MicroevolutionMicroevolution

Chapter 17Chapter 17

Selective Breeding & Selective Breeding & EvolutionEvolution

• Evolution is genetic change in a Evolution is genetic change in a line of descent through successive line of descent through successive generationsgenerations

• Selective breeding practices yield Selective breeding practices yield evidence that heritable changes do evidence that heritable changes do occuroccur

Domestication of DogsDomestication of Dogs

• Began about 50,000 years agoBegan about 50,000 years ago

• 14,000 years ago - artificial 14,000 years ago - artificial selectionselection– Dogs with desired forms of traits Dogs with desired forms of traits

were bredwere bred

• Modern breeds are the result Modern breeds are the result

Results of Artificial SelectionResults of Artificial Selection

• Extremes in sizeExtremes in size– Great Dane and Chihuahua Great Dane and Chihuahua

• Extremes in formExtremes in form– Short-legged dachshundsShort-legged dachshunds

– English bulldogEnglish bulldog

•Short snout and compressed faceShort snout and compressed face

•Extreme traits lead to health problemsExtreme traits lead to health problems

Evolutionary TheoriesEvolutionary Theories

• Widely used to interpret the Widely used to interpret the past and present, and even to past and present, and even to predict the futurepredict the future

• Reveal connections between the Reveal connections between the geological record, fossil record, geological record, fossil record, and organism diversityand organism diversity

Early Scientific TheoriesEarly Scientific Theories

• Hippocrates - All aspects of nature Hippocrates - All aspects of nature can be traced to their underlying can be traced to their underlying causescauses

• Aristotle - Each organism is distinct Aristotle - Each organism is distinct from all the rest and nature is a from all the rest and nature is a continuum or organizationcontinuum or organization

Confounding EvidenceConfounding Evidence

• BiogeographyBiogeography

• Comparative Comparative

anatomyanatomy

• Geologic discoveriesGeologic discoveries

BiogeographyBiogeography

• Size of the known world expanded Size of the known world expanded

enormously in the 15th centuryenormously in the 15th century

• Discovery of new organisms in Discovery of new organisms in

previously unknown places could not previously unknown places could not

be explained by accepted beliefsbe explained by accepted beliefs– How did species get from center of How did species get from center of

creation to all these places?creation to all these places?

Comparative MorphologyComparative Morphology

• Study of similarities and differences Study of similarities and differences in body plans of major groupsin body plans of major groups

• Puzzling patterns:Puzzling patterns:– Animals as different as whales and bats Animals as different as whales and bats

have similar bones in forelimbs have similar bones in forelimbs

– Some parts seem to have no function Some parts seem to have no function

Geological DiscoveriesGeological Discoveries

• Similar rock layers throughout worldSimilar rock layers throughout world

• Certain layers contain fossilsCertain layers contain fossils

• Deeper layers contain simpler fossils Deeper layers contain simpler fossils

than shallow layersthan shallow layers

• Some fossils seem to be related to Some fossils seem to be related to

known speciesknown species

19th Century - New Theories19th Century - New Theories

• Scientists attempt to reconcile Scientists attempt to reconcile evidence of change with traditional evidence of change with traditional belief in a single creation eventbelief in a single creation event

• Two examplesTwo examples– Georges Cuvier - multiple catastrophesGeorges Cuvier - multiple catastrophes– Jean Lamarck - inheritance of acquired Jean Lamarck - inheritance of acquired

characteristicscharacteristics

The Theory of UniformityThe Theory of Uniformity

• Lyell’s Lyell’s Principles of GeologyPrinciples of Geology

• Subtle, repetitive processes of Subtle, repetitive processes of change had shaped Earthchange had shaped Earth

• Challenged the view that Earth was Challenged the view that Earth was only 6,000 years oldonly 6,000 years old

Darwin’s VoyageDarwin’s Voyage

• At age 22, Charles Darwin began a At age 22, Charles Darwin began a five-year, round-the-world voyage five-year, round-the-world voyage aboard the aboard the BeagleBeagle

• In his role as ship’s naturalist, he In his role as ship’s naturalist, he collected and examined the species collected and examined the species that inhabited the regions the ship that inhabited the regions the ship visitedvisited

Voyage of the BeagleVoyage of the Beagle

EQUATOR

GalapagosIslands

Figure 17.4ePage 275

GalapagosGalapagosIslandsIslands

Isabela

Darwin

Wolf

Pinta

Marchena Genovesa

Fernandia

SantiagoBartolomé

RabidaPinzon

SeymourBaltra

Santa Cruz

Santa Fe

Tortuga

Española

San Cristobal

Floreana

Volcanic islands far off coast of Ecuador

All inhabitants are descended from species that arrived on islands from elsewhere

Figure 17.4dPage 275

Malthus - Struggle to Malthus - Struggle to SurviveSurvive

• Thomas Malthus, a clergyman and Thomas Malthus, a clergyman and economist, wrote essay that Darwin economist, wrote essay that Darwin read on his return to Englandread on his return to England

• Argued that as population size Argued that as population size increases, resources dwindle, the increases, resources dwindle, the struggle to live intensifies, and conflict struggle to live intensifies, and conflict increasesincreases

Galapagos FinchesGalapagos Finches

• Darwin observed finches with a Darwin observed finches with a variety of lifestyles and body formsvariety of lifestyles and body forms

• On his return, he learned that there On his return, he learned that there were 13 specieswere 13 species

• He attempted to correlate variations He attempted to correlate variations in their traits with environmental in their traits with environmental challengeschallenges

Darwin’s TheoryDarwin’s Theory

A population can change over time A population can change over time

when individuals differ in one or when individuals differ in one or

more heritable traits that are more heritable traits that are

responsible for differences in the responsible for differences in the

ability to survive and reproduce.ability to survive and reproduce.

Alfred WallaceAlfred Wallace

• Naturalist who arrived at the same Naturalist who arrived at the same

conclusions Darwin didconclusions Darwin did

• Wrote to Darwin describing his viewsWrote to Darwin describing his views

• Prompted Darwin to finally present Prompted Darwin to finally present

his ideas in a formal paper his ideas in a formal paper

Populations EvolvePopulations Evolve

• Biological evolution does not Biological evolution does not change individualschange individuals

• It changes a populationIt changes a population

• Traits in a population vary among Traits in a population vary among individualsindividuals

• Evolution is change in frequency Evolution is change in frequency of traitsof traits

The Gene PoolThe Gene Pool

• All of the genes in the population All of the genes in the population

• Genetic resource that is shared (in Genetic resource that is shared (in theory) by all members of theory) by all members of populationpopulation

Variation in PhenotypeVariation in Phenotype

• Each kind of gene in gene pool Each kind of gene in gene pool may have two or more allelesmay have two or more alleles

• Individuals inherit different allele Individuals inherit different allele combinationscombinations

• This leads to variation in This leads to variation in phenotypephenotype

• Offspring inherit genes, not Offspring inherit genes, not phenotypesphenotypes

What Determines Alleles in What Determines Alleles in New Individual? New Individual?

• Mutation Mutation

• Crossing over at meiosis ICrossing over at meiosis I

• Independent assortmentIndependent assortment

• FertilizationFertilization

• Change in chromosome Change in chromosome

number or structurenumber or structure

Genetic EquilibriumGenetic Equilibrium

• Allele frequencies at a Allele frequencies at a

locus are not changinglocus are not changing

• Population is not Population is not

evolvingevolving

Five Conditions Five Conditions

• No mutationNo mutation

• Random matingRandom mating

• Gene doesn’t affect survival Gene doesn’t affect survival

or reproductionor reproduction

• Large populationLarge population

• No immigration/emigrationNo immigration/emigration

Microevolutionary ProcessesMicroevolutionary Processes

• Drive a population away from Drive a population away from genetic equilibriumgenetic equilibrium

• Small-scale changes in allele Small-scale changes in allele frequencies brought about by:frequencies brought about by:– Natural selectionNatural selection

– Gene flowGene flow

– Genetic driftGenetic drift

Gene MutationsGene Mutations

• Infrequent but inevitableInfrequent but inevitable

• Each gene has own mutation Each gene has own mutation raterate

• Lethal mutations Lethal mutations

• Neutral mutationsNeutral mutations

• Advantageous mutationsAdvantageous mutations

Hardy-Weinberg RuleHardy-Weinberg Rule

At genetic equilibrium, proportions At genetic equilibrium, proportions of genotypes at a locus with two of genotypes at a locus with two alleles are given by the equation:alleles are given by the equation:

pp22 AAAA + 2 + 2pq Aapq Aa + + qq22 aaaa = 1 = 1

Frequency of allele Frequency of allele A A = = pp

Frequency of allele Frequency of allele aa = = qq

Punnett SquarePunnett Square

AA(p2) Aa(pq)

Aa(pq) aa(q2)

Ap aq

Ap

aq

In-text figurePage 280

Conditions for Hardy-Conditions for Hardy-WeinbergWeinberg• Single gene , there can be no sex-Single gene , there can be no sex-

linkage or mutliple alleleslinkage or mutliple alleles• Mating must be randomMating must be random• No migration into or out of populationNo migration into or out of population• No gene changes through mutationsNo gene changes through mutations• All genotypes must be viable, survive All genotypes must be viable, survive

and produce the same number of and produce the same number of offspringoffspring

• Population must be of infinite sizePopulation must be of infinite size

Frequencies in GametesFrequencies in Gametes

A A A a a a

0.49 AA 0.42 Aa 0.09 aa

0.49 + 0.21 0.21 + 0.09

0.7A 0.3a

F1 genotypes:

Gametes:

In-text In-text figurefigurePage 280Page 280

No Change No Change through through GenerationGenerationss

STARTING POPULATION

490 AA butterfliesDark-blue wings

420 Aa butterfliesMedium-blue wings

90 aa butterfliesWhite wings

490 AA butterflies

THE NEXT GENERATION

420 Aa butterflies

90 aa butterflies

THE NEXT GENERATION

490 AA butterflies

420 Aa butterflies

90 aa butterflies

NO CHANGE

NO CHANGEFigure 17.9Figure 17.9Page 281Page 281

Natural SelectionNatural Selection

• A difference in the survival and A difference in the survival and

reproductive success of different reproductive success of different

phenotypesphenotypes

• Acts directly on phenotypes and Acts directly on phenotypes and

indirectly on genotypesindirectly on genotypes

Reproductive Capacity Reproductive Capacity & Competition& Competition

• All populations have the capacity All populations have the capacity to increase in numbersto increase in numbers

• No population can increase No population can increase indefinitelyindefinitely

• Eventually the individuals of a Eventually the individuals of a population will end up competing population will end up competing for resourcesfor resources

Variation in PopulationsVariation in Populations

• All individuals have the same genes All individuals have the same genes that specify the same assortment of that specify the same assortment of traitstraits

• Most genes occur in different forms Most genes occur in different forms (alleles) that produce different (alleles) that produce different phenotypesphenotypes

• Some phenotypes compete better Some phenotypes compete better than othersthan others

Change over TimeChange over Time

• Over time, the alleles that produce Over time, the alleles that produce the most successful phenotypes will the most successful phenotypes will increase in the populationincrease in the population

• Less successful alleles will become Less successful alleles will become less commonless common

• Change leads to increased fitnessChange leads to increased fitness– Increased adaptation to environmentIncreased adaptation to environment

Results of Natural SelectionResults of Natural Selection

Three possible outcomes:Three possible outcomes:

• A shift in the range of values for a A shift in the range of values for a given trait in some directiongiven trait in some direction

• Stabilization of an existing range of Stabilization of an existing range of valuesvalues

• Disruption of an existing range of Disruption of an existing range of valuesvalues

Directional Directional SelectionSelection

• Allele Allele frequencies frequencies shift in one shift in one directiondirection

Nu

mb

er o

f in

div

idu

als

in t

he

po

pu

lati

on

Range of values for the trait at time 1

Range of values for the trait at time 2

Range of values for the trait at time 3

Nu

mb

er o

f in

div

idu

als

in t

he

po

pu

lati

on

Nu

mb

er o

f in

div

idu

als

in t

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po

pu

lati

on

Figure 17.10Page 282

Peppered MothsPeppered Moths

• Prior to industrial revolution, Prior to industrial revolution, most common phenotype was most common phenotype was light coloredlight colored

• After industrial revolution, dark After industrial revolution, dark phenotype became more phenotype became more commoncommon

Pesticide ResistancePesticide Resistance

• Pesticides kill susceptible Pesticides kill susceptible insectsinsects

• Resistant insects survive and Resistant insects survive and reproducereproduce

• If resistance has heritable basis, If resistance has heritable basis, it becomes more common with it becomes more common with each generation each generation

Antibiotic ResistanceAntibiotic Resistance

• First came into use in the 1940sFirst came into use in the 1940s

• Overuse has led to increase in Overuse has led to increase in resistant formsresistant forms

• Most susceptible cells died out Most susceptible cells died out and were replaced by resistant and were replaced by resistant formsforms

Stabilizing Stabilizing SelectionSelection

• Intermediate Intermediate forms are forms are favored and favored and extremes are extremes are eliminatedeliminated

Nu

mb

er o

f in

div

idu

als

in t

he

po

pu

lati

on

Range of values for the trait at time 1

Range of values for the trait at time 2

Range of values for the trait at time 3Figure 17.12Page 284

Selection for Gall SizeSelection for Gall Size

• Gall-making fly has two major Gall-making fly has two major predatorspredators

• Wasps prey on larvae in small Wasps prey on larvae in small gallsgalls

• Birds eat larvae in large gallsBirds eat larvae in large galls

• Flies that cause intermediate-Flies that cause intermediate-sized galls have the highest sized galls have the highest fitnessfitness

Disruptive Disruptive SelectionSelection

• Forms at both Forms at both ends of the ends of the range of range of variation are variation are favored favored

• Intermediate Intermediate forms are forms are selected againstselected against

Nu

mb

er o

f in

div

idu

als

in t

he

po

pu

lati

on

Range of values for the trait at time 1

Range of values for the trait at time 2

Range of values for the trait at time 3

Nu

mb

er o

f in

div

idu

als

in t

he

po

pu

lati

on

Nu

mb

er o

f in

div

idu

als

in t

he

po

pu

lati

on

Figure 17.14Page 285

African FinchesAfrican Finches

• Selection favors Selection favors birds with very birds with very large or very large or very small billssmall bills

• Birds with Birds with intermediate-intermediate-sized bill are less sized bill are less effective feederseffective feeders

10

20

30

40

50

60

Nu

mb

er o

f in

div

idu

als

10 12.8 15.7 18.5

Widest part of lower bill(millimeters)

nestlings

drought survivors

Figure 17.15Page 285

Sexual SelectionSexual Selection

• Selection favors certain Selection favors certain secondary sexual characteristicssecondary sexual characteristics

• Through nonrandom mating, Through nonrandom mating, alleles for preferred traits alleles for preferred traits increaseincrease

• Leads to increased sexual Leads to increased sexual dimorphismdimorphism

Balanced PolymorphismBalanced Polymorphism

• Polymorphism - “having Polymorphism - “having many forms”many forms”

• Occurs when two or more Occurs when two or more alleles are maintained at alleles are maintained at frequencies greater than 1 frequencies greater than 1 percentpercent

Sickle-Cell Trait: Sickle-Cell Trait: Heterozygote AdvantageHeterozygote Advantage

• Allele Allele HbHbSS causes causes sickle-cell anemia sickle-cell anemia when heterozygous when heterozygous

• Heterozygotes are Heterozygotes are more resistant to more resistant to malaria than malaria than homozygoteshomozygotes

less than 1 in 1,600

1 in 400-1,600

1 in 180-400

1 in 100-180

1 in 64-100

more than 1 in 64

Malaria case

Sickle-cell trait

Figure 17.17Page 286-287

Gene FlowGene Flow

• Physical flow of alleles into a Physical flow of alleles into a populationpopulation

• Tends to keep the gene pools of Tends to keep the gene pools of populations similarpopulations similar

• Counters the differences that result Counters the differences that result from mutation, natural selection, from mutation, natural selection, and genetic driftand genetic drift

Genetic DriftGenetic Drift

• Random change in allele frequencies Random change in allele frequencies brought about by chancebrought about by chance

• Effect is most pronounced in small Effect is most pronounced in small populationspopulations

• Sampling error - Fewer times an Sampling error - Fewer times an event occurs, greater the variance in event occurs, greater the variance in outcomeoutcome

BottleneckBottleneck

• A severe reduction in population sizeA severe reduction in population size

• Causes pronounced driftCauses pronounced drift

• Example Example – Elephant seal population hunted down Elephant seal population hunted down

to just 20 individuals to just 20 individuals – Population rebounded to 30,000Population rebounded to 30,000– Electrophoresis revealed there is now no Electrophoresis revealed there is now no

allele variation at 24 genesallele variation at 24 genes

Founder EffectFounder Effect

• Effect of drift when a small number Effect of drift when a small number of individuals starts a new populationof individuals starts a new population

• By chance, allele frequencies of By chance, allele frequencies of founders may not be same as those founders may not be same as those in original populationin original population

• Effect is pronounced on isolated Effect is pronounced on isolated islandsislands

Inbreeding Inbreeding

• Nonrandom mating between related Nonrandom mating between related individualsindividuals

• Leads to increased homozygosityLeads to increased homozygosity

• Can lower fitness when deleterious Can lower fitness when deleterious recessive alleles are expressedrecessive alleles are expressed

• Amish, cheetahsAmish, cheetahs