Objective:

Understand how allele frequencies can show evolution in a population.

Individuals have variations but they do not evolve.

The smallest scale (microevolution) shows change in allele frequencies in a population over time.

Causes of evolution:Natural selection (fittest organisms)Genetic drift (random catastrophes change

allele frequencies)Gene flow (movement of alleles between

populations)

Genetic VariationDiverse, inheritable traits set the stage for

evolution Variation within a population Variation between populations

Sources of Genetic VariationFormation of new alleles by mutationChromosomal alterations (deletion,

duplication, translocation, etc.)Sexual reproduction (crossing over,

independent assortment, and fertilization).

A gene pool is a summative of a population’s genes.

Allele frequency is the number of times one allele appears in the gene pool. # of time allele is present/total # of alleles

Ex: 500 flowers = 320 Red (RR) + 160 pink (RW) + 20 White (WW)Total alleles = 1,000 (each individual has 2

alleles)640 + 160 = 800 red alleles160 + 40 = 200 white allelesFrequency of red = 800/1000 = 80%Frequency of white = 200/1000 = 20%

A control to compare evolving populations to.

H-W Equilibrium1. Large population2. No movement into/out of population3. No mutations4. Random mating5. No natural selection (no beneficial/lethal

alleles)

p = dominant allele frequency q = recessive allele frequency

p + q = 1 p2 + 2pq + q2 = 1 AA + Aa + aA + aa = 1 Applying the H-W Equation

See if evolution is happening (allele frequencies change.)

This can be used to calculate the number of heterozygotes vs homozygous dominant individuals

Natural Selection Nonrandom mating

Traits allow you to have more offspring.Ex: sexual dimorphism: males are elaborately

decorated to attract mates.

© 2011 Pearson Education, Inc.

Animation: Causes of Evolutionary ChangeCauses of Evolutionary Change

Right-click slide / select “Play”Right-click slide / select “Play”

Genetic Drift Random changes in allele frequencies over

time, reducing variation. Bottleneck Effect

An event causes a loss of the majority of a population. Founder Effect

A few individuals leave to start a new population

Gene Flow Reduces differences between populations by

sharing of gametes across them.

Modes of Selection At any moment, populations show a

normal curve for most traits. This curve can change in 3 ways

depending on how the environment selects for a trait.

Phenotypes (fur color)

Original population

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divi

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Directional Selection Common when environment changes. One of two extremes is favored.

EX: Increase size of bears during ice ages

Originalpopulation

Evolvedpopulation

Disruptive Selection Both extremes are favored while average

disappears.EX: beak size in finches (large for hard seeds

and small for soft)

Stabilizing Selection Extremes disappear increasing the

intermediate.EX: birth weight of babies