Download Chapter 1 Art Slides

Population Genetics:
Populations change in genetic
characteristics over time
 Ways to measure change:


Allele frequency change (B and b)
 Genotype frequency change (BB, Bb, & bb)
Hardy-Weinberg Equation
p+q=1
 p = the frequency of dominant allele (B)
 q = the frequency of recessive allele (b)
 Total frequency = 1

 Assumes
a 2 allele simple traits
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
2
How to Calculate Frequencies



Imagine there are 1,000 plants in a population
Only two flower colors are possible:
red (R = dominant) and white (r = recessive).
Let there be
 160
RR individuals (red homozygotes)
 480 Rr individuals (red heterozygotes)
 360 rr individuals (white homozygotes)
How to Calculate Frequencies
Total population = 1,000
 The genotype frequency is:


RR = 160/1000 = .16 or 16%
 Rr = 480/1000 = .48 or 48%
 rr = 360/1000 = .36 or 36%
How to Calculate Frequencies


Each plant has two alleles for flower color
Since there are 1,000 individuals, there are 2,000
alleles
 160
RR + 480 Rr + 360 rr = 1000 plants
 2x160 RR + 2x480 Rr + 2x360 rr = 2000 alleles

Allele frequency:
R
= (2 x 160 + 480 + 0) = 800/2000 = .40 (40%)
 r = (0 + 480 + 2 x 360) = 1200/2000 = .60 (60%)
How to Calculate Frequencies


In 1908, Hardy & Weinberg independently
discovered another way to calculate gene
frequencies in an equilibrium population
Equilibrium populations do not change, and
assume:





No mutation
No gene flow (no movement of alleles in or out)
No selection
Mating is random
The population is large
Given a Hardy-Weinberg equilibrium:

Allele frequencies and genotype frequencies do not
change in a population
p + q = 1
 p2 + 2pq + q2 = 1
160 RR + 480 Rr + 360 rr = 1000 plants
 p2= freq of genotype RR (p is freq of allele R)
 2pq = freq of Rr
 q2 = freq of rr =.36
 q = √¯ q2 = .6 (p = 1 – q)

Genetic Variation: The Raw
Material of Change
Genetic variation - differences in DNA
sequences in a population
 Population changes occur because of
these differences
 Two major sources of variation:


Mutations
 Recombination
Four Mechanisms of Change

Changes in allele frequencies in a
population occur in four ways:

Mutations
 Gene flow
 Genetic drift
 Natural selection

Animation on mechanisms of change
Mutations
Mutations are random events
 Ultimately, all new genetic variation comes
from mutations
 Mutations can be harmful, beneficial, or
neutral

Gene Flow
Gene flow -movement of alleles between
two different populations
 May result in new alleles being introduced
or rare alleles being lost by migration

Genetic Drift



The result of chance
events
Leads to random changes
in allele frequencies
The smaller the
population, the greater
the genetic drift
 By
chance, the white allele
at right decreased
Genetic Drift
May reduce genetic variation
 May fix alleles that are neutral, harmful, or
beneficial in a population
 Occurs in a genetic bottleneck

Genetic Bottlenecks

Occur when there is a drop in population
size

Environmental disaster
 Founder effect

New population is less genetically diverse
than original population
Natural Selection



When individuals with favorable traits
have a higher reproductive rate
Artificial selection is similar, but people
pick the favorable trait not nature.
Three types of Natural Selection:
1.
2.
3.
Directional selection
Stabilizing selection
Disruptive selection
Directional Selection


Individuals of one
extreme phenotype
are favored
Peppered moths
Stabilizing Selection



Individuals with an
intermediate phenotype
are favored
The extreme phenotypes
are selected against
Human birth weights
Disruptive Selection



Where both extreme
phenotypes are favored
The intermediate
phenotypes are selected
against
African seed cracker birds
Three Types of Natural Selection
Sexual Selection


A form of natural
selection
Females place
selective pressure on
males
 Peacock’s
feathers
 Lion’s mane
How Populations Change
Mutations and recombination occur randomly
 These events create genetic variation
 Gene flow, genetic drift, and natural selection
change allele frequencies


Gene flow tends to equalize allele frequencies
between 2 pops
 Genetic drift an natural selection tend to diverge
allele frequencies between 2 pops
Animations on the causes of differences in
the alpine skypilot population
1.
2.
3.
4.
5.
6.
Differences in the population
Variation in the population
The variation is inherited
There is variable survival
There is variation in offspring production
Conclusion
Concept Quiz
The average litter size for a squirrel is three.
Above and below three is not favored. What
type of selection is occurring?
A.
B.
C.
Disruptive selection
Stabilizing selection
Directional selection
Concept Quiz
A population of slugs has 230 individuals that are
AA, 570 that are Aa, and 200 that are aa. What is
the frequency of the A allele?
A.
B.
C.
.50 or 50%
.65 or 65%
.52 or 52%
Concept Quiz
A few birds manage to start their own
population on a new island. What best
describes this situation?
A.
B.
C.
Genetic drift
Gene flow
Natural selection
Process Animation 17.1
The Hardy-Weinberg Equation
Macintosh
Windows