Download Hardy-Weinberg principle

Microevolutionary Processes
Chapter 21
Microevolutionary Processes
 Microevolution is a change in allele frequencies in a population
over generations

Drive a population away from genetic equilibrium
 Small-scale changes in allele frequencies brought about by:

Natural selection

Gene flow

Genetic drift
Populations Evolve
 Biological evolution does not change individuals
 It changes a population
 Traits in a population vary among individuals
 Evolution is change in frequency of traits
Gene Pools and Allele Frequencies
 Population
localized group of individuals capable of interbreeding and producing
fertile offspring
 Gene Pool
 consists of all the alleles in a population

Gene Mutations
 Infrequent but inevitable
 Each gene has own mutation rate
The average is about one mutation in every 100,000 genes per
generation
 Mutation rates are often lower in prokaryotes and higher in viruses
 Short generation times allow mutations to accumulate rapidly in
prokaryotes and viruses
 Lethal mutations
 Neutral mutations
 Advantageous mutations

Variation in Phenotype
 Each kind of gene in gene pool may have two or more alleles
 Individuals inherit different allele combinations
 This leads to variation in phenotype
 Offspring inherit genes, not phenotypes
Variation in Phenotype
Variation in Phenotype
Variation in Phenotype
What Determines Alleles in
New Individuals?
 Mutation
 Crossing over at meiosis I
 Independent assortment
 Fertilization
 Change in chromosome number or structure
The Hardy-Weinberg Principle
 The Hardy-Weinberg principle describes a population that is not evolving
 If a population does not meet the criteria of the Hardy-Weinberg principle,
it can be concluded that the population is evolving
Hardy-Weinberg Equilibrium
 The Hardy-Weinberg principle states that frequencies of alleles and
genotypes in a population remain constant from generation to generation
 In a given population where gametes contribute to the next generation
randomly, allele frequencies will not change
 Mendelian inheritance preserves genetic variation in a population
Hardy-Weinberg Rule
At genetic equilibrium, proportions of genotypes at a locus with
two alleles are given by the equation:
p2 + 2pq + q2 = 1
Frequency of allele A = p
Frequency of allele a = q
Conditions for Hardy-Weinberg Equilibrium
 The Hardy-Weinberg theorem describes a hypothetical population that is
not evolving
 In real populations, allele and genotype frequencies do change over time
Conditions for Hardy-Weinberg Equilibrium
 The five conditions for non-evolving populations are rarely met in nature
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
Applying the Hardy-Weinberg Principle
 We can assume the locus that causes phenylketonuria (PKU) is in Hardy-
Weinberg equilibrium given that
1. The PKU gene mutation rate is low
2. Mate selection is random with respect to whether or not an individual
is a carrier for the PKU allele
3. Natural selection can only act on rare homozygous individuals who do
not follow dietary restrictions
4. The population is large
5. Migration has no effect, as many other populations have similar allele
frequencies
Applying the Hardy-Weinberg Principle
 The occurrence of PKU is 1 per 10,000 births
 q2  0.0001

q  0.01
 The frequency of normal alleles is

p  1 – q  1 – 0.01  0.99
 The frequency of carriers is
2pq  2  0.99  0.01  0.0198
 or approximately 2% of the U.S. population

Altering Allele Frequencies
in a Population
 Three major factors alter allele frequencies and bring about most
evolutionary change
 Natural selection
 Genetic drift
 Gene flow
1.) Natural Selection
 Differential success in reproduction results in certain alleles being passed to
the next generation in greater proportions
 Evolution by natural selection involves both chance and “sorting”

New genetic variations arise by chance
Beneficial alleles are “sorted” and favored by natural selection
 Only natural selection consistently results in adaptive evolution, an
increase in the frequency of alleles that improve fitness

Results of Natural Selection
Three possible outcomes:
 A shift in the range of values for a given trait in some direction
 Stabilization of an existing range of values
 Disruption of an existing range of values
in the population
Number of individuals
phenotypic range
 Allele frequencies shift in one
direction
Range of values for the trait at time 1
Number of individuals
in the population
 Favors individuals at one end of the
Range of values for the trait at time 2
Number of individuals
in the population
Directional Selection
Range of values for the trait at time 3
Peppered Moths
 Prior to industrial revolution, most common phenotype was light
colored
 After industrial revolution, dark phenotype became more common
 PEPPERED MOTHS - KETTLEWELL
Adaptation
 Darwin noticed that every species seemed well adapted to the life
it leads
 Adaptations
 physical and behavioral characteristics combine to help
the organism catch food, handle harsh conditions and
reproduce
Examples of Adaptations
Camouflage:
a structural adaptation enabling an organism to blend in with its
environment
 Octopus
 Cuttlefish

 Mimicry:

a structural adaptation that provides protection for an organism by
copying the appearance of another species
black on yellow
kill fellow
red on black
friend of jack
Pesticide Resistance
 Pesticides kill susceptible insects
 Resistant insects survive and reproduce
 If resistance has heritable basis, it becomes more common with
each generation
Antibiotic Resistance
 First came into use in the 1940s
 Overuse has led to increase in resistant forms
 Most susceptible cells died out and were replaced by resistant forms
 Intermediate forms are favored and
extremes are eliminated
Number of individuals
in the population
Stabilizing Selection
Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3
Stabilizing Selection
 Heterozygote advantage occurs when heterozygotes have a higher fitness
than do both homozygotes
 Natural selection will tend to maintain two or more alleles at that locus
 For example, the sickle-cell allele causes deleterious mutations in
hemoglobin but also confers malaria resistance
Figure 21.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
5.0–7.5%
7.5–10.0%
10.0–12.5%
12.5%
variation are favored
 Intermediate forms are selected
against
Number of individuals Number of individuals
in the population
in the population
 Forms at both ends of the range of
Range of values for the trait at time 1
Range of values for the trait at time 2
Number of individuals
in the population
Disruptive Selection
Range of values for the trait at time 3
Sexual Selection
 Sexual selection
natural selection for mating success
 can result in sexual dimorphism
 marked differences between the sexes in secondary sexual
characteristics

2.) Genetic Drift
 Genetic Drift
how allele frequencies fluctuate unpredictably from one generation to
the next
 tends to reduce genetic variation through losses of alleles, especially in
small populations

Effects of Genetic Drift
1. Genetic drift is significant in small populations
2. Genetic drift can cause allele frequencies to change at random
3. Genetic drift can lead to a loss of genetic variation within populations
4. Genetic drift can cause harmful alleles to become fixed
Founder Effect
 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 due to chance

Bottleneck
 Bottleneck effect
results from a drastic reduction in population size due to a sudden
environmental change
 if the population remains small, it may be further affected by genetic
drift
 understanding the bottleneck effect can increase understanding of
how human activity affects other species

3.) Gene Flow
 Gene flow
movement of alleles among populations
 alleles can be transferred through the movement of fertile individuals
or gametes (for example, pollen)
 tends to reduce genetic variation among populations over time

Barriers to Gene Flow
Whether or not a physical barrier deters gene flow depends upon:

Organism’s mode of dispersal or locomotion

Duration of time organism can move
Inbreeding
 Nonrandom mating between related individuals
 Leads to increased homozygosity
 Can lower fitness when deleterious recessive alleles are expressed
 Amish, cheetahs