Pouplation Genetics

8/14/2019 Pouplation Genetics

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Copyright (c) The McGraw-Hill Com 1

CHAPTER 24

POPULATIONGENETICS

Prepared by

Brenda Leady, University of Toledo


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Population genetics

Study of genes and genotypes in a

population

Want to know extent of genetic variation,

why it exists and how it changes over time

Helps us understand how genetic variation

is related to phenotypic variation


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Gene pool

All of the genes in a population

Study genetic variation within the gene

pool and how variation changes from one

generation to the next

Emphasis is often on variation in alleles

between members of a population


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Population

Group of individuals of the same species

that can interbreed with one another

Some species occupy a wide geographic

range and are divided into discrete

populations


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Genes in Natural Populations Are

Usually Polymorphic Polymorphism many traits display variation within

a population Due to 2 or more alleles that influence phenotype

Polymorphic gene- 2 or more alleles Monomorphic predominantly single allele Single nucleotide polymorphism (SNPs)

Smallest type of genetic change in a gene

Most common 90% of variation in human genesequences Large, healthy populations exhibit a high level of

genetic diversity

Raw material for evolution


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Allele and genotype frequencies

Related but distinct calculations


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Example

49 red-flowered RR

42 pink-flowered Rr

9 white-flowered rr

Allele frequency of r

1.0 - 0.3 = 0.7 frequency of R

Genotype frequency of rr


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Hardy-Weinberg equation

Relates allele and genotype frequencies

under certain conditions


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Conditions The population is so large that allele frequencies do

not change due to random sampling error The members of the population mate with each other

without regard to their phenotypes and genotypes No migration occurs between different populations No survival or reproductive advantage exists for any

of the genotypesin other words, no natural selectionoccurs

No new mutations occur

In reality, no population meets these conditions If frequencies are not in equilibrium, an

evolutionary mechanism is at work


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Microevolution

Changes in a populations gene pool fromgeneration to generation

Change because Introduce new genetic variation (mutations, gene

duplication, exon shuffling, horizontal gene transfer) Population will not evolve with mutations as the only source

Evolutionary mechanisms that alter the prevalence ofan allele or genotype (natural selection, randomgenetic drift, migration, nonrandom mating) Potential for widespread genetic change


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Selective survival of genotypes that confergreater reproductive success

Natural selection acts onCharacteristics with a survival advantage

Make organisms better adapted, more likely tosurvive, greater chance to reproduce

Favors individuals that produce viableoffspring


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Modern description of natural selection

1. Allelic variation arises from random mutations that mayalter the function of the protein.

2. Some alleles may encode proteins that enhance anindividuals survival or reproductive success compared to

that of other members of the population3. Individuals with beneficial alleles are more likely to

survive and contribute their alleles to the gene pool ofthe next generation

4. Over the course of many generations, allele frequencies

of many different genes may change through naturalselection, thereby significantly altering the characteristicsof a population Net result of natural selection is a population that is better

adapted to its environment and/or more successful atreproduction.


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Darwinian fitness

Relative likelihood that a genotype will

contribute to the gene pool of the next

generation as compared with othergenotypes

Measure of reproductive success

Hypothetical gene with alleles A and aAA, Aa, aa


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Suppose average reproductive successes

areAA 5 offspring

Aa 4 offspring

Aa 1 offspring Fitness is W and maximum is 1.0 for

genotype with highest reproductive ability

Fitness of AA: WAA = 5/5 = 1.0Fitness of Aa: WAa = 4/5 = 0.8

Fitness of aa: Waa = 1/5 = 0.2


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Mean fitness of population

Average reproductive success of

members of a population

As individuals with higher fitness valuesbecome more prevalent, natural selection

increases the mean fitness of the

population


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Natural selection patterns

Directional selection

Stabilizing selection

Disruptive selection

Balancing selection


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Directional selection

Favors individuals at one extreme of a

phenotypic distribution that have greater

reproductive success in a particularenvironment

Initiators

New favored allele introducedProlonged environmental change


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Stabilizing selection

Favors the survival of individuals with

intermediate phenotypes

Extreme values of a trait are selectedagainst

Clutch sizeToo many eggs and offspring die due to lack

of care and food

Too few eggs does not contribute enough to

next generation


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Balancing selection

Maintains genetic diversity Balanced polymorphism

Two or more alleles are kept in balance, andtherefore are maintained in a population overthe course of many generations

2 common ways

For a single gene, heterozygote favored Heterozygote advantage HS alleleNegative frequency-dependent selection

Rare individuals have a higher fitness


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Sexual selection

Form of natural selection Directed at certain traits of sexually

reproducing species that make it morelikely for individuals to find or choose amate and/or engage in successful mating

In many species, affects malecharacteristics more intensely than it doesfemale


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Intrasexual selection Between members of the same sex

Horns in male sheep, antlers in male moose, malefiddler crab enlarged claws

Males directly compete for mating opportunities or

territories

Intersexual selection Between members of the opposite sex

Female choice

Often results in showy characteristics for males Cryptic female choice

Genital tract or egg selects against genetically related sperm Inhibits inbreeding


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Explains traits that decrease survival but increase

reproductive success Male guppy (Poecilia reticulata) is brightly colored

compared to the female Females prefer brightly colored males In places with few predators, the males tend to be

brightly colored In places where predators are abundant, brightly colored

males are less plentiful because they are subject topredation

Relative abundance of brightly and dully colored malesdepends on the balance between sexual selection, whichfavors bright coloring, and escape from predation, whichfavors dull coloring


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Hypothesized that females choose males

for mates based on males coloration Male were in glass enclosures to avoid

direct competition Goal to determine which of 2 males a

female would prefer Females preference for males dramatically

different under different lights

Mating preference lost undermonochromatic light

Sexual selection followed a diversifying

mechanism


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Random genetic drift

Changes allelic frequency due to random

sampling error

Random events unrelated to fitness Favors either loss or fixation of an alleleFrequency reaches 0% or 100%

Faster in smaller populations


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Bottleneck

Population reduced dramatically and thenrebuilds

Randomly eliminated members without

regard to genotype Surviving members may have allele

frequencies different from originalpopulation

Allele frequencies can drift substantiallywhen population is small

New population likely to have less genetic

variation


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Founder effect

Small group of individuals separates from

a larger population and establishes a new

colony

Relatively small founding population

expected to have less genetic variation

than original population

Allele frequencies in founding population

may differ markedly from original

population


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Neutral theory of evolution

Non-Darwinian evolution Neutral variation

Much of the variation seen in natural populations iscaused by genetic drift Does not preferentially select for any particular allele

Most genetic variation is due to the accumulationof neutral mutations that have attained highfrequencies due to genetic drift

Neutral mutations do not affect the phenotype sothey are not acted upon by natural selection


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Main idea is that much of the modern

variation in gene sequences is explainedby neutral variation rather than adaptivevariation

Sequencing data supports this idea Nucleotide substitutions much more likely

in 3rd base of codon (usually dont changeamino acid) than 1st or 2nd (usually does

change amino acid) Changing the amino acid is usually

harmful to the coded protein


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Migration

Gene flow occurs when individualsmigrate between populations having

different allele frequencies Migration tends to reduce differences in

allele frequencies between the 2populations

Tends to enhance genetic diversity withina population


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Nonrandom mating

One of the conditions required to establishthe Hardy-Weinberg equilibrium is random

mating Individuals choose their mates irrespective oftheir genotypes and phenotypes

Forms of nonrandom matingAssortative/disassortative Inbreeding


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Assortative mating Individuals with similar phenotypes are more

likely to mate Increases the proportion of homozygotes

Disassortative mating

Dissimilar phenotypes mate preferentiallyFavors heterozygosity


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InbreedingChoice of mate based on genetic historyDoes not favor any particular allele but it does

increase the likelihood the individual will be

homozygousMay have negative consequences with regard

to recessive allelesLower mean fitness of a population if

homozygous offspring have a lower fitnessvalue

Inbreeding depression


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