LU2 MA1.ppt

7/28/2019 LU2 MA1.ppt

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7/28/2019 LU2 MA1.ppt


History• Evolutionary synthesis integratedDarwinian selection & Mendelianinheritance

• Emergence of population geneticsfield

• Modern synthesis: integrateddiscoveries from paleontology,taxonomy, biogeography & popgenetics

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Tenets of the Modern Synthesis

1. Importance of populations as unitsof evolution

2. Natural selection as primarymechanism of evolutionary change3. Gradualism explains large changesoccuring over long periods of time

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Components of populations

Population = localized group of individuals of a species which are morelikely to interbred with each other thanwith other species

Species = a group of populations whose

members have the potential to interbredin nature

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Population genetics & thegene pool

Interbreeding within populations reducesgene flow

Gene flow = migration of fertileindividuals between populations

Gene pool = total aggregate of genes in a

population at any given time

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Population genetics and

genetic equilibrium•A population whose allele & genotype

frequencies do not change downgenerations is in genetic equilibrium

•Hardy-Weinberg theorem states that

normal processes of sexual reproductiondo not change allele & genotypefrequencies

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Hardy-Weinberg Theorem

concepts• Recombination & segregation associatedwith sexual reproduction do not alter the

gene pool of population• Used to describe non-evolving

populations; can identify presence of

evolution (change in gene frequencies)• Mathematics of theorem can be used todetermine expected allele & gene

frequencies in populations

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Hardy-Weinberg equilibrium

is maintained if 1. Very large population size is maintained

2. Isolation from other populations ismaintained

3. No net mutations alter the gene pool

4. Individuals in population mate randomly

5. Natural selection is not influencing population

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Microevolution

Five causes of microevolution are:1. Genetic drift; occurs in small

populations

2. Gene flow; migration among populations

3. Mutation; alters allele frequencies4. Nonrandom mating

5. Natural selection; differential survival &reproduction

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Genetic drift

In small populations, chance events causeallele frequencies to change randomly bysampling errors

Smaller the population the moreimportant genetic drift

Two types : I) bottleneck effect2) Founder effect

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Genetic drift (contd)

Bottleneck effect

Occurs when a population is reducedto very small size rapidly & thenrecovers

•Small numbers of survivors not likely to be representative

•Reduction effects all individualsrandomly

•Longer population remains small the

greater is the effect

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Genetic drift (contd)Founder events

Genetic drift following founding of a new population by a few individuals (islands,continents)

•Smaller the founding population, less likely itsgene pool will be representative of original population

•Genotype of founders determines genetic makeupof population

• If colony survives, random drift will affect allelefrequencies until population becomes relativelylarge

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Nonrandom mating

2 types: 1) Inbreeding2) Assortative mating

Inbreeding• Individuals mate with close neighbors rather than with more distant members of a population

•Effect is reduced frequency of heterozygousgenotypes, increased frequency of homozygotes

• Inbreeding do not necessarily change allelefrequencies but does change genotype frequencies

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Assortative matingsIndividuals mate with partners that are likethemselves in certain phenotypic characters

Examples:

1. Toads pair up according to size relationships;males must be able to achieve amplexus with larger females

2. In humans tall women commonly pair with tallmen

3. Humans tend to mate within races, therebyreinforcing differences among ethnic groups

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Gene Flow

Mutation

•Tends to reduce between population differences,thereby homogenizing population genetic structure

•Can counteract effects of natural selection, geneticdrift in relatively small, isolated populations

•Can immediate change the gene pool by allele

substitutions

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M i & S l bi i

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Mutation & Sexual recombinationare processes that generate genetic

variation•Mutations: rare, random but ultimate source of allelic variation•Usually occur in somatic cells, so most are notheritable•Point mutations that alter protein enough to affectfunction are more often harmful than beneficial, notretained•Chromosomal mutations usually affect many geneloci & tend to disrupt developmental processes•Mutation can produce adequate genetic variation in

bacteria due to their very short generation times

Di l id & B l d

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Diploidy & BalancedPolymorphism preserve genetic

variation•Diploidy : hides much genetic variation fromselection by presence of recessive alleles inheterozygotes

•Because recessive alleles not expressed inheterozygotes, less favored or harmful alleles may

be retained in populations

•The more rare the recessive allele is, the greater its protection by heterozygosity•Result of heterozygote protection is maintenance of

large pool of alleles which may be beneficial if conditions change

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• Balanced polymorphism = the ability of natural selection to maintain diversity in a

population Natural selection can preserve geneticvariation due to heterozygote advantage• Example: Sickle-cell anemia caused byrecessive allele• Heterozygotes are resistant to malaria sohave fitness advantage over both

homozygotes in regions with malaria• Distribution of recessive allele closely tiedto malaria incidence

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Effect of natural selection oncharacteristics in population varies

depending on which phenotypes aremost favored

1) Directional selectionNatural selection operates to shift underlying allelefrequencies in a consistent direction.

Directional selection occurs when their is adirectional change in the environment or a new

mutation produces a new phenotype that is moreadaptive than existing phenotypes.

There are many examples of directional selection --resistance of bacteria and other organisms tochemicals designed to control them (e.g. antibiotics,DDT)

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2) Stabilizing selection

Selection operates against extremephenotypes and favors intermediate ones.

Stabilizing selection often results whenenvironmental conditions have beenconstant. Example: Human birth weight -- newbornswith birth weights much more or much lessthan the average (ca. 7 lbs.) haveincreased mortality.

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3) Disruptive selection

Selection against intermediate phenotypesin favor of extreme phenotypes

Disruptive selection can result in theformation of genetically distinct"subpopulations/"

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Sexual selection: form of natural

selection which can result in pronounced differences betweensexes

•Sexual selection: selection for secondary sexualcharacteristics advantageous in acquiring mates

•Results in sexual dimorphism: male & female

differ in size or other traits advantageous in matingcompetition or female choice

•Examples: body sizes, plumages, horns & antlersvary between sexes

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LU2 MA1.ppt