The Evolution of Populations. Hardy-Weinberg Theorem Serves as a model for the genetic structure of...

The Evolution of PopulationsThe Evolution of Populations

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 populationnonevolving population (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

Population geneticsPopulation genetics

PopulationPopulation: : a localized group of individuals a localized group of individuals

belonging to the same speciesbelonging to the same species SpeciesSpecies: :

a group of populations whose a group of populations whose individuals have the potential to individuals have the potential to interbreed and produce fertile offspringinterbreed and produce fertile offspring

Gene poolGene pool: : all the genes in a population at all the genes in a population at

any one timeany one time Population geneticsPopulation genetics: :

the study of genetic changes in the study of genetic changes in populationspopulations

““Individuals are selected, but Individuals are selected, but populations evolve.”populations evolve.”

Hardy-Weinberg EquationHardy-Weinberg Equation

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

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

P2=frequency of AA genotype; 2pq=frequency P2=frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q2=frequency of aa of Aa plus aA genotype; q2=frequency of aa genotype; genotype;

p2 + 2pq + q2 = 1.0p2 + 2pq + q2 = 1.0

Microevolution, IMicroevolution, I

A change in the A change in the gene pool of a gene pool of a population over a population over a succession of succession of generationsgenerations

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

Microevolution, IIMicroevolution, II

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

Microevolution, IIIMicroevolution, III

Founder Effect:Founder Effect: the colonization the colonization of a new habitat of a new habitat by a few by a few individualsindividuals

Microevolution, IVMicroevolution, IV

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

Microevolution, VMicroevolution, V

3- 3- MutationsMutations: : a change in an a change in an organism’s DNA organism’s DNA (gametes; many (gametes; many generations); original generations); original source of genetic source of genetic variation (raw material variation (raw material for naturalfor natural selection)selection)

Microevolution, VIMicroevolution, VI

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

Microevolution, VIIMicroevolution, VII

5- 5- Natural Natural SelectionSelection: : differential success differential success in reproduction; in reproduction; only form of only form of microevolution that microevolution that adapts a adapts a population to its population to its environmentenvironment

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

Natural selectionNatural selection

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

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

Peppered Moths & Peppered Moths & Industrial MelanismIndustrial Melanism

Until the mid-nineteenth century, peppered Until the mid-nineteenth century, peppered moths had mostly light-colored wings.moths had mostly light-colored wings.

Later, darker individuals became predominant.Later, darker individuals became predominant. Industrial smog helped turn tree trunks dark.Industrial smog helped turn tree trunks dark. Contrasting colors between trunk color and Contrasting colors between trunk color and

moth color led to differential predation by moth color led to differential predation by birds.birds.

Mutations and chance continued to create or Mutations and chance continued to create or permit survival of SOME lighter moths, though.permit survival of SOME lighter moths, though.

As pollution controls increased, frequencies As pollution controls increased, frequencies reversed again.reversed again.

Peppered MothsPeppered Moths

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 characteristics that leads to sexual leads to sexual dimorphismdimorphism

Reproductive Isolation Reproductive Isolation

Events that lead to reproductive isolation of Events that lead to reproductive isolation of populations of the same species cause new populations of the same species cause new species to appear. Barriers to reproduction species to appear. Barriers to reproduction that prevent mating between populations that prevent mating between populations are called are called prezygotic prezygotic (before fertilization)(before fertilization)

Examples are:Examples are:– isolation of habitatsisolation of habitats– a difference in breeding season or mating a difference in breeding season or mating

behaviorbehavior– an incompatibility of genitalia or gametes. an incompatibility of genitalia or gametes.

Reproductive Isolation IIReproductive Isolation II PostzygoticPostzygotic (after (after

fertilization) barriers fertilization) barriers that prevent the that prevent the development of development of viable, fertile hybrids viable, fertile hybrids exist because of exist because of – genetic genetic

incompatibility incompatibility between the between the populationspopulations

– hybrid sterilityhybrid sterility– hybrid breakdown. hybrid breakdown.

The sterile hybrid offspring of a male donkey and a female horse, characterized by long ears and a short mane.

Reproductive Isolation IIIReproductive Isolation III These isolation events can occur within These isolation events can occur within

the geographic range of a parent the geographic range of a parent population population ((sympatric speciationsympatric speciation))

Sympatric speciation is much more Sympatric speciation is much more common in plants than in animals. common in plants than in animals.

Allopatric SpeciationAllopatric Speciation -geographic isolation of a small -geographic isolation of a small population from its parent population population from its parent population

Occurs in animal evolution when geographically isolated Occurs in animal evolution when geographically isolated populations adapt to different environmental conditions.populations adapt to different environmental conditions.

In addition, the rate is faster in small populations than in In addition, the rate is faster in small populations than in large ones because of greater genetic drift.large ones because of greater genetic drift.