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Genes Within Populations

Chapter 21

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Outline

• Gene Variation• Hardy Weinberg Principle• Agents of Evolutionary Change• Measuring Fitness• Interactions Among Evolutionary Forces• Forms of Selection

– Selection on Color in Guppies• Limits to Selection

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Gene Variation is Raw Material

• Evolution - change over time• Evolution is descent with modification

– Darwin Through time, species accumulate

differences such that ancestral and descendent species are not identical.

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Gene Variation is Raw Material

• Natural selection and evolutionary change– Some individuals in a population possess

certain inherited characteristics that play a role in producing more surviving offspring than individuals without those characteristics.

The population gradually includes more individuals with advantageous characteristics.

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Darwin versus Lamarck

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Gene Variation In Nature

• Measuring levels of genetic variation– blood groups – 30 blood grp genes– Enzymes – 5% heterozygous

• Enzyme polymorphism– A locus with more variation than can be

explained by mutation is termed polymorphic. Natural populations tend to have more

polymorphic loci than can be accounted for by mutation.

15% Drosophila 5-8% in vertebrates

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Hardy-Weinberg Principle

• Population genetics - study of properties of genes in populations

– blending inheritance phenotypically intermediate (phenotypic inheritance) was widely accepted

new genetic variants would quickly be diluted

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Hardy-Weinberg Principle

• Hardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generation

– Sexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions.

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Hardy-Weinberg Equilibrium

Population of cats n=10016 white and 84 blackbb = whiteB_ = black

Can we figure out the allelic frequencies of individuals BB and Bb?

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Hardy-Weinberg Principle

• Necessary assumptions

Allelic frequencies would remain constant if…

– population size is very large– random mating– no mutation– no gene input from external sources– no selection occurring

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Hardy-Weinberg Principle

• Calculate genotype frequencies with a binomial expansion

(p+q)2 = p2 + 2pq + q2

• p2 = individuals homozygous for first allele• 2pq = individuals heterozygous for alleles• q2 = individuals homozygous for second allele

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p2 + 2pq + q2

and

p+q = 1 (always two alleles)

• 16 cats white = 16bb then (q2 = 0.16)• This we know we can see and count!!!!!• If p + q = 1 then we can calculate p from q2

• Q = square root of q2 = q √.16 q=0.4• p + q = 1 then p = .6 (.6 +.4 = 1)• P2 = .36• All we need now are those that are heterozygous

(2pq) (2 x .6 x .4)=0.48

• .36 + .48 + .16

Hardy-Weinberg Principle

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Hardy-Weinberg Equilibrium

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Five Agents of Evolutionary Change

• Mutation– Mutation rates are generally so low they

have little effect on Hardy-Weinberg proportions of common alleles.

ultimate source of genetic variation• Gene flow

– movement of alleles from one population to another

tend to homogenize allele frequencies

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Five Agents of Evolutionary Change

• Nonrandom mating– assortative mating - phenotypically similar

individuals mate Causes frequencies of particular

genotypes to differ from those predicted by Hardy-Weinberg.

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Five Agents of Evolutionary Change

• Genetic drift – statistical accidents.– Frequencies of particular alleles may

change by chance alone. important in small populations

founder effect - few individuals found new population (small allelic pool)

bottleneck effect - drastic reduction in population, and gene pool size

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Genetic Drift - Bottleneck Effect

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Five Agents of Evolutionary Change

• Selection – Only agent that produces adaptiveevolutionary change

– artificial - breeders exert selection – natural - nature exerts selection

variation must exist among individuals variation must result in differences in

numbers of viable offspring produced variation must be genetically inherited

natural selection is a process, and evolution is an outcome

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Five Agents of Evolutionary Change

• Selection pressures:– avoiding predators– matching climatic condition– pesticide resistance

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Measuring Fitness

• Fitness is defined by evolutionary biologists as the number of surviving offspring left in the next generation.

– relative measure Selection favors phenotypes with the

greatest fitness.

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Interactions Among Evolutionary Forces

• Levels of variation retained in a population may be determined by the relative strength of different evolutionary processes.

• Gene flow versus natural selection– Gene flow can be either a constructive or

a constraining force. Allelic frequencies reflect a balance

between gene flow and natural selection.

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Natural Selection Can Maintain Variation

• Frequency-dependent selection– Phenotype fitness depends on its frequency

within the population. Negative frequency-dependent selection

favors rare phenotypes. Positive frequency-dependent selection

eliminates variation.• Oscillating selection

– Selection favors different phenotypes at different times.

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Heterozygote Advantage

• Heterozygote advantage will favor heterozygotes, and maintain both alleles instead of removing less successful alleles from a population.

– Sickle cell anemia Homozygotes exhibit severe anemia,

have abnormal blood cells, and usually die before reproductive age.

Heterozygotes are less susceptible to malaria.

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Sickle Cell and Malaria

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Forms of Selection

• Disruptive selection– Selection eliminates intermediate types.

• Directional selection– Selection eliminates one extreme from a

phenotypic array.• Stabilizing selection

– Selection acts to eliminate both extremes from an array of phenotypes.

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Kinds of Selection

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Selection on Color in Guppies

• Guppies are found in small northeastern streams in South America and in nearby mountainous streams in Trinidad.

– Due to dispersal barriers, guppies can be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.

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Evolution of Coloration in Guppies

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Selection on Color in Guppies

• High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.

• Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.

– In the absence of predators, larger, more colorful fish may produce more offspring.

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Evolutionary Change in Spot Number

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Limits to Selection

• Genes have multiple effects– pleiotropy

• Evolution requires genetic variation– Intense selection may remove variation

from a population at a rate greater than mutation can replenish.

thoroughbred horses• Gene interactions affect allelic fitness

– epistatic interactions

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Summary

• Gene Variation• Hardy Weinberg Principle• Agents of Evolutionary Change• Measuring Fitness• Interactions Among Evolutionary Forces• Forms of Selection

– Selection on Color in Guppies• Limits to Selection

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