Evolution of PopulationsEvolution of Populations

Chapter 16Chapter 16

16-1 Genes and Variation16-1 Genes and Variation

Darwin’s handicap while developing Darwin’s handicap while developing theory of evolutiontheory of evolution

Discoveries since DarwinDiscoveries since Darwin Commonality of Genetic VariationCommonality of Genetic Variation

At least 2 allelesAt least 2 alleles ““invisible” variation involving small invisible” variation involving small

differences in biochemical processesdifferences in biochemical processes Heterozygous for many genes (4-8% in Heterozygous for many genes (4-8% in

mammals)mammals)

Variation and Gene PoolsVariation and Gene Pools

Population is a group of individuals of the same Population is a group of individuals of the same species that interbreedspecies that interbreed Share the same Share the same gene poolgene pool- all genes, including all - all genes, including all

the different alleles, that are present in a populationthe different alleles, that are present in a population Relative frequencyRelative frequency- the number of times that - the number of times that

the allele occurs in a gene pool, compared with the allele occurs in a gene pool, compared with the number of times other alleles for the same the number of times other alleles for the same gene occurgene occur

In genetic terms, evolution is any change in the In genetic terms, evolution is any change in the relative frequency of alleles in a populationrelative frequency of alleles in a population

Gene Pools When scientists determine whether a population is evolving, they may look at the sum of the population's alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice.  Calculating  Here, in a total of 50 alleles, 20 alleles are B (black), and 30 are b (brown). How many of each allele would be present in a total of 100 alleles?

Sources of Genetic VariationSources of Genetic Variation

The two main sources of genetic variation The two main sources of genetic variation are mutations and the genetic shuffling that are mutations and the genetic shuffling that results from sexual reproductionresults from sexual reproduction Mutation- any change in a sequence of DNAMutation- any change in a sequence of DNA

Mistake in replication of DNA Mistake in replication of DNA Result of radiation or chemicals in environmentResult of radiation or chemicals in environment

Gene shuffling- different combinations of genesGene shuffling- different combinations of genes During production of gametesDuring production of gametes Crossing overCrossing over

Sexual reproduction does not change the Sexual reproduction does not change the relative frequency of alleles in a populationrelative frequency of alleles in a population

Single-Gene and Polygenic Single-Gene and Polygenic TraitsTraits

The number of phenotypes produced The number of phenotypes produced for a given trait depends on how for a given trait depends on how many genes control the traitmany genes control the trait Single-gene trait- trait controlled by a Single-gene trait- trait controlled by a

single gene that has two allelessingle gene that has two alleles i.e. widow’s peak- 2 distinct phenotypesi.e. widow’s peak- 2 distinct phenotypes

Polygenic traits- traits controlled by two Polygenic traits- traits controlled by two or more genes that have two or more or more genes that have two or more allelesalleles

i.e. human height- many possible genotypes i.e. human height- many possible genotypes and phenotypesand phenotypes

16-2 Evolution as Genetic 16-2 Evolution as Genetic ChangeChange

Evolutionary fitness/ Evolutionary Evolutionary fitness/ Evolutionary adaptationadaptation

Natural selection doesn’t act directly Natural selection doesn’t act directly on geneson genes

Natural selection on single-gene Natural selection on single-gene traits can lead to changes in allele traits can lead to changes in allele frequencies and thus to evolutionfrequencies and thus to evolution Lizard color mutations and the effectsLizard color mutations and the effects

Color Mutations   Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors.

Natural Selection on Polygenic Natural Selection on Polygenic TraitsTraits

Effects are more complexEffects are more complex Natural selection can affect the Natural selection can affect the

distributions of phenotypes in any of three distributions of phenotypes in any of three ways:ways: Directional selection- when individuals at one Directional selection- when individuals at one

end of the curve have higher fitness than end of the curve have higher fitness than individuals in the middle or at the other end individuals in the middle or at the other end

Stabilizing selection- when individuals near the Stabilizing selection- when individuals near the center of the curve have higher fitness than center of the curve have higher fitness than individuals at either end of the curveindividuals at either end of the curve

Disruptive selection- when individuals at the Disruptive selection- when individuals at the upper and lower ends of the curve have higher upper and lower ends of the curve have higher fitness than individuals near the middlefitness than individuals near the middle

Genetic DriftGenetic Drift

Natural selection is not the only source Natural selection is not the only source of evolutionary changeof evolutionary change

Genetics controlled by laws of Genetics controlled by laws of probabilityprobability

In smaller pops, the results may be In smaller pops, the results may be further from laws of probabilityfurther from laws of probability

Genetic driftGenetic drift- random change in allele - random change in allele frequencies that occurs in small frequencies that occurs in small populationspopulations

Genetic Drift cont…Genetic Drift cont…

In small populations, individuals that In small populations, individuals that carry a particular allele may leave carry a particular allele may leave more descendants than other more descendants than other individuals, just by chance. Over time, individuals, just by chance. Over time, a series of chance occurrences of this a series of chance occurrences of this type can cause an allele to become type can cause an allele to become common in a population.common in a population.

founder effect-founder effect- a situation in which a situation in which allele frequencies change as a result of allele frequencies change as a result of the migration of a small subgroup of a the migration of a small subgroup of a populationpopulation

Genetic Drift   In small populations, individuals that carry a particular allele may have more descendants than other individuals. Over time, a series of chance occurrences of this type can cause an allele to become more common in a population. This model demonstrates how two small groups from a large, diverse population could produce new populations that differ from the original group.

Hardy-Weinberg PrincipleHardy-Weinberg Principle

States that the allele frequency in a States that the allele frequency in a population will remain constant population will remain constant unless one or more factors cause unless one or more factors cause those frequencies to changethose frequencies to change

genetic equilibriumgenetic equilibrium- situation in - situation in which allele frequencies remain which allele frequencies remain constantconstant

Hardy-Weinberg cont…Hardy-Weinberg cont…

Five conditions:Five conditions: Random matingRandom mating Population must be very largePopulation must be very large No movement into or out of the No movement into or out of the

populationpopulation No mutationsNo mutations No natural selectionNo natural selection

If the conditions are not met, the If the conditions are not met, the genetic equilibrium will be disrupted, genetic equilibrium will be disrupted, and the population will evolveand the population will evolve

ConditionsConditions

Random matingRandom mating Ensures that each individual has an equal chance of Ensures that each individual has an equal chance of

passing on its alleles to offspringpassing on its alleles to offspring Large populationLarge population

Genetic drift has less effect on large populationsGenetic drift has less effect on large populations No movementNo movement

Population’s gene pool must be kept together and kept Population’s gene pool must be kept together and kept separate from the gene pools of other populationsseparate from the gene pools of other populations

No mutationsNo mutations If genes mutate from one form into another, new alleles If genes mutate from one form into another, new alleles

may be introduced into the population, and allele may be introduced into the population, and allele frequencies will changefrequencies will change

No natural selectionNo natural selection All genotypes in the population must have equal All genotypes in the population must have equal

probabilities of survival and reproductionprobabilities of survival and reproduction

16.3 The Process of 16.3 The Process of SpeciationSpeciation

SpeciationSpeciation- formation of new species- formation of new species As new species evolve, populations As new species evolve, populations

become reproductively isolated from become reproductively isolated from each othereach other

Reproductive isolationReproductive isolation- occurrence in - occurrence in which the members of two which the members of two populations cannot interbreed and populations cannot interbreed and produce fertile offspringproduce fertile offspring Populations have separate gene poolsPopulations have separate gene pools

Reproductive IsolationReproductive Isolation Behavioral Isolation- occurs when two Behavioral Isolation- occurs when two

populations are capable of interbreeding populations are capable of interbreeding but have differences in courtship rituals or but have differences in courtship rituals or other reproductive strategies that involve other reproductive strategies that involve behaviorbehavior Bird songsBird songs

Geographic Isolation- two populations are Geographic Isolation- two populations are separated by geographic barriers separated by geographic barriers i.e. rivers, mountains, or bodies of wateri.e. rivers, mountains, or bodies of water Kaibab and Albert squirrels (Colorado River)Kaibab and Albert squirrels (Colorado River) May not isolate all species (bird populations)May not isolate all species (bird populations)

Temporal Isolation- two or more species Temporal Isolation- two or more species reproduce at different timesreproduce at different times Orchid pollen releaseOrchid pollen release