15.3 Shaping Evolutionary Theory15.3 Shaping Evolutionary Theory

Main idea: Main idea: The theory of evolution continues to The 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 can Allele: alternative forms of a character trait that can

be inherited.be inherited.

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

Mechanisms of EvolutionMechanisms of Evolution

Darwin’s theory of natural selection Darwin’s theory of natural selection remainsremains the central theme of the central theme of evolutionevolutionScientists of today know that natural Scientists of today know that natural selection is not the only mechanism selection is not the only mechanism of evolutionof evolutionEvolution occurs at the Evolution occurs at the populationpopulation level, with genes as the level, with genes as the raw material.raw material.

Mechanisms of EvolutionMechanisms of Evolution

Hardy-Weinberg Principle states that when Hardy-Weinberg Principle states that when allelic frequencies remain constant, a population allelic frequencies remain constant, a population is in genetic is in genetic equilibriumequilibrium..

Population GeneticsPopulation Genetics

Even though the number of owls doubled, the Even though the number of owls doubled, the ratioratio of of gray to red owls remained the gray to red owls remained the same.same.

pp2 2 + 2pq + q+ 2pq + q2 2 = 1= 1

This equation allows us to determine the This equation allows us to determine the equilibriumequilibrium frequency of each frequency of each genotypegenotype in in the population: the population:

homozygoushomozygous dominant (p dominant (p22))

heterozygousheterozygous (2pq) (2pq)

homozygoushomozygous recessive (q recessive (q22))Note the sum of these frequencies equals Note the sum of these frequencies equals one.one.

Mechanisms of EvolutionMechanisms of EvolutionPopulation GeneticsPopulation Genetics

A population in genetic equilibrium must meet all A population in genetic equilibrium must meet all fivefive conditions conditions

1.1.The population is very The population is very largelarge

2.2.No No immigration immigration or or emigrationemigration

3.3.Mating is Mating is randomrandom

4.4.MutationsMutations do not occur (no new variations) do not occur (no new variations)

5.5.Natural selectionNatural selection does not occur does not occur

Population GeneticsPopulation Genetics

Mechanisms of EvolutionMechanisms of Evolution

Genetic Drift is a change in the allelic Genetic Drift is a change in the allelic frequencies in a population that is due to frequencies in a population that is due to chancechance..In In smallersmaller populations, the effects of populations, the effects of genetic drift becomes more genetic drift becomes more pronouncedpronounced, , and the chance of losing an allele and the chance of losing an allele becomes greater.becomes greater.Examples: Founder Effect & BottleneckExamples: Founder Effect & Bottleneck

Mechanisms of EvolutionMechanisms of EvolutionGenetic DriftGenetic Drift

Founder Effect Founder Effect occurs when a small occurs when a small sample of a population settles in a location sample of a population settles in a location separatedseparated from the rest of the from the rest of the populationpopulation

Genetic DriftGenetic Drift

Alleles that were Alleles that were uncommonuncommon in the in the original population original population might be might be commoncommon in the new in the new population.population.

Mechanisms of EvolutionMechanisms of Evolution

Bottleneck occurs Bottleneck occurs when a population when a population declinesdeclines to a very to a very low number and low number and then then reboundsrebounds

Mechanisms of EvolutionMechanisms of EvolutionGenetic DriftGenetic Drift

Can Can decrease decrease genetic genetic variety.variety.

A population in genetic equilibrium A population in genetic equilibrium experiences no experiences no gene flowgene flow,, however, few however, few populations are populations are isolatedisolated

RandomRandom movement of individuals between movement of individuals between populations populations reducesreduces differences between differences between populationspopulations

Mechanisms of EvolutionMechanisms of EvolutionGene FlowGene Flow

RarelyRarely is mating is mating completely random in a completely random in a population.population.Usually individuals mate Usually individuals mate with individuals in close with individuals in close proximityproximity..

Nonrandom matingNonrandom mating

Mechanisms of EvolutionMechanisms of Evolution

This promotes inbreeding and could lead to a This promotes inbreeding and could lead to a change in allelic proportions favoring individuals change in allelic proportions favoring individuals that are that are homozygoushomozygous for particular traits. for particular traits.

A mutation is a A mutation is a random random changechange in genetic in genetic material.material.Occasionally mutations Occasionally mutations provide an provide an advantageadvantage to to organisms and become organisms and become more common in more common in subsequent generations.subsequent generations.Basis upon which Basis upon which natural natural selectionselection works. works.

Mechanisms of EvolutionMechanisms of EvolutionMutationMutation

Select the individuals that are Select the individuals that are best adapted for best adapted for survivalsurvival and and reproductionreproduction

StabilizingStabilizing selection selection eliminate extreme eliminate extreme

expressionsexpressionsBirth WeightsBirth Weights

DirectionalDirectional selection selection makes organisms more fit.makes organisms more fit.

Peppered MothsPeppered MothsDisruptiveDisruptive selection selection

process that splits a process that splits a population into two groups. population into two groups.

Lake Erie Water Lake Erie Water SnakesSnakes

Mechanisms of EvolutionMechanisms of EvolutionNatural SelectionNatural Selection

Sexual selectionSexual selection operates in operates in populations where males and populations where males and females differ significantly in females differ significantly in appearanceappearance.. MalesMales are largest and most are largest and most

colorful in the groupcolorful in the group More likely target for More likely target for predatorspredators

yet bodies enhance reproductive yet bodies enhance reproductive successsuccess

Examples: Peacocks & DeerExamples: Peacocks & Deer

Natural SelectionNatural Selection

Mechanisms of EvolutionMechanisms of Evolution

preventsprevents reproduction reproduction by making fertilization by making fertilization unlikely.unlikely. Eastern and Western Eastern and Western

meadowlarks similar meadowlarks similar in appearance but in appearance but their mating songs their mating songs separate them separate them behaviorally.behaviorally.

Reproductive IsolationReproductive IsolationPrezygotic IsolationPrezygotic Isolation

Occurs when Occurs when fertilization has fertilization has occurredoccurred but a but a hybrid offspring hybrid offspring cannot develop cannot develop or reproduce.or reproduce.

Reproductive IsolationReproductive IsolationPostzygotic IsolationPostzygotic Isolation

A Liger is the offspring of a male lion A Liger is the offspring of a male lion and a female tiger. Ligers are and a female tiger. Ligers are sterile.sterile.

A A physicalphysical barrier divides one population into two or barrier divides one population into two or more populations.more populations.

SpeciationSpeciation

Allopatric speciationAllopatric speciation

For speciation to occur, a population must For speciation to occur, a population must divergediverge and then be reproductively and then be reproductively isolatedisolated..

Species evolves into a new species Species evolves into a new species withoutwithout a physical barriera physical barrier

SpeciationSpeciation

Sympatric speciationSympatric speciation

Apple maggot flies Apple maggot flies lay eggs on apples or lay eggs on apples or hawthorns depending hawthorns depending on the type of fruit on the type of fruit the grew up on.the grew up on.

For speciation to occur, a population must For speciation to occur, a population must diverge and then be reproductively isolated.diverge and then be reproductively isolated.

Can occur in a relatively Can occur in a relatively shortshort time when one time when one species gives rise to species gives rise to manymany different species different species in response to the creation of new in response to the creation of new habitathabitat or or some other ecological opportunity.some other ecological opportunity.

Patterns of EvolutionPatterns of EvolutionAdaptive Radiation (aka divergent evolution)Adaptive Radiation (aka divergent evolution)

Follows Follows large-scale large-scale extinctionextinction eventsevents Cichlid fish Cichlid fish

of Lake of Lake Victoria in Victoria in Africa.Africa.

The relationship between two species might be The relationship between two species might be so close that the evolution of one species so close that the evolution of one species affects affects 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

The plant evolves a The plant evolves a chemical defensechemical defense

The insects evolve the The insects evolve the biochemistry to biochemistry to resistresist the the defensedefenseResponse continues to escalate.Response continues to escalate.

Patterns of EvolutionPatterns of EvolutionCoevolutionCoevolution

Coevolutionary arms race – one species can Coevolutionary arms race – one species can evolve a parasitic evolve a parasitic dependencydependency on another on another species.species.

A plant and an insect that is dependent on A plant and an insect that is dependent on the plant for foodthe plant for food

UnrelatedUnrelated species species evolve similar traits evolve similar traits even though they live even though they live in in differentdifferent parts of parts of the world.the world.Occurs in Occurs in environments that are environments that are geographically far geographically far apart but have similar apart but have similar ecologyecology and and climate.climate.

Patterns of EvolutionPatterns of EvolutionConvergent EvolutionConvergent Evolution

Punctuated Punctuated equilibrium equilibrium explains explains rapidrapid spurts of genetic spurts of genetic change causing change causing species to species to diverge diverge quicklyquickly..

Rate of SpeciationRate of Speciation

Patterns of EvolutionPatterns of Evolution

Evolution proceeds in Evolution proceeds in smallsmall, , gradualgradual steps = steps = gradualism.gradualism.