Evolution of Evolution of PopulationsPopulations

Variation and Gene Variation and Gene PoolsPools

Genetic variation is studied in Genetic variation is studied in populationspopulations. . A population is a group of individuals of the A population is a group of individuals of the

same species that interbreedsame species that interbreed. . A A gene poolgene pool consists of all genes, including consists of all genes, including

all the different all the different allelesalleles, that are present in a , that are present in a population.population.

The The relative frequencyrelative frequency of an allele is the of an allele is the number of times the allele occurs in a gene number of times the allele occurs in a gene pool, compared with the number of times pool, compared with the number of times other alleles for the same gene occur. other alleles for the same gene occur.

Relative frequency is often expressed as a Relative frequency is often expressed as a percentagepercentage..

Gene Pools:Gene Pools:

Allele Frequency:Allele Frequency:

Gene Pool for Fur Color in Mice:Gene Pool for Fur Color in Mice:

Microevolution: Microevolution: Evolution as Genetic ChangeEvolution as Genetic Change

Natural selection affects which Natural selection affects which individuals individuals survivesurvive and and reproducereproduce and which do not. and which do not.

If an individual dies without If an individual dies without reproducing, it does not contribute its reproducing, it does not contribute its allelesalleles to the population’s to the population’s gene poolgene pool. .

If an individual produces many If an individual produces many offspring, its alleles stay in the gene offspring, its alleles stay in the gene pool and may pool and may increaseincrease in frequency. in frequency.

Genetic Drift:Genetic Drift:– A random change in allele frequency is A random change in allele frequency is

called called genetic drift genetic drift – Random change means that an allele Random change means that an allele

might become more common in a might become more common in a population by chance, not because it population by chance, not because it provides an advantage. provides an advantage.

– In In small populationssmall populations, individuals that , individuals that carry a particular allele may leave more carry a particular allele may leave more descendants than other individuals do, descendants than other individuals do, just by just by chancechance. .

– Gene flow Gene flow

The Founder & Bottleneck The Founder & Bottleneck EffectsEffects

If genetic drift occurs when a If genetic drift occurs when a small group of individuals small group of individuals colonizes a new habitat colonizes a new habitat it is it is often called a often called a founder effectfounder effect; if ; if it occurs after a natural disaster it occurs after a natural disaster wipes out a large proportion of wipes out a large proportion of the original population it is often the original population it is often called a called a bottleneck effect.bottleneck effect.

Individuals may carry alleles in Individuals may carry alleles in different different relative frequenciesrelative frequencies than did the larger population than did the larger population from which they came. from which they came.

The new population will be The new population will be genetically differentgenetically different from the from the parent population.parent population.

Genetic DriftGenetic Drift

Sources of Genetic Sources of Genetic VariationVariation

In genetic terms, evolution is any In genetic terms, evolution is any change in the change in the relative frequency of relative frequency of allelesalleles in a population. in a population.

Genetic Variation (2 or more alleles Genetic Variation (2 or more alleles in the gene pool) in a population is in the gene pool) in a population is essential for natural selection to essential for natural selection to work:work:– mutations mutations – genetic shuffling that results from genetic shuffling that results from

sexual reproduction.sexual reproduction.

Mutations:Mutations:

• Any change in a Any change in a sequence of DNAsequence of DNA

• Occur because of Occur because of mistakes in DNA mistakes in DNA replicationreplication or as a or as a result of result of radiationradiation or or chemicalschemicals in the in the environmentenvironment

• Do not always affect Do not always affect an organisms an organisms phenotypephenotype

Gene Shuffling:Gene Shuffling:

Most heritable differences are due to Most heritable differences are due to gene shufflinggene shuffling..

Crossing-over Crossing-over increases the number of increases the number of genotypesgenotypes that can appear in offspring. that can appear in offspring.

Sexual reproduction produces different Sexual reproduction produces different phenotypesphenotypes, but it does not change the , but it does not change the relative frequency of alleles in a relative frequency of alleles in a population.population.

Gene Shuffling:Gene Shuffling:

Genetic EquilibriumGenetic Equilibrium A population is in genetic equilibrium if allele A population is in genetic equilibrium if allele

frequencies are not changing from one frequencies are not changing from one generation to the nextgeneration to the next

According to the Hardy-Weinberg theory, a According to the Hardy-Weinberg theory, a population is in genetic equilibrium if the population is in genetic equilibrium if the following conditions are met simultaneously:following conditions are met simultaneously:– Large population sizeLarge population size– Random matingRandom mating– No mutationsNo mutations– No migrationNo migration– No natural selection No natural selection

The Hardy-Weinberg EquationsThe Hardy-Weinberg Equations Although genetic equilibrium cannot be Although genetic equilibrium cannot be

maintained, it can be assumed to be occurring maintained, it can be assumed to be occurring at a particular moment. at a particular moment.

Based on this premise, the H-W equations can Based on this premise, the H-W equations can be used to estimate allele frequency and/or be used to estimate allele frequency and/or percentage of the population that is either percentage of the population that is either homozygous recessive, homozygous dominant homozygous recessive, homozygous dominant or heterozygous. or heterozygous.

The equations: The equations: – p + q = 1 p + q = 1 (p = dominant allele frq; q = recessive allele frq.)(p = dominant allele frq; q = recessive allele frq.)

– pp22 + 2pq + q + 2pq + q22 = 1 = 1 (hom. dom.; hetero; hom. rec.)(hom. dom.; hetero; hom. rec.)

Single-Gene and Polygenic Single-Gene and Polygenic TraitsTraits

Many traits are controlled by two or more Many traits are controlled by two or more genes and are called genes and are called polygenic traitspolygenic traits..

One polygenic trait can have many possible One polygenic trait can have many possible genotypesgenotypes and and phenotypesphenotypes..

HeightHeight in humans is a polygenic trait. in humans is a polygenic trait. A A bell-shaped curvebell-shaped curve is typical of polygenic is typical of polygenic

traits.traits. A bell-shaped curve is also called A bell-shaped curve is also called normal normal

distributiondistribution..

Natural Selection on Polygenic Natural Selection on Polygenic TraitsTraits

3 categories3 categories::Directional: favors one Directional: favors one extremeextreme

Stabilizing: favors the Stabilizing: favors the middlemiddle

Disruptive: favors both Disruptive: favors both extremesextremes

Types of Natural Types of Natural SelectionSelection

What type of selection?What type of selection?

Divergent v. Convergent EvolutionDivergent v. Convergent Evolution

DivergentDivergent

One species gives rise One species gives rise to many speciesto many species

Also known as Also known as adaptive adaptive radiationradiation

Many species with Many species with common ancestorcommon ancestor

Many Many homologous homologous structuresstructures

ConvergentConvergent

Similar looking species Similar looking species that do not have a that do not have a common ancestorcommon ancestor

Similar behavior and Similar behavior and appearance due to appearance due to environmental environmental similaritiessimilarities

Many Many analogous analogous structuresstructures

Convergent EvolutionConvergent Evolution

CoevolutionCoevolutionThe evolution of one species is directly influenced by the evolution of another

Punctuated EquilibriumPunctuated Equilibrium Slow background evolution (stasis) is interrupted Slow background evolution (stasis) is interrupted

by rapid bursts of changeby rapid bursts of change Rapid bursts of change usually occur after a mass Rapid bursts of change usually occur after a mass

extinctionextinction

Speciation:Speciation:

SpeciationSpeciation is the formation of new species. is the formation of new species. A species is a group of organisms that A species is a group of organisms that breedbreed

with one another and produce with one another and produce fertile fertile offspringoffspring. .

The The gene poolsgene pools of two populations must of two populations must become become separatedseparated for them to become new for them to become new speciesspecies

When the members of two populations cannot When the members of two populations cannot interbreedinterbreed and produce fertile offspring, and produce fertile offspring, reproductive isolationreproductive isolation has occurred and has occurred and speciationspeciation will result. will result.

Types of Reproductive Types of Reproductive IsolationIsolation

Behavioral IsolationBehavioral Isolation – Different – Different mating rituals prevent mating rituals prevent reproductionreproduction

Geographic IsolationGeographic Isolation – barriers – barriers such as rivers or mountains such as rivers or mountains prevent reproductionprevent reproduction

Temporal IsolationTemporal Isolation – different – different mating times (seasonal, nocturnal mating times (seasonal, nocturnal v. diurnal) prevent reproductionv. diurnal) prevent reproduction