Evolution of Evolution of PopulationsPopulations

How Common Is Genetic How Common Is Genetic Variation?Variation?

Many genes have at least Many genes have at least twotwo forms, or forms, or allelesalleles..

All organisms have genetic variation that is All organisms have genetic variation that is “invisible” because it involves “invisible” because it involves small small differences in biochemical processes.differences in biochemical processes.

An individual organism is An individual organism is heterozygousheterozygous for for many genesmany genes..

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:

Genetic Drift:Genetic Drift:

– A random change in allele frequency is A random change in allele frequency is called called genetic driftgenetic drift

– 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. .

– Over time, a series of Over time, a series of chance chance occurrencesoccurrences of this type can cause an of this type can cause an allele to become common in a population.allele to become common in a population.

The Founder EffectThe Founder Effect Genetic drift may occur Genetic drift may occur

when a small group of when a small group of individuals individuals colonizes a colonizes a new habitatnew habitat. .

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

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

Genetic DriftGenetic Drift

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.

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 relative frequency of allelesfrequency of alleles in a in a population.population.

Sources of Genetic Variation:Sources of Genetic Variation:– 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:

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?

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

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

Speciation in Darwin's Speciation in Darwin's FinchesFinches

founding of a new populationfounding of a new populationgeographic isolationgeographic isolationchanges in new population's changes in new population's gene poolgene pool

reproductive isolationreproductive isolationecological competitionecological competition

STEP 1: Founders STEP 1: Founders ArriveArrive

•A few finches, “species A”, travel from S. America to one of the Galápagos Islands.

•There, they survive and reproduce.

STEP 2: Geographic STEP 2: Geographic IsolationIsolation

•Some birds from species A cross to a second island.

•The two populations no longer share a gene pool.

STEP 3: Changes in the Gene STEP 3: Changes in the Gene PoolPool

•Seed sizes on the second island favor birds with large beaks.

•The population on the second island evolves into population “B”, with larger beaks.

STEP 4: Reproductive STEP 4: Reproductive IsolationIsolation

• If population B birds cross back If population B birds cross back to the first island, they to the first island, they will not will not matemate with birds from with birds from population A.population A.

• Populations A and B are Populations A and B are separate species.separate species.

STEP 5: Ecological STEP 5: Ecological CompetitionCompetition

As species A and B As species A and B competecompete for available for available seeds on the first island, they continue to seeds on the first island, they continue to evolve in a way that increases the evolve in a way that increases the differencesdifferences between them. between them.

A new speciesA new species—C—may evolve.—C—may evolve.

Continued EvolutionContinued Evolution

This process of This process of isolation, genetic isolation, genetic change, and change, and reproductive isolation reproductive isolation probably repeated probably repeated itself often across the itself often across the entire Galápagos entire Galápagos island chain.island chain.

Post-Darwin Evolutionary Post-Darwin Evolutionary StudiesStudies

Scientific evidence supports the theory that Scientific evidence supports the theory that living species descended with living species descended with modificationmodification from from common ancestorscommon ancestors that lived in the that lived in the ancient past.ancient past.

Scientists predict that as new fossils are Scientists predict that as new fossils are found, they will continue to found, they will continue to expandexpand our our understanding of how species understanding of how species evolvedevolved..

As our knowledge of As our knowledge of DNADNA and and genomesgenomes grows we are better able to understand grows we are better able to understand relationshipsrelationships between species. between species.