Chapter 23~ Chapter 23~ The Evolution of The Evolution of PopulationsPopulations

Population genetics provides Population genetics provides foundation for studying evolutionfoundation for studying evolution

MicroevolutionMicroevolution– Evolutionary change on the smallest Evolutionary change on the smallest

scalescale– A change in the genetic makeup of a A change in the genetic makeup of a

population from generation to population from generation to generationgenerationChange in the allele frequenciesChange in the allele frequencies

How do you determine if allele How do you determine if allele frequencies are changing?frequencies are changing?– Hardy-Weinberg TheoryHardy-Weinberg Theory

What is population genetics?What is population genetics? PopulationPopulation: a localized group of : a localized group of

individuals belonging to the same speciesindividuals belonging 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: the total aggregate of genes : the total aggregate of genes in a population at any one timein a population at any one time

Modern synthesis/neo-DarwinismModern synthesis/neo-Darwinism– Combination of Darwin’s, Mendel’s, and Combination of Darwin’s, Mendel’s, and

many others work many others work – ““Individuals are selected, but Individuals are selected, but

populations evolve.”populations evolve.”

Microevolution I- What is it and how Microevolution I- What is it and how does it happen?does it happen?

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 gene pool of a small population small population due to chance due to chance (usually reduces (usually reduces genetic variability)genetic 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)disaster)

Microevolution, IIIMicroevolution, III Founder Effect:Founder Effect:

a a cause of cause of genetic drift genetic drift attributable to attributable to colonization by colonization by a limited a limited number of number of individuals individuals from a parent from a parent populationpopulation

MicroevolutionMicroevolution 2- 2- Gene FlowGene Flow: :

genetic exchange genetic exchange due to the due to the migration of fertile migration of fertile individuals or individuals or gametes between gametes between populations populations (reduces (reduces differences differences between between populations)populations)

Microevolution, VMicroevolution, V 3- 3- MutationsMutations: :

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

Microevolution, VIMicroevolution, VI

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

5- 5- Natural Natural SelectionSelection: : differential differential success in success in reproductionreproduction

How is natural selection the primary How is natural selection the primary mechanism of adaptive evolution?mechanism of adaptive evolution?

Causes genetic variationsCauses genetic variations– Polymorphisms, geographic variation, Polymorphisms, geographic variation,

clinesclines Evolutionary fitness and selectionEvolutionary fitness and selection

– Fitness- contribution of genes to the Fitness- contribution of genes to the next generations gene poolnext generations gene pool

– Relative fitnessRelative fitnessContribution of genotype to next generationContribution of genotype to next generation

– Basically- unless you reproduce you Basically- unless you reproduce you have not done your job!!!!!have not done your job!!!!!

Modes of Natural SelectionModes of Natural Selection

(a) Directional selection (b) Disruptive selection (c) Stabilizing selection

What is sexual selection?What is sexual selection?

Natural selection for mating successNatural selection for mating success Usually causes sexual dimorphismsUsually causes sexual dimorphisms

– Marked differences within speciesMarked differences within species Intrasexual selection selects males Intrasexual selection selects males

that best please femalesthat best please females– Females are choosy ( for many Females are choosy ( for many

reasons) and will usually mate with reasons) and will usually mate with only the best of the best malesonly the best of the best males

Fig. 23-15

How to you use the Hardy-How to you use the Hardy-Weinberg Theorem look at a Weinberg Theorem look at a

population?population? Serves as a model for the genetic Serves as a model for the genetic

structure of a nonevolving structure of a nonevolving populationpopulation (equilibrium) (equilibrium)– Rarely if ever happens in a natural Rarely if ever happens in a natural

populationpopulation 5 conditions:5 conditions:

– 1- Very large population size1- Very large population size– 2- No migration2- No migration– 3- No net mutations3- No net mutations– 4- Random mating4- Random mating– 5- No natural selection5- No natural selection

Hardy-Weinberg EquationsHardy-Weinberg Equations

pp=frequency of one allele (A) =frequency of one allele (A) qq=frequency of the other allele (a) =frequency of the other allele (a)

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

P2P2=frequency of AA genotype=frequency of AA genotype 2pq2pq=frequency of Aa plus aA genotype; =frequency of Aa plus aA genotype; q2q2=frequency of aa genotype; =frequency of aa genotype;

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

• Heterozygote advantage Heterozygote advantage occurs when occurs when heterozygotes have a higher fitness than heterozygotes have a higher fitness than do both homozygotesdo both homozygotes

• Natural selection will tend to maintain Natural selection will tend to maintain two or more alleles at that locustwo or more alleles at that locus

• The sickle-cell allele causes mutations in The sickle-cell allele causes mutations in hemoglobin but also confers malaria hemoglobin but also confers malaria resistanceresistance

Heterozygote Advantage

Fig. 23-17

0–2.5%

Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)

Frequencies of thesickle-cell allele

2.5–5.0%

7.5–10.0%

5.0–7.5%

>12.5%

10.0–12.5%