ch 21 - genes within populations

8/2/2019 Ch 21 - Genes Within Populations

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Warm-Up 12/13/11

If a population has the following genotype

frequenciesAA = 0.42, Aa = 0.46, and aa =

0.12 what are the allele frequencies?A = 0.42 a =0.12

A =0.6 a = 0.4

A = 0.65 a = 0.35

A = 0.76 a = 0.24

A = 0.88 a = 0.12


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Chapter 21Genes Within

Populations

21.1Genes vary in natural populations

21.2Why do allele frequencies change in populations?21.3Selection can act on traits affected by many genes.


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Natural Selection


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18.1 MICROEVOLUTION

Population -- all the members of a single species

Evolution that occurs within a population = microevolution

Population genetics studies variations in gene pools


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Gene pool total of all the alleles in the population

Alleles chromosome sections that code for specific proteinstraits

Examples: Humans have alleles for

blue eyes / brown eyes /green eyescurly/straight hairblood type A / B / O / AB


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Hardy-Weinberg Principle

Why do allele frequencies change in populations?

They wont as long as the following assumptions are

met. The population size is very large

Random mating is occurring

No mutation takes place

No genes are input from other sources (no immigrationtakes place)

No selection occurs


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Hardy-Weinberg Equation

(p + q) 2 = p2 + 2pq + q2

p2 = individuals homozygous for the dom allele

2pq = individuals heterozygous

q2 = individuals homozygous for the rec allele


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5 Factors that alter proportions enough to

deviate from Hardy-Weinburg1. Mutation

2. Gene Flow (immigration into and emigration

out of a population)3. Non-random mating

4. Genetic drift (random change in allele

frequencies)5. Selection


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1. Mutation

Generally so low that they DO NOT effect

Hardy-Weinburg

1 in 100,000 cells mutates Ultimate source of genetic variation


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2. Gene Flow

1) movement of alleles from one population to another, by

migration2) Increases variation3) Continued gene flow decreases diversity, gene poolsbecome more similar4) Can prevent speciation from occurring


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Example of GENE FLOW

Each rat snake represents a separate population

of snakes These snake remain similar and can interbreed This keeps their gene pools somewhat similar They are considered subspecies

Figure 18.3


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Figure 18.3

3 N d M i


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3. Nonrandom Mating

1. Nonrandom mating individuals of certain genotypes mate with oneanother more commonly than would be expected on a random basis

2. Inbreeding is mating between relatives to a greater extent than bychance.a. Inbreeding does not change the allele frequencies.b. However, inbreeding decreases the proportion of heterozygotes.


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3. Assortative mating occurswhen individuals mate with thosethat have the same phenotype.

5. Sexual selection occurs whenmales compete for the right toreproduce and the female selectsmales of a particular phenotype.

(guppies, lions)

4 GENETIC DRIFT


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4. GENETIC DRIFT In a small population

particular alleles change drastically by chance alone.

Occurs when founders start a new population or after abottleneck


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Bottleneck Effect caused by a severe reduction inpopulation, reduces overall diversity. Ex Cheetah

FOUNDER EFFECT


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FOUNDER EFFECT

The founder effect is anexample of genetic driftwhere rare alleles orcombinations occur inhigher frequency in apopulation isolated from

the general population.

Dwarfism in Amishcommunities

Polydactylism in Amishcommunities


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5. Selection

Artificial Selection

Traits are selected

Natural SelectionEnvironment determines which individuals will

produce the most offspring


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3 Conditions for Natural Selection

1. Variation must exist among individuals in a

population

2. Variation among individuals results indifferences in the number of offspring surviving

in the next generation

3. Variation must be genetically inherited


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Fitness The number of surviving offspring left inthe next generation. Most fit phenotype is assigned a fitness value of 1

Relative fitness compares the fitness of onephenotype to another

Measuring Fitness


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Frequency-Dependent Selection

The fitness of a phenotype depends on its

frequency within a population

Negative Frequency-Dependent Rare phenotype is favored by selection

Promotes variation

Positive Frequency-Dependent

Common phenotype is favored by selection Eliminates variation

Heterozygous Advantage


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Heterozygous Advantage

Forms of Selection


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Forms of Selection

1. Directional Selection

One phenotype isfavored over another

Causes a shift in theoverall appearance ofthe species

Ex: horses get larger

2 STABILIZING SELECTION


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2. STABILIZING SELECTION

occurs when extremephenotypes areeliminated and theintermediatephenotype is favored.

Human babies have an average size


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Human babies have an average size

Too big and they can't get through birthcanal

Too small and they have lowsurvivability

DISRUPTIVE SELECTION -- occurs when extreme


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phenotypes are favored and can lead to more than one distinctform.

CAN LEAD TO SPECIATION

I i thi i


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Imagine this scenario....

Sleebos come in many sizes, the most common Sleebo is 4

inches long, but some can be 10 inches and others can be assmall as 1 inch. A new predator is introduced to the Sleeboisland. Small sleebos are able to hide under rocks and avoidbeing eaten. Large sleebos are too big for the predator to eat.

What will happen to the Sleebo population?

Types of Selection


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MACROEVOLUTION


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MACROEVOLUTION

Evolutionary change above the

species level Speciation the splitting of one

species into two or morespecies

speciation is the final result ofchanges in gene pool

What is a Species?


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What is a Species? A group of actually or potentially interbreeding

populations (isolated from other groups)

Gene flow can occur between populations of thesame species

Hybrids occur when members of


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Hybrids occur when members ofdifferent species produceoffspring...

Lion + Tiger = Liger

Tiger + Lion = Tigon

Biochemical genetics uses DNA hybridization


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g ytechniques to determine relatedness of organisms;the phylogenetic species concept uses DNA/DNA

comparisons

Hyenas are now placed with the catfamily due to DNA sequencing.

SPECIATION


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Flycatcher species

Empidonax minimus Empidonax virescens Empidonax tralli

What stops these speciesfrom mating with each other?

Figure 18.10a


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Figure 18.10b


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Figure 18.10c


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B. REPRODUCTIVE ISOLATING MECHANISMS


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PREZYGOTIC

Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation

Gamete isolation

Damselfly penises

B. REPRODUCTIVE ISOLATING MECHANISMS


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POSTZYGOTIC

Zygote mortality Hybrid sterility F2 fitness

TEMPORAL ISOLATION


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MODES OF SPECIATION


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Allopatric Speciation

Populationsseparatedgeographically

Variations

accumulate Reproductiveisolation

separates thepopulation

SYMPATRIC SPECIATION


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SYMPATRIC SPECIATION

Sympatric speciation

would occur whenmembers of a singlepopulation develop adifference without

geographic isolation

Ex. Apple Maggot flieschoosing a particular type

of apple (SympatricSpeciation)

Ex. Mate preference

Sympatric vs Allopatric


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y p p

ADAPTIVE RADIATION


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A single ancestral species

become several differentspecies

Speciation


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Evolutionary Pace


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WORD ATTACK!


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1. Phyletic Speciation / Divergent Speciation2. Mechanical Incompatibility

3. Gamete Incompatibility4. Hybrid Inviability5. Behavioral / Temporal / Habitat / Geographic Isolation6. Adaptive Radiation7. Fitness / Relative Fitness

8. Premating Isolation / Postmating Isolation9. *Allopatric / Sympatric Speciation10. Disruptive / Directional / Stabilizing Selection11. Polymorphism

12. Sexual Selection / Nonrandom Mating / Assortive Mating13. Gene Pool / Gene Flow / Genetic Drift14. Microevolution / Macroevolution15. Founder Effect / Bottleneck Effect

ch 21 - genes within populations

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