1 Lecture #2 – Evolution of Populations. 2 Key Concepts: The Modern Synthesis Populations and the...

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Lecture #2 – Evolution of Populations

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

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

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

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

Critical Thinking

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

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

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

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”

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

• 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

• 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!!!

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

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

Critical Thinking

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

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???

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

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

The environment is the wall; natural selection is the gate

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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

• Genetic Drift

• Gene Flow

• Selective Mating

• Mutation

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

Natural Selection – the essence of Darwin’s theory

ter…

. More on this later…

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

favorable traits in a population

frequencies

• Genetic Drift

• Gene Flow

• Mutation

• 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)

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

Diagram – bottlenecking

Bottlenecking = extreme genetic drift

Critical Thinking

• What events could cause a bottleneck???

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

Image – cheetah

Conservation implications – cheetahs are a bottlenecked species

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

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”

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

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

Critical Thinking

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

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

frequencies

• Genetic Drift

• Gene Flow

• Mutation

Gene Flow

• Mixes alleles between populationsImmigrationEmigration

• Most populations are NOT completely isolated

Critical Thinking

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

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

frequencies

• Genetic Drift

• Gene Flow

• Mutation

Image – peacock with mating display

Selective Breeding

Critical Thinking

• Animal behaviors are obvious examples

• Can you think of others???

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

frequencies

• Genetic Drift

• Gene Flow

• Mutation

Diagram – mutations

Cartoon - jackalope

Mutations• Random, rare, but

regular events• The only source of

completely new traits

just for fun…..

Evolution = random events

x“the gate”

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

frequencies

• Genetic Drift

• Gene Flow

• Mutation

Sources of Genetic Variation

• Natural selection acts on natural variation

• Where does this variation come from???MeiosisMutation

• Additional mechanisms help preserve variation (later)

Diagram – meiosis I

Meiosis = key source of variation

Diagram – meiosis II

Diagram – results of meiosis with n=2

Random, Independent Assortment of Homologous Chromosomes

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

Diagram – results of meiosis with n=2

Random, Independent Assortment of Homologous Chromosomes

Four possible

distributions

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!

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

Diagram – recombinationRecombination increases the

potential variation to

infinity

Critical Thinking

• Can meiosis produce totally new traits???

Critical Thinking

• Can meiosis produce totally new traits???

• No – remember, normal meiosis just shuffles the alleles

• Only mutation can make entirely new alleles

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 = ???

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

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

Critical Thinking

• Can evolution respond to “needs”???

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

Diagram – patterns of natural selection

Patterns of Change by Natural Selection

• Directional Selection

• Diversifying Selection (AKA disruptive)

• Stabilizing Selection

Diagram – patterns of natural selection

Remember, all populations exhibit a range of natural variation

Diagram – directional selection

Directional Selection

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

• The population shifts to favor the successful phenotype

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

Diagram – stabilizing selection

Stabilizing Selection

• The intermediate phenotypes are most successfulHomogenous environmentsStable conditions

• The range of variation within the population is reduced

Critical Thinking

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

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

Diagram – patterns of selectiondirectional diversifying

Critical Thinking

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

Critical Thinking

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

• Everything!Variation is randomSelection is adaptive

Images – natural variation in flower color

Preservation of Natural Variation

• Diploidy

• Balanced Polymorphism

• Neutral Variation

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

Balanced Polymorphism

• Heterozygote advantage

• Frequency dependent selection

• Phenotypic variation

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

Graph – frequency dependent selection results

Balanced Polymorphisms – Frequency Dependent Selection

rare clone is less infected

Images – balanced polymorphisms in asters and snakes

Balanced Polymorphisms – Phenotypic Variationmultiple morphotypes are favored by heterogeneous

(patchy) environment

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

Key Concepts: QUESTIONS???

• The Modern Synthesis

• Populations and the Gene Pool

• The Hardy-Weinberg Equilibrium

• Micro-evolution

• Sources of Genetic Variation

• Preservation of Genetic Variation

1 Lecture #2 – Evolution of Populations. 2 Key Concepts: The Modern Synthesis Populations and the...

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