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Biology 102Biology 102

Lecture Lecture 15: Population 15: Population GeneticsGenetics

•• Natural variation occurs in every population of Natural variation occurs in every population of every speciesevery species

•• Due to different alleles for genesDue to different alleles for genes

•• Evolution is measured as changes in allele and Evolution is measured as changes in allele and genotype frequency in a populationgenotype frequency in a population

Species VariationSpecies Variation

•• If evolution is not at If evolution is not at

work, allele and genotypework, allele and genotype

frequencies will stay the frequencies will stay the

same from one generation same from one generation

to the nextto the next

HardyHardy--Weinberg PrincipleWeinberg Principle

•• Without evolution, alleles in a population will Without evolution, alleles in a population will quickly reach equilibriumquickly reach equilibrium

•• Once the frequency of alleles in a population is Once the frequency of alleles in a population is known, we can use the known, we can use the HardyHardy--Weinberg equation Weinberg equation

•• p = frequency of the dominant allele in the populationp = frequency of the dominant allele in the population

•• q = frequency of the recessive allele in the populationq = frequency of the recessive allele in the population

HardyHardy--Weinberg PrincipleWeinberg Principle

•• Assume that 2 heterozygous individuals produce Assume that 2 heterozygous individuals produce offspring offspring

Where does it come from?Where does it come from?

p

p

q

q

pp

(p2)

qq

(q2)

pq

pq

•• We can calculate what proportion of individuals We can calculate what proportion of individuals in the next generation will have a given genotype in the next generation will have a given genotype

and phenotypeand phenotype

•• You will do this in Lab 9You will do this in Lab 9

What does it mean?What does it mean?

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•• Two alleles exist for color in a certain type of Two alleles exist for color in a certain type of beetle. Red (R) is dominant to blue (r).beetle. Red (R) is dominant to blue (r).

•• In a specific population of beetles, 51% arIn a specific population of beetles, 51% are red e red and 49% are blue.and 49% are blue.

•• Assuming the population is in HAssuming the population is in H--W equilibrium, W equilibrium, what are the frequencies of the red and blue what are the frequencies of the red and blue alleles in the gene pool? alleles in the gene pool?

ExampleExample

= RR = RR andand RrRr = = rrrr onlyonly

pp22 andand pqpq qq22 onlyonly

qq22 = 0.49= 0.49

q = 0.7q = 0.7

p + q = 1p + q = 1

1 1 –– 0.7 = p0.7 = p

p = 0.3p = 0.3

ExampleExample

•• Cystic fibrosis is an Cystic fibrosis is an autosomalautosomal recessive disorder recessive disorder that affects 1/2500 (0.0004) Caucasians.that affects 1/2500 (0.0004) Caucasians.

•• Assuming the population is in HAssuming the population is in H--W equilibrium, W equilibrium, what percentage of Caucasians are carriers?what percentage of Caucasians are carriers?

ExampleExample

= CC = CC andand CcCc = cc only= cc only

pp22 andand pqpq qq22 onlyonly

qq22 = 0.0004= 0.0004

q = 0.02q = 0.02

p + q = 1p + q = 1

1 1 –– 0.02 0.02 = p= p

p = 0.98p = 0.98

ExampleExample

= CC = CC andand CcCc = cc only= cc only

pp22 andand pqpq qq22 onlyonly

Frequency of Frequency of heterozygotesheterozygotes = 2pq= 2pq

2(2(0.980.98)(0.02) = 2pq)(0.02) = 2pq

0.0392 or 3.92%0.0392 or 3.92%

ExampleExample

pp22 + 2pq + q+ 2pq + q22 = 1= 1

p = 0.98 q = p = 0.98 q = 0.020.02

(0.98)2 + 2(0.98)(0.02) + (0.02)2 = ?(0.98)2 + 2(0.98)(0.02) + (0.02)2 = ?

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Check Your Work!Check Your Work!

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•• The HardyThe Hardy--Weinberg equation makes some Weinberg equation makes some pretty big assumptionspretty big assumptions

•• Organism is diploidOrganism is diploid

•• Only sexual reproduction is usedOnly sexual reproduction is used

•• Only 2 alleles exist for a given geneOnly 2 alleles exist for a given gene

•• Complete dominanceComplete dominance

•• Not a sexNot a sex--linked traitlinked trait

ProblemsProblems

•• Adjustments can be made for these assumptions, Adjustments can be made for these assumptions, but not for the biggest problem of all…but not for the biggest problem of all…

IT ASSUMES NO EVOLUTION!!IT ASSUMES NO EVOLUTION!!

•• Assumes…Assumes…

•• Very large sample sizeVery large sample size

•• No migrationNo migration

•• No mutationNo mutation

•• Random matingRandom mating

•• No differential reproductive successNo differential reproductive success

ProblemsProblems

•• The HardyThe Hardy--Weinberg equation is great for… Weinberg equation is great for…

•• Getting a snapshot of alleles in timeGetting a snapshot of alleles in time

•• Tracking evolutionary changes over timeTracking evolutionary changes over time

•• It is NOT good for predicting evolutionary It is NOT good for predicting evolutionary changeschanges

ProblemsProblems

•• Five agents of evolutionary changeFive agents of evolutionary change

•• All bring about changes in allele frequency and All bring about changes in allele frequency and

disrupt Hardydisrupt Hardy--Weinberg equilibriumWeinberg equilibrium

•• Genetic driftGenetic drift

•• Gene flowGene flow

•• MutationsMutations

•• Sexual selectionSexual selection

•• Natural selectionNatural selection

ContibutingContibuting FactorsFactors

•• Changes in Changes in allelallele frequency due to e frequency due to chance eventschance eventsin a in a small populationsmall population

•• Example: Flipping a coinExample: Flipping a coin3 out of 103 out of 10

300 out of 1000300 out of 1000•• Drift tends to reduce variationDrift tends to reduce variation•• 2 main types2 main types

•• Founder effectFounder effect•• Bottlenecking eventBottlenecking event

Genetic DriftGenetic Drift

•• When a small group separates from a larger When a small group separates from a larger populationpopulation

•• Small group might…Small group might…

•• Carry alleles that are rare in the original Carry alleles that are rare in the original populationpopulation

•• Be missing alleles that are present in the Be missing alleles that are present in the original populationoriginal population

Founder EffectFounder Effect

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•• Example:Example:

•• A population of red and yellow beetles existsA population of red and yellow beetles exists

•• The wind randomly blows only red beetles to a The wind randomly blows only red beetles to a

new islandnew island

Founder EffectFounder Effect

•• Example:Example:

•• EllisEllis--Van Van CreveldCreveld syndrome is common among syndrome is common among old order Amish in Lancaster County, old order Amish in Lancaster County, PennsylvaniaPennsylvania

•• All are descendents All are descendents

of one couple that of one couple that

founded the communityfounded the community

in 1744in 1744

Founder EffectFounder Effect

•• A catastrophe (e.g. earthquake, tsunami, etc)A catastrophe (e.g. earthquake, tsunami, etc)kills off most of a populationkills off most of a population

•• Handful of survivors may not represent the full Handful of survivors may not represent the full genetic diversity of the original populationgenetic diversity of the original population

•• Reduction in variation and adaptabilityReduction in variation and adaptability

Bottlenecking EventBottlenecking Event

•• A bottlenecking event is randomA bottlenecking event is random

•• A plague that kills off individuals lacking a A plague that kills off individuals lacking a particular allele is natural selectionparticular allele is natural selection

•• Natural selection kills individuals due to genetic Natural selection kills individuals due to genetic makemake--upup

•• Bottlenecking kills indiscriminatelyBottlenecking kills indiscriminately

IMPORTANT!!IMPORTANT!!

•• Example:Example:

•• Endangered speciesEndangered species

•• Genetic variation lowGenetic variation low

•• SuceptibilitySuceptibility highhigh

Bottlenecking EventBottlenecking Event

•• It’s important to remember that all sources of It’s important to remember that all sources of genetic drift…genetic drift…

•• Occur in small populationsOccur in small populations

•• Are RANDOMAre RANDOM

Genetic DriftGenetic Drift

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•• Transfer of alleles from one population to Transfer of alleles from one population to anotheranother

•• Due to individuals moving into or out of a Due to individuals moving into or out of a populationpopulation

Gene FlowGene Flow

•• Examples:Examples:

•• A migrating bird changes flocksA migrating bird changes flocks

•• GrolarGrolar bears (gene flow between species)bears (gene flow between species)

Gene FlowGene Flow

•• Permanent changes in the DNA sequencePermanent changes in the DNA sequence

•• Multiple sources (UV radiation, chemicals, etc)Multiple sources (UV radiation, chemicals, etc)

•• We’ve talked about this beforeWe’ve talked about this before

MutationsMutations

•• If mutations occur in the gametes, changes can If mutations occur in the gametes, changes can be passed to the next generationbe passed to the next generation

•• Frequency = 1/10,000 to 1/100,000,000Frequency = 1/10,000 to 1/100,000,000

•• Almost always leads to early death or lowered Almost always leads to early death or lowered reproductive reproductive fitnessfitness

MutationsMutations

MutationsMutations

•• But not alwaysBut not always

•• Examples:Examples:

Sexual SelectionSexual Selection

•• Mating is not randomMating is not random

•• Certain characteristics increase an organism’s Certain characteristics increase an organism’s chances of reproductionchances of reproduction

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Sexual SelectionSexual Selection

•• In many species, males and females look very In many species, males and females look very differentdifferent

•• Sexual dimorphismSexual dimorphism

Sexual SelectionSexual Selection

•• Some of these differences give a competitive Some of these differences give a competitive advantage when competing with each other for advantage when competing with each other for

matesmates

•• IntraIntrasexualsexual selectionselection

Sexual SelectionSexual Selection

•• Some of these differences give a competitive Some of these differences give a competitive advantage when attempting to attract matesadvantage when attempting to attract mates

•• InterIntersexual selectionsexual selection