The document discusses several ways that the theory of evolution continues to be shaped, including: 1) Scientists now recognize that natural selection is not the only mechanism of evolution, with genetic drift, gene flow, mutation, and non-random mating also influencing changes within populations over time. 2) Two theories for the rate of speciation discussed are gradualism, where evolution proceeds in small gradual steps, and punctuated equilibrium, where species diverge rapidly during sporadic periods of genetic change. 3) Various patterns of evolution are examined, including adaptive radiation, coevolution, convergent evolution, and factors that can influence speciation like genetic isolation and the formation of new habitats.

1. 15.3 Shaping Evolutionary Theory15.3 Shaping Evolutionary Theory
Main idea:Main idea: The theory of evolution continues toThe theory of evolution continues to
be refined as scientists learn new information.be refined as scientists learn new information.
Objectives:Objectives:

Discuss patterns observed in evolution.Discuss patterns observed in evolution.

Describe factors that influence speciation.Describe factors that influence speciation.

Compare gradualism with punctuated equilibrium.Compare gradualism with punctuated equilibrium.
Review VocabularyReview Vocabulary

Allele: alternative forms of a character trait that canAllele: alternative forms of a character trait that can
be inherited.be inherited.

2. 15.3 Shaping Evolutionary Theory15.3 Shaping Evolutionary Theory
Hardy-Weinberg PrincipleHardy-Weinberg Principle
Genetic driftGenetic drift
Founder effectFounder effect
BottleneckBottleneck
Punctuated EquilibriumPunctuated Equilibrium
Sexual selectionSexual selection
New Vocabulary

3. Mechanisms of EvolutionMechanisms of Evolution
Darwin’s theory of natural selectionDarwin’s theory of natural selection
remainsremains the central theme ofthe central theme of
evolutionevolution
Scientists of today know that naturalScientists of today know that natural
selection is not the only mechanismselection is not the only mechanism
of evolutionof evolution
Evolution occurs at theEvolution occurs at the populationpopulation
level, with genes as thelevel, with genes as the raw material.raw material.

4. Mechanisms of EvolutionMechanisms of Evolution
Hardy-Weinberg Principle states that whenHardy-Weinberg Principle states that when
allelic frequencies remain constant, a populationallelic frequencies remain constant, a population
is in geneticis in genetic equilibriumequilibrium..
Population GeneticsPopulation Genetics

Even though the number of owls doubled, theEven though the number of owls doubled, the ratioratio ofof
gray to red owls remained thegray to red owls remained the same.same.

5. pp22
+ 2pq + q+ 2pq + q22
= 1= 1
This equation allows us to determine theThis equation allows us to determine the
equilibriumequilibrium frequency of eachfrequency of each genotypegenotype inin
the population:the population:
homozygoushomozygous dominant (pdominant (p22
))
heterozygousheterozygous (2pq)(2pq)
homozygoushomozygous recessive (qrecessive (q22
))
Note the sum of these frequencies equalsNote the sum of these frequencies equals one.one.
Mechanisms of EvolutionMechanisms of Evolution
Population GeneticsPopulation Genetics

6. A population in genetic equilibrium must meet allA population in genetic equilibrium must meet all
fivefive conditionsconditions
1.1.The population is veryThe population is very largelarge
2.2.NoNo immigrationimmigration oror emigrationemigration
3.3.Mating isMating is randomrandom
4.4.MutationsMutations do not occur (no new variations)do not occur (no new variations)
5.5.Natural selectionNatural selection does not occurdoes not occur
Population GeneticsPopulation Genetics
Mechanisms of EvolutionMechanisms of Evolution

7. Genetic Drift is a change in the allelicGenetic Drift is a change in the allelic
frequencies in a population that is due tofrequencies in a population that is due to
chancechance..
InIn smallersmaller populations, the effects ofpopulations, the effects of
genetic drift becomes moregenetic drift becomes more pronouncedpronounced,,
and the chance of losing an alleleand the chance of losing an allele
becomes greater.becomes greater.
Examples: Founder Effect & BottleneckExamples: Founder Effect & Bottleneck
Mechanisms of EvolutionMechanisms of Evolution
Genetic DriftGenetic Drift

8. Founder EffectFounder Effect occurs when a smalloccurs when a small
sample of a population settles in a locationsample of a population settles in a location
separatedseparated from the rest of thefrom the rest of the populationpopulation
Genetic DriftGenetic Drift
Alleles that wereAlleles that were
uncommonuncommon in thein the
original populationoriginal population
might bemight be commoncommon
in the newin the new
population.population.
Mechanisms of EvolutionMechanisms of Evolution

9. Bottleneck occursBottleneck occurs
when a populationwhen a population
declinesdeclines to a veryto a very
low number andlow number and
thenthen reboundsrebounds
Mechanisms of EvolutionMechanisms of Evolution
Genetic DriftGenetic Drift

CanCan decreasedecrease
geneticgenetic variety.variety.

10. A population in genetic equilibriumA population in genetic equilibrium
experiences noexperiences no gene flowgene flow,, however, fewhowever, few
populations arepopulations are isolatedisolated
RandomRandom movement of individuals betweenmovement of individuals between
populationspopulations reducesreduces differences betweendifferences between
populationspopulations
Mechanisms of EvolutionMechanisms of Evolution
Gene FlowGene Flow

11. RarelyRarely is matingis mating
completely random in acompletely random in a
population.population.
Usually individuals mateUsually individuals mate
with individuals in closewith individuals in close
proximityproximity..
Nonrandom matingNonrandom mating
Mechanisms of EvolutionMechanisms of Evolution
This promotes inbreeding and could lead to aThis promotes inbreeding and could lead to a
change in allelic proportions favoring individualschange in allelic proportions favoring individuals
that arethat are homozygoushomozygous for particular traits.for particular traits.

12. A mutation is aA mutation is a randomrandom
changechange in geneticin genetic
material.material.
Occasionally mutationsOccasionally mutations
provide anprovide an advantageadvantage toto
organisms and becomeorganisms and become
more common inmore common in
subsequent generations.subsequent generations.
Basis upon whichBasis upon which naturalnatural
selectionselection works.works.
Mechanisms of EvolutionMechanisms of Evolution
MutationMutation

13. Select the individuals that areSelect the individuals that are
best adapted forbest adapted for survivalsurvival andand
reproductionreproduction
StabilizingStabilizing selectionselection

eliminate extremeeliminate extreme
expressionsexpressions
Birth WeightsBirth Weights
DirectionalDirectional selectionselection

makes organisms more fit.makes organisms more fit.
Peppered MothsPeppered Moths
DisruptiveDisruptive selectionselection

process that splits aprocess that splits a
population into two groups.population into two groups.
Lake Erie WaterLake Erie Water
SnakesSnakes
Mechanisms of EvolutionMechanisms of Evolution
Natural SelectionNatural Selection

14. Sexual selectionSexual selection operates inoperates in
populations where males andpopulations where males and
females differ significantly infemales differ significantly in
appearanceappearance..

MalesMales are largest and mostare largest and most
colorful in the groupcolorful in the group

More likely target forMore likely target for predatorspredators
yet bodies enhance reproductiveyet bodies enhance reproductive
successsuccess
Examples: Peacocks & DeerExamples: Peacocks & Deer
Natural SelectionNatural Selection
Mechanisms of EvolutionMechanisms of Evolution

15. preventsprevents reproductionreproduction
by making fertilizationby making fertilization
unlikely.unlikely.

Eastern and WesternEastern and Western
meadowlarks similarmeadowlarks similar
in appearance butin appearance but
their mating songstheir mating songs
separate themseparate them
behaviorally.behaviorally.
Reproductive IsolationReproductive Isolation
Prezygotic IsolationPrezygotic Isolation

16. Occurs whenOccurs when
fertilization hasfertilization has
occurredoccurred but abut a
hybrid offspringhybrid offspring
cannot developcannot develop
or reproduce.or reproduce.
Reproductive IsolationReproductive Isolation
Postzygotic IsolationPostzygotic Isolation

A Liger is the offspring of a male lionA Liger is the offspring of a male lion
and a female tiger. Ligers areand a female tiger. Ligers are sterile.sterile.

17. AA physicalphysical barrier divides one populationbarrier divides one population
into two or more populations.into two or more populations.
SpeciationSpeciation
Allopatric speciationAllopatric speciation
For speciation to occur, a population mustFor speciation to occur, a population must
divergediverge and then be reproductivelyand then be reproductively isolatedisolated..

18. Species evolves into a new speciesSpecies evolves into a new species
withoutwithout a physical barriera physical barrier
SpeciationSpeciation
Sympatric speciationSympatric speciation

Apple maggot fliesApple maggot flies
lay eggs on apples orlay eggs on apples or
hawthorns dependinghawthorns depending
on the type of fruiton the type of fruit
the grew up on.the grew up on.
For speciation to occur, a population mustFor speciation to occur, a population must
diverge and then be reproductively isolated.diverge and then be reproductively isolated.

19. Can occur in a relativelyCan occur in a relatively shortshort time when onetime when one
species gives rise tospecies gives rise to manymany different speciesdifferent species
in response to the creation of newin response to the creation of new habitathabitat oror
some other ecological opportunity.some other ecological opportunity.
Patterns of EvolutionPatterns of Evolution
Adaptive Radiation (aka divergent evolution)Adaptive Radiation (aka divergent evolution)
FollowsFollows
large-scalelarge-scale
extinctionextinction
eventsevents

Cichlid fishCichlid fish
of Lakeof Lake
Victoria inVictoria in
Africa.Africa.

20. The relationship between two species might beThe relationship between two species might be
so close that the evolution of one speciesso close that the evolution of one species affectsaffects
the evolutionthe evolution of the other species.of the other species.
MutualismMutualism – occurs when two species benefit– occurs when two species benefit
each other.each other.
CoevolutionCoevolution
Patterns of EvolutionPatterns of Evolution

21. The plant evolves aThe plant evolves a
chemical defensechemical defense
The insects evolve theThe insects evolve the
biochemistry tobiochemistry to resistresist thethe
defensedefense
Response continues to escalate.Response continues to escalate.
Patterns of EvolutionPatterns of Evolution
CoevolutionCoevolution
Coevolutionary arms race – one species canCoevolutionary arms race – one species can
evolve a parasiticevolve a parasitic dependencydependency on anotheron another
species.species.
A plant and an insect that is dependent onA plant and an insect that is dependent on
the plant for foodthe plant for food

22. UnrelatedUnrelated speciesspecies
evolve similar traitsevolve similar traits
even though they liveeven though they live
inin differentdifferent parts ofparts of
the world.the world.
Occurs inOccurs in
environments that areenvironments that are
geographically fargeographically far
apart but have similarapart but have similar
ecologyecology andand climate.climate.
Patterns of EvolutionPatterns of Evolution
Convergent EvolutionConvergent Evolution

23. PunctuatedPunctuated
equilibriumequilibrium
explainsexplains rapidrapid
spurts of geneticspurts of genetic
change causingchange causing
species tospecies to
divergediverge quicklyquickly..
Rate of SpeciationRate of Speciation
Patterns of EvolutionPatterns of Evolution
Evolution proceeds inEvolution proceeds in smallsmall,, gradualgradual steps =steps =
gradualism.gradualism.