Evolution and Gene Evolution and Gene FrequenciesFrequencies

Chapter 5Chapter 5

Gene FrequenciesGene Frequencies Populations and gene Populations and gene

poolspools PopulationsPopulations

Groups of individuals of Groups of individuals of the same species that the same species that occupy a given area at occupy a given area at the same time and share the same time and share a unique set of genesa unique set of genes

AlleleAllele – varying – varying expressions of genesexpressions of genes

Ex. – Hair or fur colorEx. – Hair or fur color

Gene FrequenciesGene Frequencies Gene poolGene pool – the sum – the sum

of all alleles for all of all alleles for all traits in a sexually traits in a sexually reproducing population reproducing population

Reasons for variationReasons for variation Independent Independent

assortmentassortment Crossing-over Crossing-over Chance of a particular Chance of a particular

sperm fertilizing an eggsperm fertilizing an egg Rearrangements of Rearrangements of

number and structure number and structure of chromosomesof chromosomes

MutationsMutations

Gene FrequenciesGene Frequencies Hardy-Weinberg Hardy-Weinberg

theoremtheorem The mixing of alleles at The mixing of alleles at

meiosis and their meiosis and their subsequent subsequent recombination do not recombination do not alter the relative alter the relative frequencies of the alleles frequencies of the alleles in future generationsin future generations

Requires that certain Requires that certain assumptions be metassumptions be met

Gene FrequenciesGene Frequencies AssumptionsAssumptions

Population size must Population size must be largebe large

Sexual reproduction Sexual reproduction in the population in the population must be randommust be random

Migration can not Migration can not occuroccur

Mutations must not Mutations must not occuroccur

Gene FrequenciesGene Frequencies Evolutionary mechanismEvolutionary mechanism

Evolution is not bad, nor Evolution is not bad, nor good…it is just a factgood…it is just a fact

Evolution is a result of some Evolution is a result of some individuals in a population individuals in a population surviving and being more surviving and being more effective at reproducing than effective at reproducing than others in the population others in the population ((Fitness)Fitness)

This leads to changes in the This leads to changes in the allelic frequency, which is allelic frequency, which is the definition of…the definition of…

EVOLUTION!EVOLUTION!

Gene FrequenciesGene Frequencies Population sizePopulation size

The smaller the size the The smaller the size the more significance chance more significance chance may bemay be

Survival may not be Survival may not be because of being the best, because of being the best, but because of chancebut because of chance

Gene FrequenciesGene Frequencies Genetic DriftGenetic Drift

Chance events influencing Chance events influencing the frequencies of genes in the frequencies of genes in populations – populations – Neutral Neutral EvolutionEvolution

Flipping a coinFlipping a coin Founder EffectFounder Effect

When individuals from a When individuals from a group colonize new habitatsgroup colonize new habitats

Seldom carry the same allelic Seldom carry the same allelic frequenciesfrequencies

New colony is likely to have New colony is likely to have distinctive genetic makeupdistinctive genetic makeup

Gene FrequenciesGene Frequencies Bottleneck effectBottleneck effect

Example of the Founder Example of the Founder EffectEffect

Population becomes very Population becomes very small and reduces the small and reduces the amount of variationamount of variation

Traditional view – Traditional view – bottlenecks decrease the bottlenecks decrease the genetic variation and makes genetic variation and makes the species less likely to the species less likely to withstand environmental withstand environmental stressesstresses

Populations with high Populations with high diversity are likely to have diversity are likely to have individuals with genes that individuals with genes that allow them to withstand allow them to withstand environmental pressureenvironmental pressure

Gene FrequenciesGene Frequencies Neutral evolution Neutral evolution

When gene frequencies change When gene frequencies change independently of natural independently of natural selectionselection

These changes do not affect These changes do not affect the fitness of the organismsthe fitness of the organisms

Gene FrequenciesGene Frequencies Gene FlowGene Flow

Changes in relative allele Changes in relative allele frequency from the frequency from the migration of individuals migration of individuals (Evolution)(Evolution)

ImmigrateImmigrate Individuals enter a Individuals enter a

population from the outsidepopulation from the outside EmigrateEmigrate

Individuals leave a Individuals leave a populationpopulation

Can result in the Can result in the separation of one species separation of one species into two different speciesinto two different species

Gene FrequenciesGene Frequencies MutationMutation

Changes in the Changes in the structure of genes and structure of genes and chromosomeschromosomes

Mutations are a fact of Mutations are a fact of lifelife

Source of variation Source of variation Origin of all new allelesOrigin of all new alleles Make extinction less Make extinction less

likelylikely Most are harmfulMost are harmful

Gene FrequenciesGene Frequencies Modes of selectionModes of selection

Directional selectionDirectional selection When one extreme When one extreme

phenotypic group of phenotypic group of individuals within a individuals within a species are at a species are at a disadvantage to the other disadvantage to the other extremeextreme

Bad allele becomes Bad allele becomes extinctextinct

Peppered MothsPeppered Moths

Gene FrequenciesGene FrequenciesDisruptive Disruptive selectionselection

Selection in which Selection in which individuals in the individuals in the middle of the range middle of the range of phenotypes is of phenotypes is selected against or selected against or will become extinctwill become extinct

Gene FrequenciesGene FrequenciesStabilizing Stabilizing selectionselection

When the two extreme When the two extreme phenotypes are not an phenotypes are not an advantage.advantage.

Gene FrequenciesGene Frequencies PolymorphismPolymorphism

Two or more distinct Two or more distinct forms with no forms in forms with no forms in betweenbetween

Balanced polymorphismBalanced polymorphism Maintaining each distinct Maintaining each distinct

form at about the same form at about the same amountamount

Gene FrequenciesGene Frequencies SpeciesSpecies

A group of populations A group of populations in which genes are in which genes are actually, or actually, or potentially, exchanged potentially, exchanged through interbreedingthrough interbreeding

Gene FrequenciesGene Frequencies SpeciationSpeciation

The formation of new speciesThe formation of new species Reproductive isolationReproductive isolation

Population must be isolatedPopulation must be isolated Premating isolationPremating isolation

Prevents mating from taking Prevents mating from taking placeplace

Temporal isolationTemporal isolation Behavioral isolationBehavioral isolation

Postmating isolationPostmating isolation Prevents successful fertilization Prevents successful fertilization

and development even though and development even though mating has occurredmating has occurred

Gene FrequenciesGene Frequencies Allopatric speciationAllopatric speciation

When subpopulations When subpopulations become geographically become geographically isolatedisolated

Most common form of Most common form of speciationspeciation

Also results in adaptive Also results in adaptive radiationradiation

Ex. Gallapgos finchesEx. Gallapgos finches

Gene FrequenciesGene Frequencies Parapatric speciationParapatric speciation

Speciation in a small, local Speciation in a small, local populationpopulation

DemesDemes

Gene FrequenciesGene Frequencies Sympatric speciationSympatric speciation

Speciation that occurs in Speciation that occurs in populations that have populations that have overlapping rangesoverlapping ranges