Chapter 16 Chapter 16

Population Genetics and Population Genetics and SpeciationSpeciation

Variation of TraitsVariation of Traits

In nature many quantitative traits in In nature many quantitative traits in a population tend to follow a bell a population tend to follow a bell curve patterncurve pattern

Average (ex height and weight) Average (ex height and weight)

The report, The report, Mean Body Weight, Height, and Body Mass Index (BMI) 1960-2002: United States, shows that the average height of a man aged , shows that the average height of a man aged 20-74 years increased from just over 5'8" in 1960 20-74 years increased from just over 5'8" in 1960 to 5'9½" in 2002, while the average height of a to 5'9½" in 2002, while the average height of a woman the same age increased from slightly over woman the same age increased from slightly over 5'3" 1960 to 5'4" in 2002. 5'3" 1960 to 5'4" in 2002.

Meanwhile, the average weight for men aged 20-Meanwhile, the average weight for men aged 20-74 years rose dramatically from 166.3 pounds in 74 years rose dramatically from 166.3 pounds in 1960 to 191 pounds in 2002, while the average 1960 to 191 pounds in 2002, while the average weight for women the same age increased from weight for women the same age increased from 140.2 pounds in 1960 to 164.3 pounds in 2002.140.2 pounds in 1960 to 164.3 pounds in 2002.

Causes of VariationCauses of Variation

EnvironmentEnvironment heredityheredity

• MutationMutation• Recombination of genesRecombination of genes• Random pairing of gametesRandom pairing of gametes

Theoretically…how many Theoretically…how many genetically unique, brothers and genetically unique, brothers and

sisters could your parents sisters could your parents produce? produce?

For humans, the number of different For humans, the number of different gametes is 223 * 223, or 8,388,6082, gametes is 223 * 223, or 8,388,6082, giving 70,368,744,177,664 giving 70,368,744,177,664 (70 (70 trillion) trillion) possible combinations. possible combinations.

The Gene PoolThe Gene Pool Total genetic information available in Total genetic information available in

a populationa population Ex. In a homogeneous population like Ex. In a homogeneous population like

in South Korea….the dark skin alleles in South Korea….the dark skin alleles are just not in that gene pool. are just not in that gene pool.

Calculate Allele frequency ?Calculate Allele frequency ? Divide number of certain allele by Divide number of certain allele by

the total number of allelesthe total number of alleles

Phenotype frequency- number of Phenotype frequency- number of individuals with a particular individuals with a particular phenotype divided by the total phenotype divided by the total number of individuals in the number of individuals in the populationpopulation

Frequency of a pair? Multiply the Frequency of a pair? Multiply the individual frequency numbers….individual frequency numbers….

Hardy & WeinbergHardy & Weinberg

Genotype frequencies tend to remain Genotype frequencies tend to remain the same in a population unless the same in a population unless acted on upon outside forces. acted on upon outside forces. (genetic equilibrium)(genetic equilibrium)

5 things must happen for this to 5 things must happen for this to occur…occur…

1.1. No MutationsNo Mutations

2.2. No individuals enter of leaveNo individuals enter of leave

3.3. Population is largePopulation is large

4.4. Mating is randomMating is random

5.5. Natural Selection does not occurNatural Selection does not occur

*** Where on Earth does this happen?*** Where on Earth does this happen?

Hardy Weinberg EquationHardy Weinberg Equation

Calculate frequencies of alleles in Calculate frequencies of alleles in populationpopulation

P dominant alleleP dominant allele Q recessive alleleQ recessive allele Problems 1, 2, 3 – go over together Problems 1, 2, 3 – go over together

Disrupting Genetic EquilibriumDisrupting Genetic Equilibrium

Mutation:Mutation: Can increase with mutagensCan increase with mutagens Spontaneous mutationsSpontaneous mutations

• Usually remain fairly low in numberUsually remain fairly low in number• Introduce new alleles into pop.Introduce new alleles into pop.• Many are harmful, some are beneficial Many are harmful, some are beneficial

some are neithersome are neither

Polydactyly--mutationPolydactyly--mutation

MutationsMutations

Provide new alleles in gene poolProvide new alleles in gene pool AdaptationsAdaptations

Gene FlowGene Flow

Emigration- out Emigration- out Immigration- in Immigration- in This maintains gene flowThis maintains gene flow Ex.- Male lions chase away maturing Ex.- Male lions chase away maturing

cubs, they must go find another cubs, they must go find another group… (This way we can be sure that group… (This way we can be sure that the will not mate with their sister the will not mate with their sister cubs…) cubs…)

Gene flow….Gene flow….

Genetic DriftGenetic Drift

Allele frequencies change as a result Allele frequencies change as a result of random events…of random events…

This is more applicable to a smaller This is more applicable to a smaller population than to a larger one. population than to a larger one.

Causes of EvolutionCauses of Evolution Genetic Drift: change due to chanceGenetic Drift: change due to chance

• Bottleneck effect (fire, earthquake, Bottleneck effect (fire, earthquake, flood)…flood)…

An An example of a bottleneck:example of a bottleneck:Northern elephant seals have reduced genetic variation Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so population of southern elephant seals that was not so intensely hunted.intensely hunted.

Another bottleneck ex.Another bottleneck ex.

Ashkenazi Jews population reduced Ashkenazi Jews population reduced due to Genocide (perhaps) and high due to Genocide (perhaps) and high incidence of Tay Sachs Disease incidence of Tay Sachs Disease now….now….

Non random matingNon random mating

Not goodNot good Geographic proximityGeographic proximity Amplify certain traitsAmplify certain traits Some select mates based on Some select mates based on

similarities, this is assortative similarities, this is assortative mating…mating…

Founder effectFounder effect

Small population Small population breaks away from a breaks away from a larger one. Rarer larger one. Rarer alleles are over alleles are over represented…..represented…..

Amish 1770’s Amish 1770’s German population, German population, extreme isolation, extreme isolation, and intermarriage… and intermarriage…

As a result… many Amish have As a result… many Amish have polydactylypolydactyly

Sexual Selection Sexual Selection

Females choose males, based on Females choose males, based on certain traitscertain traits

Peacock: Male is brightly colored Peacock: Male is brightly colored (may attract predators, but gets the (may attract predators, but gets the girls to notice him ….) girls to notice him ….)

Sexual SelectionSexual Selection sexual dimorphism: Differences sexual dimorphism: Differences

between the females and malesbetween the females and males ““It bestows upon the females a large, It bestows upon the females a large,

pink, pillowy buttocks “pink, pillowy buttocks “

Natural SelectionNatural Selection

Some members are more likely to Some members are more likely to survive and reproduce- thus their survive and reproduce- thus their genes will be passed on to the next genes will be passed on to the next generationgeneration

When Natural Selection is at work, When Natural Selection is at work, distribution of properties may change distribution of properties may change over time over time

Stabilizing SelectionStabilizing Selection

It’s Best to have the average traitsIt’s Best to have the average traits The ones that survive are let’s say, The ones that survive are let’s say,

“not the biggest , or the smallest”“not the biggest , or the smallest”

Types of SelectionTypes of Selection

StabilizingStabilizing

Disruptive SelectionDisruptive Selection

Being one extreme or another is Being one extreme or another is better than being averagebetter than being average

Directional SelectionDirectional Selection

One extreme form of a trait is better One extreme form of a trait is better for survivalfor survival

The story of the The story of the peppered moth peppered moth in England in England during the during the Industrial Industrial Revolution…Revolution…What type of What type of selection is selection is demonstrated demonstrated here?here?

Formation of SpeciesFormation of Species

Is called speciationIs called speciation Over time animals can change a lot, Over time animals can change a lot,

or a little, from ancient ancestorsor a little, from ancient ancestors

Morphological Concept of SpeciesMorphological Concept of Species

Says…species are animals that look Says…species are animals that look alikealike

Over time, we found that dissimilar Over time, we found that dissimilar animals bred and interacted in animals bred and interacted in nature so we had to change that..nature so we had to change that..

Biological Concept of SpeciesBiological Concept of Species

Species are members that can Species are members that can interbreed…interbreed…

Modern Concept of Species?Modern Concept of Species?

Look alikeLook alike And, can interbreedAnd, can interbreed

There will always be exceptions There will always be exceptions to that rule though….to that rule though….

How can new species arise?How can new species arise?

Geographic IsolationGeographic Isolation Allopatric SpeciationAllopatric Speciation Reproductive IsolationReproductive Isolation Sympatric SpeciationSympatric Speciation

Geographic IsolationGeographic Isolation Physical IsolationPhysical Isolation Gene flow between them stopsGene flow between them stops Genetic driftGenetic drift Mating cannot occur Mating cannot occur

Allopatric SpeciationAllopatric Speciation

2 different species due to geographic 2 different species due to geographic isolationisolation

Reproductive IsolationReproductive Isolation

Successful mating is preventedSuccessful mating is prevented Prezygotic- before fertilizationPrezygotic- before fertilization Post-zygotic- after fertilizationPost-zygotic- after fertilization

Reproductive IsolationReproductive Isolation Anatomical incompatibilityAnatomical incompatibility Small male dog and large female dog Small male dog and large female dog

cannot matecannot mate Prezygotic barriers- prevent matingPrezygotic barriers- prevent mating Post-zygotic barriers- mating occurs Post-zygotic barriers- mating occurs

but offspring is infertile but offspring is infertile

Pre-zygotic barriers-Pre-zygotic barriers-

Hmmmm…just think about it…Hmmmm…just think about it…

Post-zygotic barriers- mating occurs Post-zygotic barriers- mating occurs but offspring is infertile but offspring is infertile

Sympatric SpeciationSympatric Speciation

2 sub populations become 2 sub populations become reproductively isolated within the reproductively isolated within the same geographic area. same geographic area.

Rates of SpeciationRates of Speciation

Sometimes millions of years, Sometimes millions of years, sometimes a lot less (thousand) sometimes a lot less (thousand)

Theory – millions of years Theory – millions of years “gradualism”“gradualism”

Theory- no changes for a very long Theory- no changes for a very long time then a sudden burst- time then a sudden burst- “punctuated equibrium”“punctuated equibrium”

Which side, is which?Which side, is which?