Download Chapter 21~The Evolution of Populations

Chapter 21~The Evolution of Populations
Following the Big Ideas
 Evolution-
Microevolution, or evolution within populations, is
measured as a change in allele frequencies over
generations!
 Genetic diversity allows individuals in a population to
respond differently to the same changes in environmental
conditions
 Continuity of homeostatic mechanisms reflect a common
ancestry, while changes may occur in response to
different environmental conditions.

Overview: The Smallest Unit of Evolution
 One common misconception is that organisms
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evolve during their lifetimes
Natural selection acts on individuals, but only
populations evolve
Consider, for example, a population of medium
ground finches on Daphne Major Island
During a drought, large-beaked birds were more
likely to crack large seeds and survive
The finch population evolved by natural selection
Figure 21.1
Is this finch evolving?
Concept 21.1: Genetic variation makes evolution
possible
 Sources of variation
 gene mutations- 1/10,000 gametes may have a change in
a gene, usually negative.
 Chromosomal mutations- new combinations of genes on
chromosomes in organism that can be inherited together
 Errors in mitosis and meiosis result in changes in
phenotype.
 Sexual Reproduction- Recombination- crossing over.
 Immigration or emigration- brings new genes into the
population or removes genes from the population
 Migration- tends to have the greatest affect on small
populations
Populations evolve
 Natural selection acts on individuals
 differential survival


“survival of the fittest”
differential reproductive success

who bears more offspring
 Populations evolve
 genetic makeup of
population changes
over time
 favorable traits
(greater fitness)
become more common
Mummichog
Modern Synthesis: Comprehensive Theory of
Evolution Integrating Many Ideas
 Population genetics- study of changes in genetic
make-up of populations
population- group of interbreeding organisms of the
same species living together
 individuals do not evolve, populations do
 species- a group of similar organisms that can breed to
produce fertile offspring

Allele frequencies
 each organism has a set of individual alleles, but
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individuals can carry some of the same alleles
in any population there are a certain number of
alleles of each kind
some alleles are more common than others
gene pool- total of all of the alleles in a population
at any given time, each allele occurs in the gene pool
with a certain frequency
Evolution- gradual change in the allele frequencies
in a population
Differential Reproduction (Natural Selection)
 Mutations lead to new alleles and are the primary source
of variation that can be acted on by natural selection.
 DNA mutations can be positive, negative or neutral based
on their effect or lack of effect they have on the resulting
nucleic acid or protein and the phenotypes that are
conferred by the protein.
 Alterations in the DNA sequence can lead to changes in
the type or amount of protein produced and consequent
phenotype. (Phenotypes are determined by protein
activity!)
 Whether a mutation is positive or negative (or neutral)
depends on the environmental context. If environment
changes, screening effect of natural selection will change.
 Differential reproduction will cause certain allele
frequencies to increase and others to decline
 Alleles that benefit organism are passed on and
alleles that don’t benefit organism are not passed on
due to early death
 The frequency of alleles that gives individuals an
advantage under new conditions will increase in the
population. The frequencies of alleles that are
disadvantageous will decrease
Concept 21.2: The Hardy-Weinberg equation can
be used to test whether a population is evolving
 1908- Hardy and Weinberg showed that the
segregation and recombination of genes in sexual
reproduction could not by itself change allele
frequencies
 if the frequency of allele p is 90% and allele q is 10%,
ordinary random mating would produce a new
generation with 90% p and 10% q
 law is used to figure out allele frequencies at
equilibrium (when there is no evolution) by
mathematical equation. Any deviation from the
reference point establishes evolution
Hardy- Weinberg Theorem
 The alleles are for instance, B
and b, the genotypes that could
be made are:
 the equation for phenotypes

black white
p + q = 1
 the equation for alleles
p2 + 2pq + q2 = 1
BB (p2) (homo dom.)
Bb (2pq) (heteroz.)
bb (q2) (homo rec.)
B
b
B
BB
Bb
b
Bb
bb
Hardy-Weinberg Theorem
 Serves as a model for the genetic structure of a
non-evolving population (equilibrium)
 Evolution = change in allele frequencies in a
population
 hypothetical: what conditions not would
cause allele frequencies to change?
 non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic
drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is
equally fit)
The first 2
conditions listed
could occur, but the
rest never do.
Mutations always occur
and reproduction is
seldom random or nonselective due to
differential
reproduction
Hardy-Weinberg Equation
 p=frequency of one allele (A);
other allele (a);
p+q=1.0
q=frequency of the
(p=1-q & q=1-p)
p2=frequency of AA genotype; 2pq=frequency of Aa
genotype; q2=frequency of aa genotype;

frequencies of all individuals must add to 1 (100%), so:

p2 + 2pq + q2 = 1
G.H. Hardy
mathematician
W.
Weinberg
physician
Using Hardy-Weinberg equation
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.16
bb
MustWhat
assume
arepopulation
the genotype
is in
frequencies?
H-W equilibrium!
Hardy-Weinberg Theorem
 How is the theorem useful?
 demonstrates that evolution in a pop. does occur
 theorem tells us that under certain conditions allele
frequencies would remain constant and no evolution would
occur
 that allele frequency changes tells us external factors are
causing them to change
 failure of the theorem- shows that evolution occurs and the
extent of variation from the theorem, shows the extent of
evolution
Concept 21.3: Microevolution
 Definition
 small scale evolutionary change represented by a generational
shift in a population’s relative allelic frequencies and
genotypes
 there are 5 possible agents for microevolution, but only natural
selection generally leads to an accumulation of favorable
adaptations in a population.
5 Agents of evolutionary change
Gene Flow
Genetic Drift
Mutation
Non-random mating
Selection
Microevolution I- Genetic Drift
 A change in the gene
pool of a population
over a succession of
generations
 1- Genetic drift:
changes in the gene
pool of a small
population due to
chance (usually
reduces genetic
variability)
Microevolution I: type of genetic drift
 The Bottleneck
Effect: type of genetic
drift resulting from a
reduction in population
(natural disaster) such
that the surviving
population is no longer
genetically
representative of the
original population
Cheetahs
 All cheetahs share a small number of alleles
 less than 1% diversity
 as if all cheetahs are
identical twins
 2 bottlenecks
 10,000 years ago
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Ice Age
last 100 years

poaching & loss of habitat
Conservation issues
Peregrine Falcon
 Bottlenecking is an important
concept in conservation biology of
endangered species
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loss of alleles from gene pool
reduces variation
reduces adaptability
Breeding programs must
consciously outcross
Golden Lion
Tamarin
Microevolution I: type of genetic drift
 Founder Effect:
a cause of genetic drift attributable
to colonization by a limited number
of individuals from a parent
population
 just by chance some rare alleles
may
be at high frequency;
others may be missing
 skew the gene pool of
new population
 human populations that
started from small group
of colonists
 example:
colonization of New World
Microevolution II- Gene Flow
 2- Gene Flow:
genetic exchange due to
the migration of fertile
individuals or gametes
between populations
(reduces differences
between populations)
• seed & pollen
distribution by
wind & insect
• migration of animals
Microevolution III- Mutations
 Mutations:
Mutation creates variation
a change in an organism’s DNA
(gametes; many generations);
original source of genetic
variation (raw material for
natural selection)
most are harmful, even lethal.
Some are neutral, very few are
beneficial. Always random
Microevolution IV- Nonrandom Mating
 Nonrandom mating:(selective
breeding) inbreeding- mating with close
neighbors- increases frequency of
homozygous recessive allelic
combinations
 assortive mating- individuals
mate with others with the same
phenotype (for a specific trait)
 sexual selection- males compete
to mate (collect a harem), or
females choose a mate based on
some physical characteristic
(male bird plumage) ) Those most
“fit” are chosen first!
Sexual selection
 Sexual dimorphism:
secondary sex
characteristic distinction
 Sexual selection:
selection towards
secondary sex
characteristics that leads
to sexual dimorphism
Concept 21.4: Natural selection is the only mechanism
that consistently causes adaptive evolution
Microevolution VNatural Selection
 differential
success in
reproduction;
 climate change
 food source availability
 predators, parasites, diseases
 toxins
 only form of microevolution that
adapts a population to its
environment
• combinations of alleles
that provide “fitness”
increase in the population
Natural Selection
 Selection acts on any trait that affects survival
or reproduction
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predation selection
physiological selection
sexual selection
 Natural Selection- based on relative fitness- fitness of
one phenotype (allele at one loci) compared to another.
Like all genetics, the phenotype may change depending
on the selection pressures at the time. Heterozygotes can
be a depository for recessive genes. Operates in three
ways:
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directional- the extreme phenotype is favored so allele
frequencies shift in that one direction. Ex- peppered moth
stabilizing- the intermediate phenotype is selected (~7 lb. birth
weight)
disruptive- favors 2 or more extreme phenotypes over the
intermediates (specialization gives advantage over the
generalists). Often each extreme has adapted to a specific
habitat, predator, food, etc. (takes two directions)
Effects of Selection
 Changes in the average trait of a population
DIRECTIONAL
SELECTION
giraffe neck
horse size
STABILIZING
SELECTION
human birth weight
DISRUPTIVE
SELECTION
rock pocket mice
Other interesting notes!
 Genetic diversity allows individuals in a population
to respond differently to the same changes in
environmental conditions. EX. Not all animals in a
population stampede. EX. Not all individuals in a
population in a disease outbreak are equally affected;
some may not show any symptoms, some show mild
symptoms and others show severe symptoms.
Variation & natural selection
 Variation is the raw material for natural selection
 there
have to be differences within population
 some
individuals must be more fit than others
Genetic Variation
 Variation within populations
polygenic characters- vary quantitatively within the
population along a continuum- height in humans (genetic
polymorphism)
 polymorphism (discrete characters)- contrasting formspink vs white flowers. Forms are called morphs. A
population is called polymorphic if two or more distinct
morphs are in noticeable frequency. (phenotypic
polymorphism)
 Both of the above contribute to variation
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Variation between
populations
 geographical variation- environmental factors effect natural
selection between two locales.
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Cline- graded change in some trait along a geographical transect
Characteristics of a species often are different in different parts of the
range, due to varying selective environmental pressures.
Race Circle-adjacent populations in a range can interbreed to
produce normal offspring (subspecies), but nonadjacent populations
cannot. They are still considered the same species because there is
continuous interbreeding among adjacent subspecies throughout the
range. If a population varies so much from its neighbors that it can
no longer produce normal offspring with them then speciation has
occurred.
Allopatry
Variation preservation
 Prevention of natural
selection’s reduction of
variation
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Diploidy
2nd set of chromosomes hides
variation in the heterozygote
 Balanced polymorphism
1- heterozygote advantage
(hybrid vigor; i.e.,
malaria/sickle-cell anemia);
2- frequency dependent
selection (survival &
reproduction of any 1 morph
declines if it becomes too
common; i.e., parasite/host)
Adaptations
 adaptation- any kind of inherited trait that improves the
chance of survival and reproduction of the organism in a
given environment. The selecting force is the
environment itself.
 Types of adaptations
 Structural adaptations- those that involve the body of the
organism- wings, leaves w/ large surface area, fins,
webbed feet (morphological)
 Physiological adaptations- involve the metabolism of the
organism- protein web made by spiders, poison venom
made by snakes
 Behavioral adaptations- mating behavior, migration by
birds, spawning of fish, hibernation
Coloration- An Important Adaptation!
 Camouflage- adaptation for protection (cryptic
coloration)
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counter shading- dark top, light under belly
 Warning coloration- (aposematic coloration) colors make
animal easier to see, must have a reason; venom,
poisonous, bad taste, a really bad guy! Or to protect
young.
 Mimicry- one organism is protected against enemies by
resembling another, unrelated organism- Viceroy
butterfly is tasty, but resembles the Monarch butterfly
which is bad tasting, thus birds do not attempt to eat it.
Cryptic coloration
aposemetic coloration
Mimicry
Observed Natural Selection
 Industrial Melanism- When industry emits pollutants that
change the color of the habitat leading to selective forces that
select dark colored forms of insects.
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Peppered moth B. betularia is found in wooded areas around London on
lichen-covered trees
2. Before the industrial revolution(1850) most pepper moths were a light
pepper in color, blending well into the habitat. The black form was rare.
3. 1850-1900- England became industrialized and heavy soot darkened
the tree trunks killing the lichen on them.
In 1890’s- 99% of moths were black in color, the light pepper variety was
rare.
5. In cleaner areas, the light pepper colored moths still predominated.
WHY?
6. After 1950- England has installed air pollution controls and the
pepper moth population has returned to the original color of before
1850.
Industrial Melanism- Peppered moths
 Insect resistance to DDT- Most died; those resistant
to DDT live to pass trait on.
 Bacterial resistance to antibacterials.
 Remember- Phenotypic variations are not directed
by the environment but occur through random
changes in DNA and through new gene
combinations!!!!!
Clones and Natural Selection
 Bacterial resistance to antibiotics
 clones- single-species population that has descended from one
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ancestor; all individuals are genetically identical because they
have been reproduced asexually.
When random mutations occur, a new trait appears and the
mutant will asexually produce a population with the new trait.
Such is the case with penicillin-resistant bacteria that thrive in
an environment rich with penicillin.
Penicillin destroys the cell wall of bacteria, preventing them
from carrying on normal metabolism. Penicillin-resistant
bacteria are not affected by penicillin.
Population of resistant bacteria thrives quickly as susceptible
bacteria get killed off. (Ex. Penicillin-resistant Gonorrhea
bacteria)
Why Natural Selection Cannot Fashion Perfect
Organisms
1. Selection can act only on existing variations
2. Evolution is limited by historical constraints
3. Adaptations are often compromises
4. Chance, natural selection, and the environment
interact
Connecting the Concepts with the Big Ideas
 Evolution Natural selection acts on trait variation, and trait variation is determined
by genes. Whether or not a trait gives an advantage depends on the
environment. Thus genes, traits, environment, and natural selection are all
involved in microevolution.
 Microevolution occurs when allele frequencies in a population change over
time; Hardy and Weinberg devised a mathematical method by which
geneticists can measure that change.
 If there is no gene flow or genetic drift, random mating, occurrence of
mutation, or any type of selection, microevolution should not occur.
 Small populations are especially vulnerable to genetic drift- chance events
that in a small population may remove some alleles completely and may
cause others to become more frequent.
 Some genes, such as those for sickle cell anemia, are maintained in
populations living in particular environments due to stabilizing selection.
 Mutation and genetic variation are the ultimate sources of the raw
materials for evolution.