessive alleles retained in heterozygotesNot expressedNot eliminated, even if the recessive trait is aa may be eliminated, while Aa is preserved in

the population

• Recessive alleles function as latent variation that may prove helpful if environment changes

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Balanced Polymorphism

• Heterozygote advantage

• Frequency dependent selection

• Phenotypic variation

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Map – global distribution of sickle cell allele

Images – normal and sickled red blood cells

Balanced Polymorphism – heterozygote advantage

Sickle-cell Anemia

a mutation in the gene that codes for hemoglobin causes a single amino acid substitution in the protein, RBC shape changes from round to sickle shape

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Graph – frequency dependent selection results

Balanced Polymorphisms – Frequency Dependent Selection

rare clone is less infected

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Images – balanced polymorphisms in asters and snakes

Balanced Polymorphisms – Phenotypic Variationmultiple morphotypes are favored by heterogeneous

(patchy) environment

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Neutral Variation

• Genetic variation that has no apparent effect on fitness

• Not affected by natural selection

• May provide an important base for future selection, if environmental conditions change

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Key Concepts: QUESTIONS???

• The Modern Synthesis

• Populations and the Gene Pool

• The Hardy-Weinberg Equilibrium

• Micro-evolution

• Sources of Genetic Variation

• Natural Selection

• Preservation of Genetic Variation