Population Population Genetics and Genetics and SpeciationSpeciationChapter 16Chapter 16

Variation of Traits within Variation of Traits within a Populationa Population

Microevolution: is the evolution that occurs Microevolution: is the evolution that occurs within a population or a change in the gene within a population or a change in the gene pool over a succession of generationspool over a succession of generations

Macroevolution Macroevolution is evolutionary change on a is evolutionary change on a grand scale, encompassing the origin of novel grand scale, encompassing the origin of novel designs, evolutionary trends and adaptive designs, evolutionary trends and adaptive radiation and mass extinctionradiation and mass extinction

MacroevolutionMacroevolution

Novel designs like feather & wingsNovel designs like feather & wings Trends like increasing brain size in Trends like increasing brain size in

mammalsmammals Adaptive radiation is seen in flowering Adaptive radiation is seen in flowering

plantsplants Mass extinctions like the dinosaursMass extinctions like the dinosaurs

Causes of Variation?Causes of Variation?

MutationMutation RecombinationRecombination Random pairing of gametesRandom pairing of gametes

The Gene PoolThe Gene Pool Definition: the total genetic information Definition: the total genetic information

available in a populationavailable in a population Allele frequencyAllele frequency: : determined by dividing determined by dividing

the number of a certain allele by the total the number of a certain allele by the total number of alleles of all types in the number of alleles of all types in the populationpopulation Ex. Two alleles Ex. Two alleles A, aA, a. If in a set of 100 . If in a set of 100

gametes, half are carrying allele gametes, half are carrying allele AA, then the , then the frequency of frequency of A A is .5 or 50 %. The total of all is .5 or 50 %. The total of all allele types must add up to 100.allele types must add up to 100.

Hardy-Weinberg Theorem Hardy-Weinberg Theorem

Before we can look at microevolution we must Before we can look at microevolution we must consider the H-W theorem. But first….. A few consider the H-W theorem. But first….. A few definitions:definitions: PopulationPopulation--all the members of a single species all the members of a single species

occupying a particular area at the same time.occupying a particular area at the same time. SpeciesSpecies--organisms that share a common gene pool, organisms that share a common gene pool,

interbreed with one anotherinterbreed with one another Gene Pool-Gene Pool- total of all the genes of all the total of all the genes of all the

individuals in a population.individuals in a population.

Hardy-Weinberg Genetic Hardy-Weinberg Genetic EquilibriumEquilibrium

5 conditions that must be 5 conditions that must be met to maintain equilibriummet to maintain equilibrium

Population must be largePopulation must be large Population must be isolated from othersPopulation must be isolated from others No mutationsNo mutations Random mating must occurRandom mating must occur No natural selection can occurNo natural selection can occur

Getting the Hardy-Weinberg Getting the Hardy-Weinberg Equilibrium FormulaEquilibrium Formula

In a wildflower population there are two In a wildflower population there are two alleles for color. alleles for color. AA--pinkpink and and aa--white.white.

500 plants = 1000 alleles 500 plants = 1000 alleles 20 of those plants are white = 40 20 of those plants are white = 40 aa alleles alleles 480 of those plants are pink 480 of those plants are pink

320 are 320 are AAAA = 640 = 640 AA alleles alleles 160 are 160 are AaAa = 160 = 160 AA alleles and 160 alleles and 160 aa alleles alleles

So the frequency of allele So the frequency of allele AA is 800/1000 is 800/1000 = .8 = 80%= .8 = 80%

The frequency allele The frequency allele aa is 200/1000 = .2 = is 200/1000 = .2 = 20%20%

p = the frequency of allele p = the frequency of allele AA q = the frequency of allele q = the frequency of allele aa p + q = 1 (.8 + .2 = 1)p + q = 1 (.8 + .2 = 1)

If you consider genotypic frequenciesIf you consider genotypic frequencies AA = 320/500 = .64 = 64%AA = 320/500 = .64 = 64% Aa = 160/500 = .32 = 32%Aa = 160/500 = .32 = 32% Aa = 20/500 = .04 = 4%Aa = 20/500 = .04 = 4%Hardy-Weinberg equation:Hardy-Weinberg equation:pp22 + 2pq + q + 2pq + q22 = 1 = 1 (frequency of(frequency of (frequency of (frequency of (frequency of (frequency of AA genotype) Aa genotype) aa genotype)AA genotype) Aa genotype) aa genotype)

For our example: .64 + .32 + .04 = 1For our example: .64 + .32 + .04 = 1

Disruption of Genetic Disruption of Genetic EquilibriumEquilibrium

Evolution is the change in a populationsEvolution is the change in a populations’’ genetic material over generations, that is, genetic material over generations, that is, a change of the populationa change of the population’’s allele s allele frequencies or genotype frequencies. frequencies or genotype frequencies.

ANYANY exceptions to the five conditions exceptions to the five conditions necessary for H-W equilibrium can result necessary for H-W equilibrium can result in evolution.in evolution.

Causes of MicroevolutionCauses of Microevolution

Genetic driftGenetic drift Gene flowGene flow MutationsMutations Nonrandom matingNonrandom mating Natural selectionNatural selection

If any of these occur then equilibrium is NOT If any of these occur then equilibrium is NOT present in the population!!present in the population!!

Gene FlowGene Flow

Populations may gain or lose alleles by Populations may gain or lose alleles by gene flow. This is genetic change due to gene flow. This is genetic change due to the migration of fertile individuals or the migration of fertile individuals or gametes between populationsgametes between populations Ex. Human moving around the world.Ex. Human moving around the world.

MutationsMutations

A change in an organismA change in an organism’’s DNAs DNA

Genetic Drift-changes in a gene Genetic Drift-changes in a gene pool of a small population due pool of a small population due to chanceto chance

Two situations that can lead to genetic drift:Two situations that can lead to genetic drift: Bottleneck effect: disasters such as Bottleneck effect: disasters such as

earthquakes or floods reduce the pop. earthquakes or floods reduce the pop. drastically, killing victims unselectively- drastically, killing victims unselectively- reduces genetic variability.reduces genetic variability.

Founder effect: a small number of Founder effect: a small number of individuals colonize an isolated island, lake individuals colonize an isolated island, lake or other new habitat-reduced genetic or other new habitat-reduced genetic variability.variability.

Nonrandom mating or Nonrandom mating or assortive matingassortive mating

Individual select Individual select mates because of a mates because of a particular phenotype.particular phenotype. Ex. Cardinal with the Ex. Cardinal with the

brightest red feathers. brightest red feathers. Peacocks with the Peacocks with the

most most ““eyeseyes”” in tail in tail..

Natural SelectionNatural Selection

Differential success in reproduction Differential success in reproduction because an organism is more fit for their because an organism is more fit for their environment.environment. Which colored dot Which colored dot ““mousemouse”” became most became most

common in your Adaptation Activitycommon in your Adaptation Activity??

Types of SelectionTypes of Selection

StabilizingStabilizing: individual with the average : individual with the average form of a trait have the highest fitness.form of a trait have the highest fitness.

Disruptive SelectionDisruptive Selection: individual with : individual with either extreme variation of a trait have a either extreme variation of a trait have a greater fitness than individual with the greater fitness than individual with the average form of the trait.average form of the trait.

Directional Directional Selection: Selection: individual individual that display a more that display a more extreme form of a extreme form of a trail have greater trail have greater fitness than fitness than individuals with an individuals with an average form of a average form of a trait.trait.

Formation of SpeciesFormation of Species

Biological concept of Species: Biological concept of Species: a population of organisms that can a population of organisms that can

successfully interbreed but cannot breed successfully interbreed but cannot breed with other groups.with other groups.

It all begins with being It all begins with being isolated.isolated.

GeographyGeography: barriers formed by canyons, : barriers formed by canyons, mountains, water or deserts (cities and mountains, water or deserts (cities and highways) can divide or fragment and highways) can divide or fragment and isolate parts of populations from each isolate parts of populations from each other.other.

Natural selection and genetic drift Natural selection and genetic drift cause the two subpopulations to diverge, cause the two subpopulations to diverge, eventually making them incompatible for eventually making them incompatible for mating.mating.

Allopatric SpeciationAllopatric Speciation

New species arise because of New species arise because of geographic isolation.geographic isolation. More likely to happen in small populations More likely to happen in small populations

where gene pool will change quickly.where gene pool will change quickly.

Reproductive IsolationReproductive Isolation

May happen in the May happen in the absence of absence of geographic barriers.geographic barriers.

May be caused by May be caused by disruptive selectiondisruptive selection Temporal: different Temporal: different

breeding timesbreeding times Behavioral: different Behavioral: different

mating calls.mating calls.

Sympatric SpeciationSympatric Speciation Two species develop reproductive Two species develop reproductive

isolation within the same geographic area isolation within the same geographic area by occupying different niches.by occupying different niches.

Rates of SpeciationRates of Speciation

GradualismGradualism: speciation occurs at a : speciation occurs at a regular, gradual rate. Change happens regular, gradual rate. Change happens slowlyslowly

Punctuated equilibriumPunctuated equilibrium: sudden, rapid : sudden, rapid change followed by long periods of change followed by long periods of equilibrium or little change.equilibrium or little change.