Genes and Variation

Chapter 16Chapter 16

Section 1Section 1

Darwin’s Ideas Revisited

No doubt, there is No doubt, there is variationvariationamong individuals of the among individuals of the samesame species species

Many of these differences Many of these differences are are inheritableinheritable

Such differences provide Such differences provide the the raw materialraw material for for natural selectionnatural selection

Darwin’s Ideas Revisited Differences that help organisms Differences that help organisms survivesurvive

and and reproducereproduce in their in their environmentenvironment become more become more commoncommon

Differences that are Differences that are notnot as beneficial as beneficialbecome become lessless common common

SpeedSpeed

Preferred Differences/Variations Become Adaptations

Cold ResistanceCold Resistance

Preferred Differences/Variations Become Adaptations

Intelligence Intelligence

Preferred Differences/Variations Become Adaptations

Mimicry Mimicry

Preferred Differences/Variations Become Adaptations

Heat ResistanceHeat Resistance

Preferred Differences/Variations Become Adaptations

Attractiveness to a MateAttractiveness to a Mate

Preferred Differences/Variations Become Adaptations

Darwin’s Ideas Revisited

Darwin developed his theory in 1836 with a Darwin developed his theory in 1836 with a serious disadvantageserious disadvantage Did not know how Did not know how heredityheredity

workedworked He was unable to He was unable to explain explain

two important factorstwo important factors What was the What was the source of variationsource of variation

(which was so central to his theory)(which was so central to his theory) How were How were inheritable inheritable

traitstraits passed from one passed from onegeneration to the nextgeneration to the next

Mendel’s work on Mendel’s work on inheritanceinheritance was published was published in the 1860’s in the 1860’s Did not become Did not become importantimportant until the 1900’s until the 1900’s

Darwin’s Ideas Revisited

1910 1910 Biologists realize that Biologists realize that genesgenes determine determine traits traits

CombineCombine Mendel Mendel and Darwin’s workand Darwin’s work

Today, Today, geneticsgenetics and and evolutionary theoryevolutionary theory work together work together to explain…to explain…

Darwin’s Ideas Revisited

How inheritable How inheritable variation variation appearsappears

How natural selection How natural selection operatesoperates on that variation on that variation

This more completely This more completely explains explains (than Darwin ever could)(than Darwin ever could) how evolution takes placehow evolution takes place

Darwin’s Ideas Revisited

Gene Pools

Combined Combined genetic informationgenetic information of all the of all the members of a particular populationmembers of a particular population

All of the All of the genesgenes for all of the for all of the traitstraits in a in a population population 

The sum The sum totaltotal of of all of the all of the allelesalleles

Gene Pools

Remember a population isRemember a population is A A groupgroup of individuals of the same of individuals of the same speciesspecies, in a , in a

given given areaarea, that make up a , that make up a breedingbreeding group group

Not twoNot twodifferentdifferentspeciesspeciesmixing mixing genesgenesfrom from twotwo

differentdifferentpoolspools

throughthroughartificialartificialselectionselection

(Selective (Selective Breeding)Breeding)

Recall….individuals of the same species Recall….individuals of the same species share share a common a common gene poolgene pool

Gene Pools

As genetic As genetic change change

occurs in one individual, occurs in one individual,

it can it can spreadspread through through

the population (the population (gene poolgene pool))

via its offspringvia its offspring

Gene Pools

Relative Frequency

Relative Frequency: the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

**Evolution is any change in the relative frequency of alleles in a population

MigrationMigration MovementMovement of individuals into ( of individuals into (immigrationimmigration))

and out of (and out of (emigrationemigration) a population) a population Causes a Causes a changechange in the gene pool in the gene pool

Mechanisms Affecting Gene Pools

IsolationIsolation One or more One or more portionsportions of a population are of a population are cut offcut off

from the restfrom the rest This This splitssplits a gene pool into a gene pool into smallersmaller pools pools Causes a change in the Causes a change in the numbernumber and and

frequencyfrequency of alleles of alleles in a gene pool in a gene pool

Mechanisms Affecting Gene Pools

Non-Random MatingNon-Random Mating Selection Selection of a mate causes preference for a of a mate causes preference for a

phenotypephenotype Certain genes/alleles are under Certain genes/alleles are under strong strong selection selection

pressurepressure The The alleleallele for this for this

trait becomes trait becomes more more prevalentprevalent in the gene pool in the gene pool

Mechanisms Affecting Gene Pools

Environmental Change (Affecting a Trait)Environmental Change (Affecting a Trait) Natural Natural successionsuccession Natural Natural disastersdisasters

(floods, volcanic eruption, earthquake, landslide)(floods, volcanic eruption, earthquake, landslide) ClimateClimate changes changes

(precipitation, temperature, air quality)(precipitation, temperature, air quality) DiseaseDisease Loss of Loss of habitathabitat

(deforestation, dams, urban sprawl, pollution) (deforestation, dams, urban sprawl, pollution) Invasive Invasive speciesspecies IndustrializationIndustrialization

Mechanisms Affecting Gene Pools

Peppered Moth

Lighter colored mothsLighter colored mothsblended better onblended better onlight colored treeslight colored trees

Industrial Industrial RevolutionRevolutionand productionand productionof smog…of smog…

Peppered Moth

Darkened the Darkened the trees….trees….

So dark coloredSo dark coloredmoths blendedmoths blendedbetterbetter

Peppered Moth Genetic Frequency

Year Year % of Light Moths % of Dark Moths% of Light Moths % of Dark Moths 1 90 101 90 10 2 83 172 83 17 3 3 71 29 71 29 4 4 64 36 64 36 5 5 48 52 48 52 6 6 39 61 39 61 7 7 33 67 33 67 8 8 22 78 22 78 9 9 12 88 12 88 10 10 8 92 8 92

Genetic Drift-Genetic Drift- Random Random changechange in allele frequencies in allele frequencies

Mechanisms Affecting Gene Pools

Sample of Original Population

Genetic Drift-Genetic Drift- Occurs in Occurs in smallsmall populations populations

Mechanisms Affecting Gene Pools

Sample of Original Population Founding Population A

Founding Population B

Genetic Drift-Genetic Drift- An allele becomes An allele becomes more or lessmore or less common common

Mechanisms Affecting Gene Pools

Sample of Original Population Founding Population A

Founding Population B

Descendants

Sources of Genetic Variation

In order for In order for variationvariation to occur there must be to occur there must be a way for genes to a way for genes to changechange

Sources of Genetic Variation

**The two main sources of genetic variation are mutation and the genetic shuffling that results from sexual reproduction

Sources of Genetic Variation

Most heritable differences are due to gene shuffling that occurs during the production of gametes Humans: 23 pairs of chromosomes

8.4 million different combination of genes

Crossing-over During meiosis Produce different phenotypes and genotypes

Mutations

Mutation- any Mutation- any changechange in a sequence of in a sequence of DNADNA The The ultimateultimate source of genetic source of genetic variabilityvariability Occur because ofOccur because of

Mistakes during Mistakes during replicationreplication RadiationRadiation exposure exposure Chemical Chemical exposureexposure

Mutations

A mutation that A mutation that changeschanges an organism’s an organism’s phenotype- (phenotype- (physicalphysical, behavioral, , behavioral, and/or biochemical characteristics)and/or biochemical characteristics)can affect it’s can affect it’s fitnessfitness- (ability - (ability to to survivesurvive and and reproduce)reproduce)

Mutations Bad

Some mutations are bad Some mutations are bad i.e. Albino coloring in snakes, deer, rabbits, etc. i.e. Albino coloring in snakes, deer, rabbits, etc.

makes it easier to be eaten by predatorsmakes it easier to be eaten by predators

Mutations Good

Some mutations are good Some mutations are good i.e. longer legs in deer allow them to run fasteri.e. longer legs in deer allow them to run faster Albino color can be good in snowy environmentsAlbino color can be good in snowy environments

Mutations: Good or Bad?What determines if a mutation is helpful?What determines if a mutation is helpful?

Truths about mutations:    Truths about mutations:    * Most are minor. * Most are minor. * Many are harmful. * Many are harmful. * Some are lethal. * Some are lethal. * Very few are helpful. * Very few are helpful.

Mutations: Good or Bad?

Normal

Mutations: Good or Bad?

Mutations: Good or Bad?

Mutations: Good or Bad?

Mutations: Good or Bad?

Mutations: Good or Bad?

Mutations: Good or Bad?

Mutations: Good or Bad?

Gene Shuffling

The The mixingmixing or reordering of the arrangement or reordering of the arrangement of of allelesalleles during gamete production by during gamete production by

Independent Independent AssortmentAssortment during Meiosis during Meiosis 23 23 pair of chromosomes can produce 8.4 million pair of chromosomes can produce 8.4 million

different combinations of genesdifferent combinations of genes CrossingCrossing over further mixes over further mixes

alleles alleles

Gene Shuffling

During sexual reproduction, alleles are During sexual reproduction, alleles are recombinedrecombined This This producesproduces dramatically different dramatically different phenotypesphenotypes It is a major It is a major sourcesource of of

variation within many variation within many populationspopulations

DoesDoes notnot change the change the relative relative frequencyfrequency of of each type of allele in each type of allele in a a populationpopulation

Gene Shuffling

Like shuffling a deck of cardsLike shuffling a deck of cards If each card represents an allele in a gene poolIf each card represents an allele in a gene pool Shuffling leads to Shuffling leads to

different hands different hands (combination (combination of alleles) of alleles)

But does notBut does notchange the relativechange the relativenumber (frequency)number (frequency)of 2’s, 3’s, 4’s etcof 2’s, 3’s, 4’s etc

Single-Gene Traits

**Natural Selection on a Single-gene traits can lead to changes in allele frequencies and thus to evolution

Single-Gene and Polygenic Traits

**The number of phenotypes produced for a given trait depends on how many genes control the trait.

Single-gene trait: a trait that is controlled by a single gene with two alleles

Single-Gene and Polygenic Traits

Phenotypic Ratios are determined by: Frequency of alleles in the population Alleles are dominant or recessive

Gene Expression

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Phenotype

The number of The number of phenotypesphenotypes produced produced for a given trait depends on for a given trait depends on

How many How many genesgenes control control that traitthat trait A single-geneA single-gene

traittrait 2 2 allelesalleles 22 phenotypes phenotypes

Gene Expression

A polygenic traitA polygenic trait 2 or more 2 or more genesgenes Each gene with Each gene with

2 or more 2 or more allelesalleles Many possible Many possible

genotypesgenotypes Even more Even more

possiblepossiblephenotypesphenotypes

Fre

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Phenotype (height)

Natural Selection on Polygenic Traits

How does natural selection affect Polygenic traits?

Natural selection can affect the distributions of phenotypes in three ways: 1. Directional Selection

2. Stabilizing Selection

3. Disruptive Selection

Directional Selection

When individuals at one end of the curve have higher fitness than individuals in the middle or other end

Stabilizing Selection

When the individuals near the center of the curve have higher fitness than individuals at either end of the curve.

Disruptive Selection

When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle Creates two distinct phenotypes

What are the Patterns of Evolution?

Patterns of Evolution

Divergent Evolution: (Adaptive radiation): One species or a small group of species evolves into several different forms that become less and less like each other as time passes.

Patterns of Evolution

Convergent Evolution: process by which unrelated organisms come to resemble one another. Undergo Adaptive Radiation in:

Different places Different Times Similar environments

Parallel Evolution

Parallel Evolution: two related species evolve in much the same way for long periods of time, probably in response to similar environments and selection pressure

Patterns of Evolution

Coevolution: Two species evolve in response to changes in each other over time Ex. Flowering plant and its pollinator

How do different species form?

Species Formation

Geographic Isolation: Physical separation and isolation of species caused by:

Land bridges Earth quakes Volcanic eruptions Ice caps Fire

Species Formation

Reproductive Isolation: Different populations cannot interbreed and produce fertile offspring Different mating location Mating time Different rituals

Species Formation

Behavioral Isolation: Different population are capable of interbreeding, but they do not due to difference in their:

Courtship rituals Other sexual behaviors

Species Formation

Temporal Isolation: occurs when two or more species reproduce at different times.

Extinction End of a species More than 99% of all species that ever lived

are extinct. Mass Extinction: When more than 95% of

all living species go extinct.

What effects have mass extinctions had on the history of life?Mass Extinctions have:

Provided ecological opportunities for organisms that survived

Resulted in bursts of evolution that produced many new species.

Changes to the Earth

Continental Drift: Hypothesis proposed by Alfred Wegener suggesting that throughout time the Earth’s continents have drifted apart and continue to move. This has been substantiated through science and

satellite imagery.

In the beginning….

What is the origin of organic molecules?

The First Organic Molecules