Notes – Ch. 23
Evolution of Populations
Individuals are selected
Populations evolve
• One misconception is
that organisms evolve,
in the Darwinian sense,
during their lifetimes
• Genetic variations in
populations contribute
to evolution
2007-2008
Population genetics provides a
foundation for studying evolution
• Microevolution is change in
the genetic makeup of a
population from generation
to generation
• Population genetics is the
study of how populations
change genetically over
time
• Population genetics
integrates Mendelian
genetics with the Darwinian
theory of evolution by
natural selection
Gene Pools and Allele Frequencies
• WHAT IS A SPECIES?
–
A species is a group of organisms that are
able to interbreed and produce fertile
offspring
– (there are always exceptions –
some organisms of different
species, like a dog and wolf – can
interbreed and produce fertile
offspring even through they are
different species)
• A population is a localized
group of individuals capable
of interbreeding and
producing fertile offspring
• The gene pool is the total
aggregate of genes in a
population at any one time
The Hardy-Weinberg Theorem
• The Hardy-Weinberg
theorem describes a
population that is not
evolving (this is rarely the
case)
• It states that frequencies
of alleles and genotypes
in a population’s gene
pool remain constant
from generation to
generation
Hardy-Weinberg Equilibrium
• Hardy-Weinberg equilibrium
describes a population where
allele frequencies do not change
• The five conditions are required
in order for a population to
remain at Hardy-Weinberg
equilibrium are:
– 1.A large breeding population
– 2.Random mating
– 3.No change in allelic frequency
due to mutation
– 4.No immigration or emigration
– 5.No natural selection
*The five conditions for non-evolving
populations are rarely met in
nature
• If p and q represent the
relative frequencies of
the only two possible
alleles in a population at a
particular locus, then
– p2 + 2pq + q2 = 1
– And p2 and q2 represent
the frequencies of the
homozygous genotypes
and 2pq represents the
frequency of the
heterozygous genotype
LE 23-5
Gametes for each generation are
drawn at random from the gene pool
of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
CW
(20%)
p2
pq
64%
CRCR
16%
CRCW
(20%)
CR
(80%)
CW
Eggs
(80%)
qp
4%
CWCW
16%
CRCW
q2
Conditions for Hardy-Weinberg
Equilibrium
• The Hardy-Weinberg theorem describes a
hypothetical population
• In real populations, allele and genotype
frequencies do change over time
Population Genetics and Human Health
• We can use the Hardy-Weinberg equation to
estimate the percentage of the human population
carrying the allele for an inherited disease
Mutation and sexual recombination produce the
variation that makes evolution possible
• Two processes,
mutation and sexual
recombination, produce
the variation in gene
pools that contributes
to differences among
individuals
Mutation
• Mutations are changes
in the nucleotide
sequence of DNA
• Mutations cause new
genes and alleles to
arise
Point Mutations
• A point mutation is a change in one base in a
gene
• It is usually harmless but may have significant
impact on phenotype
Mutations That Alter Gene Number or
Sequence
• Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
(frameshift)
• Gene duplication is nearly always harmful
Mutation Rates
• Mutation rates are low in animals and plants
• The average is about one mutation in every
100,000 genes per generation
• Mutations are more rapid in microorganisms
Sexual Recombination
• Sexual recombination is
far more important
than mutation in
producing the genetic
differences that make
adaptation possible
Natural selection, genetic drift, and gene flow can alter a
population’s genetic composition
• Three major factors alter allele frequencies and
bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Natural Selection
• Differential success in
reproduction results in
certain alleles being passed
to the next generation in
greater proportions
• “survival of the fittest” –
those with the traits best
suited to the environment
will survive and pass on
their DNA to their offspring
• Natural selection
accumulates and maintains
favorable genotypes in a
population
Genetic Drift
• The smaller a sample, the greater the chance
of deviation from a predicted result
– Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to
the next
– Genetic drift tends to reduce genetic variation
through losses of alleles
•
•
•
•
•
The process of genetic drift can be illustrated using 20 marbles in a jar to represent 20 organisms in
a population.
Consider this jar of marbles as the starting population. Half of the marbles in the jar are red and
half blue, and both colors correspond to two different alleles of one gene in the population.
In each new generation the organisms reproduce at random. To represent this reproduction,
randomly select a marble from the original jar and deposit a new marble with the same color as its
"parent" into a new jar. (The selected marble remains in the original jar.) Repeat this process until
there are 20 new marbles in the second jar. The second jar then contains a second generation of
"offspring", consisting of 20 marbles of various colors. Unless the second jar contains exactly 10 red
marbles and 10 blue marbles, a random shift occurred in the allele frequencies.
Repeat this process a number of times, randomly reproducing each generation of marbles to form
the next. The numbers of red and blue marbles picked each generation fluctuates: sometimes more
red, sometimes more blue. This fluctuation is analogous to genetic drift – a change in the
population's allele frequency resulting from a random variation in the distribution of alleles from
one generation to the next.
It is even possible that in any one generation no marbles of a particular color are chosen, meaning
they have no offspring. In this example, if no red marbles are selected the jar representing the new
generation contains only blue offspring. If this happens, the red allele has been lost permanently in
the population, while the remaining blue allele has become fixed: all future generations are entirely
blue. In small populations, fixation can occur in just a few generations.
LE 23-7
CWCW
CR CR
CR CR
CR CW
Only 5 of
10 plants
leave
offspring
CR CR
CWCW
CR CR
C R CW
CWCW
CR CR
C R CW
CR CW
CR CR
CWCW
CR CW
CR C R
CR CR
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
CR C W
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect
• The bottleneck effect is a sudden
change in the environment (fire,
drought – kills many in a single
generation) that may drastically
reduce the size of a population
• The resulting gene pool may no
longer be reflective of the original
population’s gene pool
– Ex – elephant seal (hunting
reduced population to just 20
individuals; has since rebounded to
3000)
– cheetahs are sufficiently closely
related to one another that
transplanted skin grafts do not
provoke immune responses
The Founder Effect
• A founder effect occurs when a
new colony is started by a few
members of the original
population. This small population
size means that the colony may
have:
– reduced genetic variation from the
original population.
– a non-random sample of the genes
in the original population.
• The founder effect occurs when a
few individuals become isolated
from a larger population
• It can affect allele frequencies in
a population
Gene Flow
• Gene flow consists of
genetic additions or
subtractions from a
population, resulting from
movement of fertile
individuals or gametes
• Gene flow causes a
population to gain or lose
alleles
• It tends to reduce
differences between
populations over time
Genetic Variation
• Genetic variation occurs in individuals in
populations of all species
• It is not always heritable (influenced by
environment - fetal alcohol syndrome)
LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white
Variation Within a Population
• Both discrete and quantitative characters contribute to
variation within a population
• Discrete characters can be classified on an either-or
basis
– discreet trait is something that is either one way or the
other, for example, trait for sickle cell anemia. if you have
the gene, you have the trait.
• Quantitative characters vary along a continuum within
a population
– quantitative trait is something that has a range, for
example, height. there are many genes that play a role and
there are not a discreet number of heights, there is a
range.
Polymorphism
• Polymorphism is when two or more clearly different
phenotypes exist in the same population of a species
– in other words, the occurrence of more than one form or morph
• Phenotypic polymorphism describes a population in which
two or more distinct morphs for a character are represented
in high enough frequencies to be readily noticeable (you can
see their traits)
• Genetic polymorphisms are the heritable components of
characters that occur along a continuum in a population
(genotypes)
Directional, Disruptive, and Stabilizing
Selection
• Selection favors certain
genotypes by acting on the
phenotypes of certain
organisms
• Three modes of selection:
– Directional- favors individuals
at one end of the phenotypic
range
– Disruptive- favors individuals
at both extremes of the
phenotypic range
– Stabilizing- favors
intermediate variants and
acts against extreme
phenotypes
The Preservation of Genetic Variation
• Various mechanisms help to preserve genetic
variation in a population
Diploidy
• Diploidy maintains
genetic variation in the
form of hidden
recessive alleles
– Diploid – pairs of genes
Balancing Selection
• Balancing selection
occurs when natural
selection maintains
stable frequencies of
two or more phenotypic
forms in a population
• Balancing selection
leads to a state called
balanced polymorphism
Heterozygote Advantage
• Some individuals who are
heterozygous at a
particular locus have
greater fitness than
homozygotes
• Natural selection will tend
to maintain two or more
alleles at that locus
• The sickle-cell allele
causes mutations in
hemoglobin but also
confers malaria resistance
LE 23-13
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
7.5–10.0%
10.0–12.5%
>12.5%
Sexual Selection
• Sexual selection is natural selection for mating
success
• It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
• Intrasexual selection is
competition among
individuals of one sex for
mates of the opposite sex
– occurs when individuals of
one sex (usually females)
are choosy in selecting
their mates from
individuals of the other sex
• Selection may depend on
the showiness of the
male’s appearance
The Evolutionary Enigma of Sexual
Reproduction
• Sexual reproduction
produces fewer
reproductive offspring
than asexual
reproduction, a socalled “reproductive
handicap”
• So why do it?
LE 23-16
Asexual reproduction
Female
Sexual reproduction
Generation 1
Female
Generation 2
Male
Generation 3
Generation 4
• Sexual reproduction produces genetic variation
that may aid in disease resistance
Why Natural Selection Cannot Fashion
Perfect Organisms
• Evolution is limited by historical constraints (what’s
happened in the past to produce current products)
• Adaptations are often compromises
• Chance and natural selection interact
• Selection can only edit existing variations