1 lecture #2 – evolution of populations image of a population of penguins

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Lecture #2 – Evolution of Populations

Image of a population of penguins

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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

Review 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???

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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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• 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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Hands On

• The equation: p2 + 2pq + q2 = 1 (Page 2)• p = the frequency of one allele• q = the frequency of the other allele• p + q MUST = 1 = 100% of the gene pool

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Hands On

• If the allele frequencies are known, the HW equilibrium can be demonstrated by a Punnett square

Assume that we know there are

½ T and ½ t alleles

Maternal Parent

T t

Paternal Parent

T TT Tt

t Tt tt

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Hands On

• What are the frequencies of each allele in the F1 generation?

Assume that we know there are

½ T and ½ t alleles

Maternal Parent

T t

Paternal Parent

T TT Tt

t Tt tt

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Hands On – Results

• What are the frequencies of each allele in the F1 generation?

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Hands On

• How do you determine the allele frequencies???

• How do you find p and q???• In this example, how do you know the

percentage of T and the percentage of t???

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Hands On

• Clasp your hands

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Hands On

• Count right thumbs up vs. left thumbs up• Right thumb up is the recessive condition!• Determine the distribution of T and t in our

class population• Type up a summary of your results and

turn in tomorrow

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• 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???

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Critical Thinking

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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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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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frequencies


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• Reproductive events are samples of the parent population

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

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2

1

2

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

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2

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

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2

1

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

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Parent pop = 10% blue

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Bottlenecking = extreme genetic drift

Diagram – bottlenecking

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Critical Thinking

• What events could cause a bottleneck???

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Critical Thinking

• What events could cause a bottleneck???

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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

44Founder 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???

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Hands On

• Genetic drift is random• Some drift is expected with every generation

Genetic drift is not necessarily extreme• Use the beads to explore this idea (Page 4)

Count out 50 beads each of 2 colorsEach bead represents an allele in the gene poolSince B and b are in equal proportion, what is

the phenotypic makeup of the diploid population???

p2 + 2pq + q2 = 1

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• Genetic drift is random• Some drift is expected with every generation

Genetic drift is not necessarily extreme• Use the beads to explore this idea

Count out 50 beads each of 2 colorsEach bead represents an allele in the gene poolSince B and b are in equal proportion, what is

the phenotypic makeup of the diploid population???

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Hands On

• Now simulate random mating by shaking up the beads and pulling out 2 beads at a time, with your eyes closedBe sure to return the beads to the “gene pool”We are sampling with replacement – random

• Record each offspring allele structureBe sure to assign a dominant and recessive

color• Repeat 50 times

Shake the pool each time to maintain random

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Hands On

• Count and record the allele structure of your second generation

• Make a new gene pool of 100 beads that reflects this new allele structure

• Repeat the bead selection for a 3rd generation

• Repeat for a total of 5 generations

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Hands On

• Is your 5th generation allele structure the same as your 1st generation???

• What is the phenotypic distribution of your 5th generation?

• What are your conclusions?• Use your lab notebook to record

observations

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Hands On

• Now simulate a bottleneck• Shake up the beads and pull out 2 beads

at a time, with your eyes closedBe sure to return the beads to the “gene pool”We are sampling with replacement – random

• Record each offspring allele structureBe sure to assign a dominant and recessive

color• Repeat 5 times

Shake the pool each time to maintain random

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Hands On

• Count and record the allele structure of your second generation

• Make a new gene pool of 100 beads that reflects this new allele structure

• Repeat the bead selection for a 3rd generation

• Repeat for a total of 5 generations

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Hands On

• Is your 5th generation allele structure the same as your 1st generation???

• What is the phenotypic distribution of your 5th generation?

• What are your conclusions?• Compare 50 vs. 5 reproductive “events”• Use your lab notebook to record

observations, and type up a summary to turn in tomorrow

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frequencies


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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???

62Gene Flow

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Hands On

• How could we demonstrate gene flow with our beads???

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• How could we demonstrate gene flow with our beads???

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frequencies


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Image – male peacock with mating display

Selective Breeding

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Critical Thinking

• Animal behaviors are obvious examples• Can you think of others???

Critical Thinking

• Animal behaviors are obvious examples• Can you think of others???

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frequencies


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Diagram – mutations in DNA strand

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


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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???

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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 =

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Natural selection has limits• Individuals vary in fitness

Natural 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”???

Beaver cartoon again

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Critical Thinking

• Can evolution respond to “needs”???

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Hands On

• Calculate the allele distribution to the F1 with the dominant phenotype resulting in a 20% decline in the reproductive success rate (Page 3, with a twist)

• The twist – start with a 50/50 distribution of dominant and recessive alleles in the gene pool

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Hands On

• Calculate the allele distribution to the F1 with the recessive phenotype resulting in 100% mortality (Page 3, with a twist)

• The twist – start with a 50/50 distribution of dominant and recessive alleles in the gene pool

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Hands On

• In either case would either the T or t allele become extinct?

• Why or why not?

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• In either case would either the T or t allele become extinct?

• Why or why not?

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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???

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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???

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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

1 lecture #2 – evolution of populations image of a population of penguins

Documents

aa slide

time slide

lab slide

population individuals

population of penguins

population x

alleles diploid individuals

distribution of alleles