Download Chapter 23 Slides

The Evolution of
Populations
Chapter 23
Important Things To Remember
About Evolution
Natural selection acts on individuals
But remember individuals do not evolve
Yet populations do evolve (over time)
Microevolution
Change in allele frequencies in a population
over generations
3 Mechanisms for Microevolution
1. Natural selection
Individuals with certain inherited traits survive and
reproduce better
Only natural selection causes adaptive evolution
2. Genetic drift
Chance events that alter allele frequency
3. Gene flow
Transfer of alleles between populations
Genetic Variation
Evolution requires variation in
heritable traits
Individuals have differences in their
genes (DNA sequences)
Genetic Variation
Genetic variation measured by:
Gene variability: Measured by average
% of loci in population that are
heterozygous (average heterozygosity)
Nucleotide variability: Measured by
comparing DNA sequences directly
Variation Between Populations
Geographic variation
Difference between genetic material of
separate populations
Some geographic separation is complete (e.g.
separate islands) while others are more gradual
Cline
Graded change in a character along a geographic axis
Species will show gradual phenotypic and/or genetic
differences over the geographic area due to gradual changes
in environment
Ldh-Bb allele frequency
1.0
0.8
0.6
0.4
0.2
0
46
44
42
Maine
Cold (6°C)
40
38
36
Latitude (ºN)
34
32
Georgia
Warm (21ºC)
Example of Cline Based On Temperature Change with Climate
30
Sources of Genetic Variation
New alleles can arise by mutation or gene
duplication
Mutations are changes in DNA nucleotide sequence
Only mutations in germ line cells passed to offspring
Many mutations are silent due to redundancy or
changes in non-coding regions
Some mutations are harmful, some may be beneficial
How can we tell if a population is evolving?
We can use the Hardy-Weinberg
Equation which allows us to compare
allele frequency between what would be
expected if evolution was not occurring.
Gene Pool and Allele Frequencies
Population: Localized group of individuals
capable of interbreeding and producing fertile
offspring
Gene pool: all the alleles for all loci in a
population
A locus is fixed if all individuals in population
homozygous for same allele
e.g. All individuals either AA or aa
If there are 2 or more alleles however, individuals in the
population can be homozygous or heterozygous
e.g. AA, aa or Aa
Allele Frequencies
Calculating allele frequency in populations
Diploid organisms: Total # of alleles at a locus is the
total # of individuals x 2
Total # of dominant alleles at a locus =
2 alleles for each homozygous dominant individual
+ 1 allele for each heterozygous
Total # of recessive alleles at a locus =
2 alleles for each homozygous dominant individual
+ 1 allele for each heterozygous
Example for Allele Frequencies
In lobsters, there is a gene C with 2 alleles: CR and CL
CR codes for right handed claws
CL codes for left handed claws
CRCR lobsters will have larger right claws
CLCL lobsters will have larger left claws
CRCL will have both claws the same size
Example for Allele Frequencies
In lobsters, there is a gene C with 2 alleles: CR and CL
CR codes for right handed claws
CL codes for left handed claws
CRCR lobsters will have larger right claws
CLCL lobsters will have larger left claws
CRCL will have both claws the same size
In a population of 10,000 lobsters, there are:
7,500 right handed
2,000 of equal size
500 left handed
Example for Allele Frequencies
If there are 10,000 lobsters, there are 20,000
copies of the C gene (remember diploid?)
Calculate the % of each allele based on the
phenotype frequency
CR = 7,500 (CRCR) x 2 = 15,000 + 2,000 (CRCL) = 17,000
CL = 500 (CLCL) x 2 = 1,000 + 2,000 (CRCL) = 3,000
17,000 + 3,000 = 20,000 (matches # of C gene seen above)
CR frequency is 0.85 or 85% (17,000/20,000)
CL frequency is 0.15 or 15% (3,000/20,000)
0.85 + 0.15 = 1 (sum of alleles is always 1)
Getting back to Hardy-Weinberg
Hardy-Weinberg principle describes a population
that is NOT evolving
• States that frequencies of alleles and genotypes in a
population remain constant from generation to
generation
• If gametes contribute to the next generation randomly,
allele frequencies will not change
• Mendelian inheritance preserves genetic variation in a
population
5 Conditions for Non-evolving Populations
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
Using Hardy-Weinberg
Consider a population of 500 wildflowers (1,000 alleles)
with the following allele frequencies:
CR (red flowers) = 0.8 = p
CW (white flowers) = 0.2 = q
p and q by convention represent the allele frequencies
Selection of Alleles at Random from a
Gene Pool
Alleles in the population
Gametes produced
Frequencies of alleles
p = frequency of
CR allele
= 0.8
Each egg:
Each sperm:
q = frequency of
CW allele
= 0.2
20%
80%
chance chance
20%
80%
chance chance
Hardy-Weinberg Equilibrium
Frequency of genotypes can be calculated
CRCR = p2 = (0.8)2 = 0.64 (64%)
CRCW = 2pq = 2 (0.8)(0.2) = 0.32 (32%)
CWCW = q2 = (0.2)2 = 0.04 (4%)
Frequencies of genotypes confirmed by Punnett
Square
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CW (20%)
CR (80%)
CR
(80%)
64% (p2)
CRCR
Eggs
CW
16% (pq)
CRCW
4% (q2)
CWCW
16% (qp)
CRCW
(20%)
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
+ 16% C R W
(from C C plants)
= 80% CR = 0.8 = p
4% CW
(from CWCW plants)
W
+ 16% C R W
(from C C plants)
= 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
5 Conditions for Non-evolving Populations
Remember Hardy-Weinberg theorem describes a
hypothetical population that is NOT evolving
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
But in real populations, allele frequencies DO change!
Hardy-Weinberg Equilibrium
If p and q represent the frequencies of the
only two possible alleles in a population at a
particular locus, then:
p2 + 2pq + q2 = 1
p2 and q2 are frequencies of homozygotes
2pq is frequency of heterozygotes
Practice Hardy Weinberg Problem
In a population of pigs, there are 4 black pigs and 12 pink pigs. The pink
allele is dominant and the black allele is recessive. What is the
percentage of the pigs that are heterozygotes?
Step 1: What is the frequency of the black pigs?
4/16 pigs or 25% (0.25) are black = q2
Step 2: What is the frequency of the black allele?
Square root of 0.25 = 0.5 = q
Step 3: What is the frequency of the pink allele?
1-0.5 = 0.5 = p
Practice Hardy Weinberg Problem
In a population of pigs, there are 4 black pigs and 12 pink pigs. The pink
allele is dominant and the black allele is recessive. What is the
percentage of the pigs that are heterozygotes?
Step 4: What is the frequency of heterozygotes?
2pq = 2 x 0.5 x 0.5 = 0.5 or 50%
Overall: 25% are homozygous recessive (black), 25% are homozygous
dominant (pink) and 50% are heterozygous (pink)
How Factors Can Alter Allele
Frequencies in Populations
Remember 3 major factors can alter allele
frequencies:
Natural selection
Genetic drift
Gene flow
Genetic Drift
Smaller a sample, the greater the chance of
deviation from a predicted result (violates
condition #4, i.e. Large populations)
Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to
the next
Genetic drift tends to reduce genetic variation
through loss of alleles
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
2
plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
Causes of Genetic Drift
Founder effect
Occurs when a few individuals become isolated
from a larger population
Allele frequencies in the small founder
population can be different from those in the
larger parent population
Causes of Genetic Drift
Bottleneck effect
Sudden reduction in population size due to a
change in the environment
The resulting gene pool may no longer be
reflective of the original population’s gene pool
If the population remains small, it may be further
affected by genetic drift
Bottleneck Effect
Original
population
Bottlenecking
event
Surviving
population
Humans Influence on Bottlenecks
Human actions can cause serious bottlenecks for
other species
Ex. Northern elephant seals have significantly reduced
genetic variation most likely due to excessive hunting by
humans
By end of 19th c., there were only about 20 individuals
Population size back up to over 30,000 but still much less genetic
variation compared to lesser hunted Southern elephant seal
Summary of Genetic Drift
Genetic drift is significant in small populations
Genetic drift causes allele frequencies to change
at random
Genetic drift can lead to a loss of genetic variation
within populations
Genetic drift can cause harmful alleles to become
fixed
Gene Flow
Gene flow is movement of alleles between
populations
Alleles can be transferred by individuals moving or
gametes (for example, pollen)
Gene flow reduces genetic variation over time
Ex. Organisms of many social species will disperse when
reaching reproductive age, leaving their original family
group and finding new territories
Gene Flow
Sometimes gene flow can increase the fitness of
population
Spread of alleles that may carry an advantage to a new
population of the species
Ex. Resistance to insecticides
Some populations of mosquitoes have evolved alleles that protect
them from insecticides
As these mosquitoes move to new areas, this beneficial allele
moves with them
Relative Fitness
Phrases “struggle for existence” and “survival of
the fittest” misleading
Imply direct competition among individuals
Reproductive success is generally more subtle and
depends on many factors
Relative fitness is contribution an individual makes
to the gene pool of the next generation, relative to
the contributions of other individuals
Sexual Selection
Sexual selection
Natural selection for mating success
May result in sexual dimorphism
Marked differences between the sexes in secondary
sexual characteristics
Types of Sexual Selection
Intrasexual selection
Competition among individuals of one sex (often
males) for mates of the opposite sex
Types of Sexual Selection
Intersexual selection (mate choice)
Occurs when individuals of one sex (usually
females) are choosy in selecting their mates
Types of Sexual Selection
Male showiness can increase a male’s chances of
attracting a female
But can also decrease his chances of survival
Why Aren’t Organisms Perfect?
Perfection unattainable
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment
interact