Chapter 23Chapter 23

Population geneticsPopulation genetics Population: Population: a localized group of individuals belonging to a localized group of individuals belonging to

the same speciesthe same species Species: Species: a group of populations whose individuals have a group of populations whose individuals have

the potential to interbreed and produce fertile offspringthe potential to interbreed and produce fertile offspring Gene pool: Gene pool: the total aggregate of genes in a population the total aggregate of genes in a population

at any one timeat any one time Population genetics: Population genetics: the study of genetic changes in the study of genetic changes in

populationspopulations Modern synthesis/neo-DarwinismModern synthesis/neo-Darwinism ““Individuals are selected, but populations evolve.”Individuals are selected, but populations evolve.”

Hardy-Weinberg TheoremHardy-Weinberg Theorem Serves as a model for the Serves as a model for the

genetic structure of a genetic structure of a nonevolving nonevolving populationpopulation (equilibrium)(equilibrium)

5 conditions:5 conditions: 1- Very large population 1- Very large population

size;size; 2- No migration;2- No migration; 3- No net mutations;3- No net mutations; 4- Random mating;4- Random mating; 5- No natural selection5- No natural selection

Hardy-Weinberg EquationHardy-Weinberg Equation

p=frequency of one allele (A); p=frequency of one allele (A); q=frequency of the other allele (a); q=frequency of the other allele (a);

p p + q=1.0+ q=1.0 (p=1-q & q=1-p) (p=1-q & q=1-p)

PP22=frequency of AA genotype; =frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; 2pq=frequency of Aa plus aA genotype; qq22=frequency of aa genotype; =frequency of aa genotype; pp2 2 + 2pq + + 2pq + qq22 = 1.0 = 1.0

Microevolution, IMicroevolution, I New definition of Evolution at the population New definition of Evolution at the population

level.level. Evolution is a generation to generation Evolution is a generation to generation

change in a population’ s frequencies of change in a population’ s frequencies of alleles.alleles.

This also can be called microevolution: A This also can be called microevolution: A change in the gene pool of a population over a change in the gene pool of a population over a succession of generationssuccession of generations

1- 1- Genetic driftGenetic drift: changes in the gene pool of a : changes in the gene pool of a small population due to chance (usually small population due to chance (usually reduces genetic variability)reduces genetic variability)

Figure 23.4 Genetic driftFigure 23.4 Genetic drift

Microevolution, IIMicroevolution, II The Bottleneck EffectThe Bottleneck Effect: :

type of genetic drift type of genetic drift resulting from a resulting from a reduction in reduction in population (natural population (natural disaster) such that the disaster) such that the surviving population is surviving population is no longer genetically no longer genetically representative of the representative of the original populationoriginal population

Microevolution, IIIMicroevolution, III

Founder Effect:Founder Effect: a cause of a cause of genetic drift genetic drift attributable to attributable to colonization by a colonization by a limited number of limited number of individuals from a individuals from a parent populationparent population

Microevolution, IVMicroevolution, IV

2- 2- Gene FlowGene Flow: : genetic exchange genetic exchange due to the migration due to the migration of fertile individuals of fertile individuals or gametes between or gametes between populations populations (reduces differences (reduces differences between between populations)populations)

Microevolution, VMicroevolution, V 3- 3- MutationsMutations: a change in an organism’s DNA : a change in an organism’s DNA

(gametes; many generations); original source (gametes; many generations); original source of genetic variation (raw material for natural of genetic variation (raw material for natural selection)selection)

4- 4- Nonrandom matingNonrandom mating: inbreeding and : inbreeding and assortive mating (both shift frequencies of assortive mating (both shift frequencies of different genotypes)different genotypes)

5- 5- Natural SelectionNatural Selection: differential success in : differential success in reproduction; only form of microevolution that reproduction; only form of microevolution that adapts a population to its environmentadapts a population to its environment

Population variationPopulation variation Polymorphism:Polymorphism:

coexistence of 2 or coexistence of 2 or more distinct forms of more distinct forms of individuals (morphs) individuals (morphs) within the same within the same populationpopulation

Geographical variation:Geographical variation: differences in genetic differences in genetic structure between structure between populations (cline)populations (cline)

Figure 23.8 Clinal variation in a plantFigure 23.8 Clinal variation in a plant

Two Random Processes that Two Random Processes that generate genetic variationgenerate genetic variation

Mutation – new alleles originate only by Mutation – new alleles originate only by mutation. Rare and random events and mutation. Rare and random events and usually occur in somatic cells and are not usually occur in somatic cells and are not passed on to the offspring.passed on to the offspring.

Sexual Recombination combines old Sexual Recombination combines old alleles with new and fresh assortments alleles with new and fresh assortments every generation.every generation.

Variation preservationVariation preservation Prevention of natural selection’s reduction Prevention of natural selection’s reduction

of variationof variation DiploidyDiploidy 2nd set of chromosomes hides 2nd set of chromosomes hides

variation in the heterozygotevariation in the heterozygote Balanced polymorphismBalanced polymorphism 1- heterozygote 1- heterozygote

advantage (hybrid vigor; i.e., malaria/sickle-advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- cell anemia); 2- frequency dependent selection (survival & frequency dependent selection (survival & reproduction of any 1 morph declines if it reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)becomes too common; i.e., parasite/host)

Natural selectionNatural selection

Fitness: Fitness: contribution an individual makes contribution an individual makes to the gene pool ofto the gene pool of the next generation the next generation

3 types3 types:: A. DirectionalA. Directional B. DiversifyingB. Diversifying C. StabilizingC. Stabilizing

Figure 23.12 Modes of selectionFigure 23.12 Modes of selection

Sexual selectionSexual selection

Sexual dimorphismSexual dimorphism: : secondary sex secondary sex characteristic distinctioncharacteristic distinction

Sexual selectionSexual selection: : selection towards selection towards secondary sex secondary sex characteristics that leads characteristics that leads to sexual dimorphismto sexual dimorphism

18

16.3 Maintenance of 16.3 Maintenance of DiversityDiversity

Genetic VariabilityGenetic Variability Populations with limited variation may not be Populations with limited variation may not be

able to adapt to new conditionsable to adapt to new conditions Maintenance of variability is advantageous to Maintenance of variability is advantageous to

the populationthe population Only exposed alleles are subject to natural Only exposed alleles are subject to natural

selectionselection

19

Maintenance of DiversityMaintenance of Diversity Natural selection causes imperfect adaptationsNatural selection causes imperfect adaptations

Depends on evolutionary historyDepends on evolutionary history Imperfections are common because of necessary Imperfections are common because of necessary

compromisescompromises The environment plays a role in maintaining The environment plays a role in maintaining

diversitydiversity Disruptive selection due to environmental Disruptive selection due to environmental

differences promotes polymorphisms in a populationdifferences promotes polymorphisms in a population If a population occupies a wide range, it may have If a population occupies a wide range, it may have

several subpopulations designated as subspeciesseveral subpopulations designated as subspecies The environment includes selecting agents that help The environment includes selecting agents that help

maintain diversitymaintain diversity

20

Subspecies Help Maintain Subspecies Help Maintain DiversityDiversityCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

(E.o. lindheimeri, E.o. quadrivittata): © Zig Leszczynski/Animals Animals/Earth Scenes; (E.o. spiloides): © Joseph Collins/Photo Researchers, Inc.;(E.o. rossalleni): © Dale Jackson/Visuals Unlimited; (E.o. obsoleta): © William Weber/Visuals Unlimited

Pantheropsis obsoleta lindheimeri Pantheropsis obsoleta rossalleni Pantheropsis obsoleta spiloides

Pantheropsis obsoleta quadrivittata

Pantheropsis obsoleta obsoleta

21

Maintenance of DiversityMaintenance of Diversity Recessive alleles:Recessive alleles:

Heterozygotes shelter recessive alleles from Heterozygotes shelter recessive alleles from selectionselection

Heterozygotes allow even lethal alleles to remain Heterozygotes allow even lethal alleles to remain in the population at low frequencies virtually in the population at low frequencies virtually foreverforever

Sometimes recessive alleles confer an advantage Sometimes recessive alleles confer an advantage to heterozygotesto heterozygotes

The sickle-cell anemia allele is detrimental in homozygoteThe sickle-cell anemia allele is detrimental in homozygote However, heterozygotes are more likely to survive malariaHowever, heterozygotes are more likely to survive malaria The sickle-cell allele occurs at a higher frequency in The sickle-cell allele occurs at a higher frequency in

malaria prone areasmalaria prone areas

22

Maintenance of DiversityMaintenance of Diversity

Heterozygote AdvantageHeterozygote Advantage Assists the maintenance of genetic, and Assists the maintenance of genetic, and

therefore phenotypic, variations in future therefore phenotypic, variations in future generations.generations.

In sickle cell disease heterozygous In sickle cell disease heterozygous individuals donindividuals don’’t die from sickle-cell t die from sickle-cell disease, and they dondisease, and they don’’t die from malaria. t die from malaria.