AP Biology Chapter 23 The Evolution of Populations

Campbell and Reece 10th Edition

AP BiologyChapter 23

The Evolution of Populations

Individuals do not evolve, populations do over time

Individuals do not evolve, populations do

Medium Ground Finch from island of Daphne Major in Galápagos Islands

Long period of drought altered their food supply to mostly larger nuts & over the years those individuals with larger beaks were more successful

Overall size of bird b/4 & after drought years

Average beak size & size of individual birds larger after the drought so….

The medium ground finch population had evolved by natural selection

Genetic Variationas

Cause of EvolutionDarwin reasoned that natural selection acted on genetic variation of populations

He knew nothing about genes

Few yrs later: Mendel’s paper on inheritance in pea plants: stage set for understanding variation

Genetic VariationGenotype

inheritable, phenotypes are not

Example: moth, Nemoria arizonaria, appears very different eating oak flowers vs. oak leaves

Phenotype is result of : genotype + environmental influence

In general, only the genetically determined part of a phenotype can affect evolution

Discrete characters = “either/or” ( Mendel’s pea) = single gene

Most heritable variations involve quantitative characters: vary along a continuum ≥ 2 genes

way to quantify gene variabilityaverage % of loci that are heterozygous

can calculate average: turns out if the average heterozygosity is 14% there is enough genetic variation for natural selection to act evolutionary change

Average Heterozygosity

Gel Electrophoresis used to calculate heterozygosity

Gel Electrophoresisdoes not show

silent mutations (DNA changes but still codes for same a.a.)

Geographic Variationdifferences in

genetic composition of separate populations

Gene Variation Between Populations

Cline: a graded change in a character along a geographic axis

MutationGene DuplicationSexual ReproductionOther process that results in new alleles or new genes

Sourcesof

Genetic Variation

Organisms with short life spans new genetic variants arise fairly rapidly

Sources of genetic variation

1. Mutations can’t predict where in genome or

what type mutation for multicellular organisms only

mutations in gametes cell line passed to new generations (most are in somatic cell line)

most point mutations silent or only slightly harmful, rarely are they beneficial

Formation of New Alleles

chromosomal changes that delete, disrupt, or rearrange usually lethal or harmful

if genes left intact they may be neutral changesTranslocation:

Part of 1 chromosome breaks off & attaches to another chromosome

Altering Gene # or Position

Translocations

if large segments duplicated usually harmful

duplications of small pieces may be beneficial

mutations accumulate over

time eventually that duplication takes on

new role

end result: expanded genome

Duplications of Chromosomes

average mutation rate in plants & animals is considered low

~ 1 mutation in every 100,000 genes /

generation

Rapid Reproduction

Prokaryotic Mutation Ratesshorter

generation spans allows for generation of genetic variation in a population

virus populations, especially retroviruses process is fastest

HIVsingle stranded RNA:less

complicated to duplicate

fewer RNA repair mechanisms in host cells

HIVmost effective treatment for a quickly mutating retrovirus has been combination protocols

Sexual Reproductionmost of genetic variation due to crossing over and independent assortment of chromosomes in meiosis and fertilization

Hardy-Weinberg Equation can tell you if a Population

is Evolving

presence of genetic variation does not guarantee that population is evolving

1 of 3 factors that cause evolution must be at work in a population

Population: group of same species in same area that interbreed, with fertile offspring

Populationsexamples of isolated populations:IslandsLakes

even populations not strictly isolated members tend to breed with own population so are genetically closer to them than other groups

Gene Poolsconsists of all copies of every allele at every locus in all members of a population

Fixed Genesif there is only 1 allele for a locus that allele is said to be fixed in the gene pool; entire population is homozygous for that gene

if there are 2 or more alleles for a locus then individuals may be homozygous or heterozygous

Each allele has a frequency in the population

to test whether natural selection is acting on a particular locus:Determine what the frequency

would be if it were not evolvingThen compare that calculation

with what you measure in the population

No difference: not evolvingdifference: evolving

The Hardy-Weinberg Principle

Hardy-Weinberg Principle 1908Hardy Weinberg

states that the frequencies of alleles & genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation & recombination of alleles are at work

If that is true the population is said to be in HARDY-WEINBERG EQUILIBRIUM

Hardy-Weinberg Principle

Hardy-Weinberg Equilibrium

Hardy-Weinberg Equilibrium

assumes random mating

http://nhscience.lonestar.edu/biol/hwe/q1d.html http://www.phschool.com/science/biology_place/labbench/lab8/intro.html

Problem 2:If 9% of an African population is born with a severe form of sickle cell anemia (ss) what % of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

Hardy-Weinberg Problems

2pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers)

Answer to problem 2

1. No Mutations2. Random Mating3. No Natural Selection4. Extremely Large Populations5. No Gene Flow

Conditions for Hardy-Weinberg Equilibrium

Departure from any of the 5 conditions usually results in evolutionary changes

A population may be evolving at some gene loci and in Hardy-Weinberg Equilibrium at other loci

Can be used to estimate the frequency of a gene causing inherited disease in a population

Must assume:No new mutationsRandom matingIgnore any effects of differential

survival & reproductive successNo genetic drift

Applying the Hardy-Weinberg Principle

How Allele Frequencies are Altered in a Population

1. No Mutations: not usually significant unless mutation produces new alleles that have a strong influence in a (+) or (-) way

2. Random Mating: not usually significant

3. No Natural Selection cause most

4. Extremely Large Populations evolutionary

5. No Gene Flow change

Conditions Necessary for H-W Equilibrium

is based on differential success in survival & reproduction

if NS consistently favoring some alleles over others, NS can cause adaptive evolution (dfn: evolution that results in a better match between organisms & their environment)

NATURAL SELECTION

process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next

the smaller the population the more pronounced the effect

GENETIC DRIFT

GENETIC DRIFT

1. Founder Effectgenetic drift that occurs when a

few individuals become isolated from a larger population & form a new population whose gene pool composition is not reflective of original population

2 Examples of Genetic Drift

Founder Effect

Founder EffectTristan da Cunha

15 British colonist in 1814

1 colonist carried recessive allele for retinitis pigmentosa

Tristan da Cunha

By late 1960’s, there were 240 descendants of the original founders

4 had retinitis pigmentosaThis frequency is 10x higher than frequency of retinitis pigmentosa in England

Tristan da Cunha

2. Bottleneck Effect: occurs when the size of a population is reduced, as by a natural disaster or human actions. The resulting population is genetically different than original population.

2nd Example of Genetic Drift

Bottleneck Effect

Genetic Drift :1. is significant in small populations2. can cause allele frequencies to

change at random3. can lead to a loss of genetic

variation w/in populations4. can cause harmful alleles to

become fixed

Summarizing Effects of Genetic Drift

the transfer of alleles from one population to another as result of movement of fertile individuals or their gametes

GENE FLOW

Gene Flow

transferred alleles may increase a population’s ability to adapt to local conditions

Culex pipiens spread of insecticide- resistant alleles used

to treat mosquitoes to prevent spread of West Nile

Gene Flow

outcome of NS is not randomNS increases frequencies of alleles that provide reproductive advantage so, leads to adaptive evolution

NS acts directly on the phenotype & indirectly on the genotype

Natural Selection is only mechanism that consistently causes adaptive evolution

the contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individuals in the population

Relative Fitness

1. DIRECTIONAL SELECTION conditions favor individuals

favoring one extreme of a phenotype

shifts curve in one direction or other

Types of Selection

2. DISRUPTIVE SELECTIONconditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes

Types of Selection

3. STABILIZING SELECTIONconditions favor the intermediate phenotype and act against both extremes

reduces variation

Types of Selection

Natural selection will increase the frequencies of alleles that enhances survival & reproduction so… over time adaptations arise

genetic drift & gene flow may cause changes that are either advantageous or disastrous

ADAPTIVE EVOLUTION

individuals with certain inherited characteristics are more likely to obtain mates

can result in sexual dimorphism: marked differences between the 2• sex characteristics

Sexual Selection

Sexual Dimorphism: House Finches

1. Intrasexual Selection selection w/in same sex Alpha male

2. Intersexual Selection aka mate choice females choosy about their mate

(often depends on male showiness)

Mechanisms of Sexual Selection

Intrasexual Selection

Intersexual Selection: Sandpiper male

1 hypothesis: females have linked “good genes”

with traitstudy: gray tree frog

2nd hypothesisfemales have linked “good health”

with these traitsstudy: birds

How do female preferences devlop?

neutral variation: differences in DNA that do not confer an advantage or disadvantage

Why don’t all genes move toward neutrality?tendency for directional or stabilizing

selection countered by mechanisms that preserve or restore variation

The Preservation of

Genetic Variation

recessive alleles hidden and carried forward in heterozygotes

heterozygote protection maintains a huge pool of alleles that might not be favored under present conditions, but could bring benefits in environment changes

Diploidy

Advantages determined by Alleles

occurs when natural selection maintains 2 or more forms in a population

2 types: 1. heterozygote advantage2. frequency-dependent selection

Balancing Selection

Heterozygotes have survival advantage

If phenotype of a heterozygote is intermediate between the 2 homozygotes then this advantage is: stabilizing selection

If phenotype of heterozygote same as dominant homozygote this advantage is directional selection

Heterozygote Advantage

Example: Sickle CellHeterozygote Advantage

SShomozygous dominantno protection against malaria

Ssheterozygousprotection against malariafew sickle cells but not harmful

sshomozygous recessivedie young of sickle cell

Sickle Cell Allele

the fitness of a phenotype depends on how common it is in the population

Frequency-Dependent Selection

Scale-Eating Fish in Lake Tanganyika

eat scales off flank of prey

some left-mouthed some right-mouthed

right-mouthed dominant to left-mouthed

Scale-eating Fishselection favors whichever mouth phenotype is least common (prey fish learn to avoid attacks from more common

Why Natural Selection does not Result in a “Perfect” Organism1. Selection can

only act on existing variationsNS favors only

the fittest available phenotypes

2. Evolution is limited by historical constraintsNS has to

work with existing structures

3. Adaptations are often compromises

each organisms must do many things: some structures are a compromise (Walrus fins great for swim, not so good for walking on rocks)

4. Chance, natural selection, & the environment interact

founding population may not carry “best” alleles for new environment; environments can change