Ch. 23 Evol of Populations 9e

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Ch. 22/23 Warm-up

1. List 5 different pieces of evidence for evolution.

2. (Review) What are the 3 ways that

sexual reproduction produces genetic diversity?

3. What is 1 thing you are grateful for today?

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Ch. 23 Warm-up

1. In a population of 200 mice, 98 are homozygous

dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb).

a) The allele frequencies of this population are: B allele: ___ b allele: ___

b) The genotype frequencies are: BB: ___ Bb: ___ bb: ___

2. Use the above info to determine the genotype frequencies of the next generation:

B (p): ___ b (q): ___

BB (p2): ___ Bb (2pq): ___

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Chapter 23

The Evolution of

Populations

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What you must know:

• _{How mutation and sexual reproduction}

each produce genetic variation.

• _{The conditions for Hardy-Weinberg}

equilibrium.

• _{How to use the Hardy-Weinburg equation}

to calculate allelic frequencies and to test whether a population is evolving.

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Smallest unit of evolution

Microevolution: change in the allele frequencies of a population over

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• _{Darwin did not know how}

organisms passed traits to offspring

• _{1866 - Mendel published his}

paper on genetics

• _{Mendelian genetics supports}

Darwin’ s theory  Evolution is based on genetic

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Sources of Genetic Variation

• Point mutations: changes in one base (eg. sickle cell)

• Chromosomal mutations: delete, duplicate, disrupt, rearrange  usually harmful

• _{Sexual recombination}_{: contributes to most} of genetic variation in a population

1.Crossing Over (Meiosis – Prophase I)

2.Independent Assortment of Chromosomes (during meiosis)

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Population genetics: study of how

populations change genetically over

time

Population: group of individuals that

live in the same area and interbreed,

producing fertile offspring

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• _{Gene pool}_{: all of the alleles for all genes}

in all the members of the population

• _{Diploid species: 2 alleles for a gene}

(homozygous/heterozygous)

• _{Fixed allele}_{: all members of a population}

only have 1 allele for a particular trait

• _{The more fixed alleles a population has,}

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Hardy-Weinberg Principle

Hardy-Weinberg Principle: The allele and

genotype frequencies of a population will remain constant from generation to

generation

…UNLESS they are acted upon by forces

other than Mendelian segregation and recombination of alleles

Equilibrium = allele and genotype frequencies remain constant

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Conditions for Hardy-Weinberg

equilibrium

1. No mutations.

2. Random mating.

3. No natural selection.

4. Extremely large population size.

5. No gene flow.

If at least one of these conditions is NOT NOT

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Allele Frequencies:

• _{Gene with 2 alleles : p, q}

p = frequency of dominant allele (A) q = frequency of recessive allele (a)

Note:

1 – p = q

1 – q = p

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Genotypic Frequencies:

•_{3 genotypes (AA, Aa, aa)}

p2 = AA (homozygous dominant) 2pq = Aa (heterozygous)

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Allele

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Genotypic

frequencies

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Strategies for solving H-W Problems:

1. If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles.

2. If you know phenotypes, then use “aa” to find q2, and then q. (p = 1-q)

3. Use p2 + 2pq + q2 to find genotype

frequencies.

4. If p and q are not constant from generation to generation, then the POPULATION IS EVOLVING!

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Hardy-weinberg practice problem #1

The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%) for a population of 1612 moths.

AA = 1469 Aa = 138 aa = 5

Allele Frequencies: A = a =

Genotypic Frequencies: AA =

Aa = aa =

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Hardy-weinberg practice problem #2: PTC Tasters

• _{Taster = AA or Aa Nontaster = aa} • _{Tasters = ____} _{Nontasters = ___}

q2 = q =

p + q = 1 p = 1 – q = p2 + 2pq + q2 = 1

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Conditions for Hardy-Weinberg

equilibrium

1. No mutations.

2. Random mating.

3. No natural selection.

4. Extremely large population size.

5. No gene flow.

If at least one of these conditions is NOT NOT

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Minor Causes of Evolution: Minor Causes of Evolution:

#1 - Mutations

• _{Rare, very small changes in allele}

frequencies

#2 - Nonrandom mating

• _{Affect genotypes, but not allele}

frequencies

Major Causes of Evolution Major Causes of Evolution:

• _{Natural selection, genetic drift, gene flow}

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Major Causes of Evolution

#3 – Natural Selection

• _{Individuals with variations better suited}

to environment pass more alleles to next generation

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Major Causes of Evolution

#4 – Genetic Drift

• Small populations have greater chance of fluctuations in allele frequencies from one generation to another

• Examples:

• Founder Effect

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Genetic Drift

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Founder Effect

• A few individuals isolated from larger population

• Certain alleles under/over represented

Polydactyly in Amish population

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Bottleneck Effect

• _{Sudden change in environment drastically}

reduces population size

Northern elephant seals hunted nearly to extinction in

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Major Causes of Evolution

#5 – Gene Flow

• _{Movement of fertile}

individuals between populations

• _{Gain/lose alleles} • _{Reduce genetic}

differences between populations

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How does natural selection bring about adaptive evolution?

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Natural selection can alter frequency

distribution of heritable traits in 3 ways:

1.Directional selection

2.Disruptive (diversifying) selection

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Directional Selection: eg. larger black bears survive extreme cold better than small ones

Disruptive Selection:

eg. small beaks for small seeds; large beaks for large seeds

Stabilizing Selection: eg. narrow range of human birth weight

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Sexual selection

• Form of natural selection – certain individuals more likely to obtain mates

• _{Sexual dimorphism}_{: difference between 2}

sexes

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Sexual selection

• Intrasexual_Intrasexual – selection within same sex (eg. M compete with other M)

• _Intersexual_Intersexual_{– mate choice (eg. F choose} showy M)

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Preserving genetic variation

• Diploidy_Diploidy: hide recessive alleles that are less favorable

• _{Heterozygote advantage}_{Heterozygote advantage}_{: greater fitness} than homozygotes

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Running Time: 14:03 min

HHMI Video:

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Natural selection cannot fashion perfect organisms.

1. Selection can act only on existing variations.

2. Evolution is limited by historical constraints.

3. Adaptations are often compromises.

4. Chance, natural selection, and the environment interact.

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Sample Problem

Define the following examples as directional, disruptive, or stabilizing selection:

a) Tiger cubs usually weigh 2-3 lbs. at birth

b) Butterflies in 2 different colors each represent a species distasteful to birds

c) Brightly colored birds mate more frequently than drab birds of same species

d) Fossil evidence of horse size increasing over time