Transcript Directional Selection

Chapter 18
Processes of
Evolution
Processes of
Evolution
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Microevolution
Microevolution pertains to the evolutionary changes
within a population.
Populations are all the members of a single species
occupying a particular area.
 Population genetics - study of genetic changes within a
population
- The various alleles at all the gene loci in all
individuals make up the gene pool of the
population.
- It is customary to describe the gene pool of a population
in terms of gene frequencies.
Processes of
Evolution
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Gene Frequencies
Suppose in a Drosophila population there are:
36% flies homozygous dominant for long wings
48% heterozygous for long wings
16% homozygous recessive for short wings
In population of 100 flies there would be:
36 LL, 48 Ll and 16 ll
Number of L alleles would be (2 X 36) + 48 = 120
Number of l alleles would be (2 X 16) + 48 = 80
 There are 120 L alleles and 80 l alleles
Processes of
Evolution
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Gene Frequencies
To determine frequency of each allele:
Calculate its percentage from total # of alleles in
population.
For dominant allele L = 120/200 = 0.6
For recessive allele l = 80/200 = 0.4
The sperm & eggs produced by this population
should have the same frequencies. You can
calculate the expected ratios of genotypes in the
next generation by using a Punnett square.
Processes of
Evolution
Gene Frequencies
Punnett Square
eggs
sperm
0.6 L
0.4 l
0.6 L
0.36LL
0.24 Ll
0.4 l
0.24 Ll
0.16 ll
Genotypes frequencies =
0.36 LL + 0.48 Ll + 0.16 ll = 1
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Processes of
Evolution
Gene Frequencies
Note that the frequency of each allele in the next
generation is the same as it was in the previous
generation.
Sexual reproduction alone cannot bring about a
change in allele frequencies.
Also, the dominant allele does not increase from
one generation to the next. It does NOT
become more common.
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Processes of
Evolution
Hardy-Weinberg
The Hardy-Weinberg principle - mathematics:
p+q=1
 p2 + 2pq + q2 = 1
p = frequency of dominant allele
q = frequency of recessive allele
p2 = frequency of homozygous dominant
individuals
q2 = frequency of homozygous recessive
individuals
2pq = frequency of heterozygous individuals
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Calculating Gene Pool Frequencies
Using the Hardy-Weinberg Equation
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Calculating Gene Pool Frequencies
Using the Hardy-Weinberg Equation
Do Practice Problems 18.1 (p. 303) now
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Processes of
Evolution
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Hardy-Weinberg
The Hardy-Weinberg principle:
Allele frequencies in a population will remain
constant assuming five conditions are met:
- No Mutations - alleles do not change
- No Gene Flow - no migration in or out of population
- Random Mating - individuals pair by chance
- No Genetic Drift - populations are large such that
gene frequencies don’t change by chance alone
- No Selection - particular genotypes not selected
Processes of
Evolution
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Hardy-Weinberg
In real life, the Hardy-Weinberg conditions are
rarely, if ever, met.
Thus, allele frequencies in a population DO
change from one generation to the next.
- The significance of the Hardy-Weinberg principle is
that it tells us what factors cause evolution:
Those that violated the conditions listed.
- Evolution can be detected by noting any deviation
from a Hardy-Weinberg equilibrium.
Processes of
Evolution
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Microevolution
The accumulation of small changes in the gene pool over
a relatively short period of time is called microevolution.
Example:
Industrial Melanism:
• Pepper moths in Great Britain
- Before Industrial Revolution, light-colored moths more
common than dark-colored moths (< 10% dark)
- After Industrial Revolution, dark-colored moth more
common than light-colored moths. (By 1950s > 80%
dark & 94% dark in 1960)
- After Clean Air Act of mid-1950s: By 1994 only 19%
dark
Industrial Melanism and Microevolution 13
When vegetation is lightcolored, dark moths are
seen & eaten by birds
When vegetation is dark
due to pollution, light
moths are seen & eaten
by birds
Processes of
Evolution
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Causes of Microevolution
1. Genetic Mutations
The raw material for evolutionary change
Provides new combinations of alleles
Some might be more adaptive than others
Many traits are polymorphic
- Two or more distinct phenotypes are present in
a population.
- Examples: Human freckles
ABO blood types
Processes of
Evolution
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Causes of Microevolution
2. Gene Flow (Gene Migration)
Movement of alleles between populations when:
- Gametes or seeds (in plants) are carried into
another population
- Breeding individuals migrate into or out of
population
Gene flow can increase variation in a population
by introducing novel alleles
Continual gene flow makes gene pools similar &
reduces differences among populations. This can
prevent speciation from happening.
Gene Flow
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There is interbreeding
between populations;
thus gene flow occurs
among the populations.
So they are sub-species
of species Elaphe
obsoleta
Processes of
Evolution
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Causes of Microevolution
3. Nonrandom Mating
When individuals do not choose mates randomly
- Inbreeding:
•Mating with relatives. Increases frequency of
recessive abnormalities.
- Assortative mating:
 Individuals
select mates with their phenotype
 Individuals reject mates with differing phenotype
 Causes population to subdivide into two phenotypic
classes
 Homozygotes increases; heterozygotes decrease
Processes of
Evolution
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Causes of Microevolution
3. Nonrandom Mating (cont’d)
- Sexual selection:
 Males
compete for the right to reproduce
 Females choose to mate with males possessing a
particular phenotype
 Example:
Elaborate tail of peacocks may be due to female
peahens choosing males with grander tails.
All of these mechanisms can cause an increase in
homozygotes
Processes of
Evolution
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Causes of Microevolution
4. Genetic Drift
Refers to changes in allele frequencies of a gene
pool due to chance.
 More
likely to have a large effect on smaller
populations where the sampling error is a larger
part of the population.
 Can cause the gene pools of two isolated
populations to become dissimilar
 Some alleles are lost and others become fixed
(unopposed)
Genetic Drift
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Processes of
Evolution
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Genetic Drift
Bottleneck Effect
Sometimes a natural disaster, or humans, might
cause a near extinction event.
This prevents a majority of individuals, and their
genotypes, from entering the next generation
Example:
- Cheetahs:
 Extreme genetic similarity is believed to be
due to a bottleneck
 They suffer from infertility because of intense
inbreeding after that time
Processes of
Evolution
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Genetic Drift
Founder Effect
When rare alleles occur at higher frequency in a
population isolated from general population
Happens when a new population is started from
just a few individuals
The alleles carried by population founders are
dictated by chance
Examples:
- Amish of Lancaster County, PA have high
incidence of dwarfism & polydactylism
- Lake Maracaibo, Venezuela has high incidence
of Huntington disease
Founder Effect
Rare recessive form of
dwarfism linked to
polydactylism is very
common in Amish of
Pennsylvania
1/14 individuals carries
recessive allele
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Processes of
Evolution
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Natural Selection
Process that results in adaptation of a population
to the biotic and abiotic environments
Requires:
- Variation - The members of a population differ from
one another
- Inheritance - Many differences are heritable genetic
differences
- Differential Adaptiveness - Some differences affect
survivability
- Differential Reproduction – Some differences affect
likelihood of successful reproduction
Processes of
Evolution
Natural Selection
Results in:
A change in allele frequencies of the gene pool
Improved fitness of the population
Natural selection is the major cause of
microevolution
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Processes of
Evolution
Types of Selection
Most traits are:
• polygenic - controlled by more than one pair of
alleles located at different loci
• variations in such traits result in a bell-shaped
curves
Three types of selection occur:
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
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Processes of
Evolution
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Types of Selection
1. Directional Selection
Occurs when one extreme phenotype is favored
- The curve shifts in one direction
- Examples:
•When bacteria become resistant to antibiotics
•Human struggle against malaria
Plasmodium & mosquito evolution of resistance to
treatments
•Gradual increase in size of horse
Directional Selection
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Processes of
Evolution
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Types of Selection
2. Stabilizing Selection
- Occurs when an intermediate phenotype is favored
- Can improve adaptation of population to a relatively
constant environment
- The peak of the curve increases and tails decrease
- Examples:
•When human babies with low or high birth weight
are less likely to survive
•Swiss starlings clutch size
Stabilizing Selection
Swiss starlings optimal
clutch size is 4-5 eggs
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Processes of
Evolution
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Types of Selection
3. Disruptive
- Two or more extreme phenotypes are favored over
any intermediate phenotypes
- The curve has two peaks
- Examples:
•When Cepaea snails vary because a wide
geographic range causes selection to vary
Disruptive Selection
Dark shells more
prevalent in forested
areas
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Light shells near
low-lying vegetation
Processes of
Evolution
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Maintenance of Variations
Genetic variability
Populations with limited variation may not be able
to adapt to new conditions & become extinct
Maintenance of variability is advantageous to
population
Only exposed alleles are subject to natural
selection:
● Thus, heterozygotes can be a protector of
recessive alleles that might otherwise be weeded
out.
Allows even lethal alleles to remain in population
at low frequencies virtually forever
Processes of
Evolution
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Maintenance of Variations
Heterozygote Advantage:
Lethal recessive alleles may confer advantage to
heterozygotes
- Sickle cell disease is detrimental in homozygote
- However, heterozygotes more likely to survive
malaria than homozygous dominants because
malaria parasite is unable to live in their red blood
cells while it destroys the RBCs of homozygotes.
- Sickle cell allele occurs at higher than expected
frequency in malaria prone areas
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Sickle-cell Disease
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Processes of
Evolution
Macroevolution
Macroevolution
- Any evolutionary change at or above the level
of the species
● Speciation
- Splitting of one species into two or more
species
- Transformation of one species into a new
species over time.
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Processes of
Evolution
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Definition of a Species
Morphological
- Can be distinguished anatomically
- Physical traits differ
- Specialist decides what criteria probably represent
reproductively isolated populations
- Most species described this way
Processes of
Evolution
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Species Definitions
Biological Species Concept
- A group of populations that can breed among
themselves to produce fertile offspring
- Are reproductively isolated from other such
populations; unable to reproduce with members of
other groups
- The organisms share a gene pool
- Very few actually tested for reproductive isolation
- Cannot be applied to asexual organisms or those
only known from the fossil record
Biological Species Definition
These three species of
flycatchers are
reproductively isolated
since they do not
reproduce with each
other
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Processes of
Evolution
Reproductive Isolating Mechanisms
Reproductive isolating mechanisms are any
structural, functional or behavioral
characteristics that prevent successful
reproduction from occurring between different
groups of organisms.
Two general types:
 Pre-zygotic Mechanisms
- Discourage attempts to mate
● Post-zygotic Mechanisms
- Prevent hybrid offspring from developing or
breeding
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Processes of
Evolution
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Prezygotic Mechanisms
1. Habitat Isolation
- Occupy different habitats & are less likely to
meet & reproduce
2. Temporal Isolation
- Species live in same habitat but reproduce at
different times
3. Behavioral Isolation
- Species have their own courtship rituals
- Firefly flashes; cricket chirping; chemical signals
Temporal Isolation
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Processes of
Evolution
Prezygotic Mechanisms
4. Mechanical Isolation
- Reproductive parts are incompatible
5. Gamete Isolation
- If gametes meet, they do not fuse to
become a zygote.
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Processes of
Evolution
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Postzygotic Mechanisms
1. Zygote Mortality
- Hybrid zygote might be created but dies
2. Hybrid Sterility
- Hybrid zygotes develop into sterile adults
- Example:
Mule is a cross between a horse & a donkey.
They are usually sterile
3. Reduced F2 Fitness
- If hybrids can reproduce, their offspring cannot.
Processes of
Evolution
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Two Modes of Speciation
1. Allopatric Speciation
- Two geographically isolated populations of one
species
- Become different species over time
- Can be due to differing selection pressures in
differing environments
- Examples:
California salamanders separated by Central Valley
Green iguanas in Galapagos Islands are thought to
be ancestors of marine & rhinoceros iguanas
Allopatric Speciation
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Processes of
Evolution
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Two Modes of Speciation
2. Sympatric Speciation
- One population develops into two or more
reproductively isolated groups
- No prior geographic isolation
- Examples:
- Tetraploid hybridization in wheat (polyploidy)
 Results
in self fertile species
 Reproductively
isolated from either parental species
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Processes of
Evolution
Adaptive Radiation
When many new species evolve from a single
ancestral species.

Occurs when members of species become
adapted to the different environments
This is an example of allopatric speciation
Examples:
1. Hawaiian honeycreepers
2. Galapagos finches
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