POPULATIONS GENETICS
AND SPECIATION
CH 17
IMPACTS, ISSUES
RISE OF THE SUPER RATS
WARFARIN RATS
 Killed By Warfarin
Survived
Why?
Resistance
Immunity
The MOST FIT
ANTIBIOTIC RESISTANCE
 Killed by Antibiotic
Left to FOUND
new population
HOW VARIATION ARISES FROM
SIMILARITIES…
What the rat example illustrates is a concept
that turns up again and again in nature and
provides proof of evolution.
Many populations contain the traits necessary
to survive when the environment turns on
them.
Which traits will be good or more common
normally can’t be predicted but through
variation populations can survive and evolve.
I. INDIVIDUALS DON’T EVOLVE,
POPULATIONS DO
Evolution is predicated on the concept that
genetic variation leads to the environment
choosing which version of a trait is best.
What mechanism(s) allow for variation?
There has to be a way for nature to allow for the
creation of new alleles.
Evolution starts with mutations in individuals;
mutation is the mechanism for the creation of
new alleles.
The penetrance of this mutation is completely
dependent on SEXUAL reproduction.
VARIATION IN POPULATIONS
 Population
 Interbreeding individuals of the same species in the same
area.
 All individuals of a species have to share certain traits
 But individuals of a population vary in the details of
these shared traits.
 These variations of course are because of alternative
versions of the alleles that cause these shared traits.
Where does VARIATION come from?
Mutations
II. HOW TO DETECT EVOLUTION:
VARIATION CAN BE CALCULATED
 Variation is distributed within a population.
 This fact is a way evolution can be studied.
 A distribution is an overview of the relative
frequency and range of a set of values.
 Often, some values in a range are more common
than others.
 A normal distribution, or bell curve, is one that
tends to cluster around an average value in the
center of the range.
 This sets the range of what’s normal and standard
for a population.
NORMAL DISTRIBUTION
Mean
Median
Mode
= average
= middle value
= the number most repeated
THE GENE POOL
 The variation is founded in alleles
 Alleles
The dominant phenotype
• Different forms of the same gene
• Determines genotype and phenotype
 Gene pool
• All alleles found in one population
 Within the gene pool, variation is due
to various alleles.
 Mutations are responsible for the
various alleles.
The Black Jaguar
6% of the South American population
VARIATION & INHERITANCE
Variation is constantly
changing because of
natural selection but
also genetic shuffling,
through new
combinations of alleles.
You have to also consider
recombination in changes in
genetics…
VARIATION & ALLELES ARE TRACKED BY
STUDYING ALLELE FREQUENCIES
 Proving evolution is partially achieved through investigating
the change in allelic frequency over several generations.
 The change is measured against the population in genetic
equilibrium.
 Allele frequencies
• Relative abundance of alleles of a given gene in a population
• Represented as percentages, much like the % composition of
compounds.
 Natural populations are never in genetic equilibrium
• Genetic equilibrium = the allele frequency stays the same.
• A theoretical state which occurs when a population is not
evolving
III. GENETIC EQUILIBRIUM
Researchers know whether or not a population is
evolving by tracking deviations from a baseline
of genetic equilibrium.
Five conditions required for a stable gene pool:
1.
2.
3.
4.
5.
Mutations do not occur
Population is infinitely large
No gene flow
Random mating
All individuals survive and reproduce equally
This doesn’t happen but the rate these change
will affect the rate of evolution.
GENOT YPE FREQUENCIES VS. ALLELE
FREQUENCIES
The thing you need to know is that genotypic
and allelic frequencies always add up to 1.
Genotype Frequency
(frequency of EE) +
(frequency of Ee) +
(frequency of ee) = 1
Allele Frequency:
(frequency of E) +
(frequency of e) = 1
MICROEVOLUTION
Genetic equilibrium is affected by several
processes that alter allelic frequency.
This is how evolution and speciation
happens.
Tracking evolution is studied by and
defined by changes in allele frequencies
over time.
THE HARDY-WEINBERG FORMULA
 Determining Genetic Equilibrium… the BASELINE
 The Hardy-Weinberg formula can be used to
determine if a population is in genetic equilibrium
p 2 (AA) + 2pq (Aa) +q 2 (aa) = 1.0
 The frequency of the dominant allele ( A) plus the
recessive allele (a) equals 1.0
p + q = 1.0
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.
You will learn more about the Hardy Weinberg formula soon…
A POPULATION IN
EQUILIBRIUM
Finding out whether a population is evolving. The frequencies of wing-color alleles among
all of the individuals in this hypothetical population of morpho butterflies are not
changing; thus, the population is not evolving.
Fig. 18-3a, p. 280
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
medium-blue wings
90 aa butterflies
white wings
90 aa butterflies
white wings
90 aa butterflies
white wings
Starting Population
Next Generation
Next Generation
Fig. 18-3b, p. 280
IV. MICROEVOLUTION
FORCES OF GENETIC CHANGE
Just as the Hardy-Weinberg principle allows us
to see a population that’s not evolving, so we
can measure changes, these forces act
against equilibrium:
1. Gene Flow
2. Genetic Drift
3. Mutation
4. Non-random Mating
5. Natural Selection
1. GENE FLOW
Gene flow
Physical movement of alleles caused by
individuals moving into and away from
populations
Immigration, etc.
Tends to counter the evolutionary effects of
mutation, natural selection, and genetic drift
on a population
Example: Movement of acorns by blue jays
GENE FLOW BETWEEN OAK POPULATIONS
2. GENETIC DRIFT—
THE CHANCE CHANGES
Genetic drift
 The random drifting of allele frequencies
 Very much dependent on population size
 Like a Pendulum…
Leads to…
Fixation has occurred when all individuals in a
population are homozygous for one allele
GENETIC DRIFT AND POPULATION SIZE
GENETIC DRIFT AND POPULATION SIZE
GENETIC DRIFT & BOTTLENECKS
 Bottleneck
 A drastic reduction in population size brought about
by severe pressure
 After a bottleneck, genetic drift is pronounced when
a few individuals rebuild a population, fumbling to
figure out beneficial traits.
 Example: Northern elephant seals
 Example: The Great Dinosaur Extinction
Catastrophic
Event
Original
Population
A
A
A
A
B
B
A
A
B
B
B
B
A
A
A
A
A
A
C
C
New
Population
Dynamics
A
A
B
B
A
A
A
A
B
B
A
C
B
A
C
GENETIC DRIFT & THE FOUNDER EFFECT
Founder effect
 Genetic drift is pronounced when a few individuals
start a new population
 (Often in conjunction with bottlenecks)
Inbreeding
 Breeding or mating between close relatives who
share a large number of alleles
 Example: Old Order Amish in Lancaster County,
Pennsylvania (Ellis-van Creveld syndrome)
3. MUTATION REVISITED
 Mutations are the source of new alleles that give rise to
differences in details of shared traits
 Lethal mutations usually result in death
 Decrease fitness but these rarely survive the gene pool.
 Neutral mutations have no effect on survival or
reproduction
 Often considered “silent” mutations.
 In humans, the average person undergoes 50-100
mutations in their lifetime.
 Less than 3% have any consequence.
 Remember, ~95% of our genome is “junk” DNA
 Beneficial mutations convey an advantage.
 Rare but does occur… has to.
4. NON-RANDOM
MATING = SEXUAL
SELECTION
SEXUAL SELECTION
With sexual selection, some version of a
trait gives an individual an advantage over
others in attracting mates
Distinct male and female phenotypes
(sexual dimorphism) is one outcome of
sexual selection
MATING DANCE OF BIRD OF PARADISE
http://youtu.be/nS1tEnfkk6M
SEXUAL SELECTION
sexual dimorphism
THE LION’S MANE…
 Females are attracted to
males with larger, dark manes
 Correlation with higher
testosterone levels
 better nutrition & health
 more muscle & aggression
 better sperm count / fertility
 more successful young
 But imposes a cost to male
 It’s HOT!
 Is it worth it??
“SEXY” = SUBCONSCIOUS
FITNESS MARKERS
The Golden Ratio
Symmetry
This contributes to
non-random
mating:
Humans have
preferences, just
like all other
sexual organisms.
BALANCED POLYMORPHISM
Balanced polymorphism
 A state in which natural selection maintains two or
more alleles at relatively high frequencies
 Occurs when environmental conditions favor
heterozygotes
Example: Sexes
Example: Sickle cell anemia and malaria
 Hb A/Hb S heterozygotes survive malaria more often
than people who make only normal hemoglobin
5. NATURAL SELECTION REVISITED
Natural selection
The variable survival and reproduction among
individuals of a population that vary in
details of their shared traits.
This is a driving force of evolution.
Occurs in recognizable patterns depending on
the organisms and their environment.
NATURAL SELECTION
Natural Selection Acts in 3 Major Ways:
All Populations Have Variation
 Sometimes hard to detect, not so in humans
Individuals Tend To Produce Too Many
Offspring
 Individuals in a population will always have a hard
time surviving
All Populations Depend on the Reproduction
of Individuals
 Fitness = the reproductive success of grandchildren.
V. Three Patterns of
Natural Selection
Natural selection
Takes 3 forms:
Directional
Stabilizing
Disruptive
We’ll take a quick peak
at all three types.
population
before selection
directional selection
stabilizing selection
disruptive selection
Fig. 18-4, p. 281
DIRECTIONAL SELECTION
Directional selection
Changing environmental conditions, the
selective PRESSURE put on by the
environment, can shift allele frequencies in
one consistent direction
Forms of traits at one end of a range of trait
variation become more common in the
population.
The other becomes less or drops out
completely.
Directional selection. These bell-shaped curves signify a range of continuous
variation in a butterfly wing-color trait. Red arrows indicate which forms are
being selected against; green, forms that are being favored.
Fig. 18-5a, p. 282
Fig. 18-5b, p. 282
Becomes this with
directional
selection
Casualties of
Natural Selection
Now extinct…
Once the norm…
Fig. 18-5c, p. 282
PREDATION AND PEPPERED MOTHS
 Light color is adaptive in areas of low pollution;
 Dark color is adaptive in areas of high pollution
Natural selection of two
forms of the same trait,
body surface coloration, in
two settings. (a) Light
moths (Biston betularia) on
a nonsooty tree trunk are
hidden from predators.
Dark ones stand out. (b)
The dark color is more
adaptive in places where
soot darkens tree trunks.
Fig. 18-6a (1), p. 283
Fig. 18-6a (2), p. 283
Fig. 18-6b (1), p. 283
Fig. 18-6b (2), p. 283
PREDATION AND ROCK-POCKET MICE
In rock-pocket mice, two alleles of a
single gene control coat color.
Night-flying owls are the selective
pressure that directionally shifts the
allele frequency.
DIRECTIONAL
SELECTION
IN ROCK-POCKET
MICE
Fig. 18-7a, p. 283
Visible evidence of directional selection in populations of rock pocket mice. (a) Rock pocket
mice that have dark fur are more common in these areas of dark basalt rock. (b, c) The two
color types of rock pocket mice, each posed on the dark and light rocks of the area.
Fig. 18-7b, p. 283
ANTIBIOTIC RESISTANT BACTERIA
A typical two-week course of antibiotics can
exert selection pressure on over a
thousand generations of bacteria.
If ANY remain, they do so because they
may have a mutated gene that causes a
resistance to these antibiotics.
Antibiotic resistant strains are now found
everywhere, in hospitals and schools.
The rate of these superbugs are
exacerbated by people not completing their
prescriptions.
SELECTION AGAINST OR
IN FAVOR OF EXTREME PHENOT YPES
Stabilizing selection
Natural selection that favors an intermediate
phenotype and eliminates extreme forms
Disruptive selection
Natural selection that favors extreme forms
of a trait and eliminates the intermediate
forms
Time 1
Number of individuals
in population
Stabilizing Selection
Range of values for the trait
Time 2
Time 3
Stepped Art
Fig. 18-8, p. 284
STABILIZING SELECTION:
BODY WEIGHT OF SOCIABLE WEAVERS
What is the optimal weight of a sociable weaver bird?
Fig. 18-9a, p. 284
Stabilizing selection in sociable weavers. Graph shows the number of birds (out of 977) that
survived a breeding season.
Figure It Out: What is the optimal weight of a sociable weaver bird?
Answer: About 29 grams
Fig. 18-9b, p. 284
DISRUPTIVE
SELECTION
Disruptive selection eliminates midrange forms of
a trait, and maintains extreme forms.
Fig. 18-10a, p. 285
Fig. 18-10b, p. 285
Fig. 18-10c, p. 285
DISRUPTIVE SELECTION:
BILL SIZE IN AFRICAN FINCHES
In African seedcracker populations, birds with bills that are about
12 or 15 millimeters wide are favored.
The difference is a result of competition for scarce food during
dry seasons.
But the middle phenotypes are not good at eating the food.
lower bill 12 mm wide = the “splitter”
Fig. 18-11a, p. 285
lower bill 15 mm wide = the “crusher”
Fig. 18-11b, p. 285
MAINTAINING VARIATION
Natural selection theory helps explain diverse
aspects of nature, including differences
between males and females, and the
relationship between sickle-cell anaemia and
malaria.
SICKLE CELL ANEMIA AND MALARIA
Distribution of malaria cases reported in Africa, Asia, and the Middle East in the 1920s,
Fig. 18-13a, p. 287
Distribution (by percentage) of people that carry the sickle-cell allele.
Fig. 18-13b, p. 287
Notice the close correlation between the maps.
Individuals with SS (normal
RBCs) get malaria and can
die.
Individuals with ss (sickle
cell anemia) usually die
early on.
The hybrid (Ss) can survive
with malaria and suffer little
side affects associated with
sickle cell anemia.
Fig. 18-13b, p. 287
The data would indicate that sickle cell
anemia and malaria respond to each other.
Both alleles have success so natural
selection favors a mix of both over one or
the other.
Malaria and sickle-cell anemia. (c) Physician searching for mosquito larvae in Southeast
Asia.
Fig. 18-13c, p. 287
NATURAL SELECTION REVISITED
 Natural Selection supports the best traits
 But Natural Selection can be limited because:
 Indirect Force: Natural selection doesn’t act on genes.
 Natural selection acts on unsuccessful physical traits…
 Not the genes that make them.
 Since traits (phenotypes) are determined by genes (genotypes),
sometimes these unsuccessful phenotypes can pop up if they
are recessive.
 Role of Mutation;
 Usually recessive alleles
 Recessive alleles can be hidden from targeting by natural
selection.
CONCEPT CHECK
 What is microevolution?
 How populations change due to changes in allele frequencies.
 Which type deals with allele frequencies that result from a
population of rats that stow away on a barge that lands on an
island?
 Founder af fect… genetic drift.
 Some years black moths are favored, in other years white
moths are favored. This form of natural selection is what?
 Directional stabilization.
VI. SPECIATION
Once Genetic Drift Ends…
SO WE’VE DISCUSSED THE MECHANISMS
THAT CAUSE ALLELE CHANGES
Now…
Once gene flow ends,
reproductive isolation occurs.
Once reproductive isolation
occurs, speciation soon
follows
REPRODUCTIVE ISOLATION
Speciation
 Evolutionary process by which new species form
 Reproductive isolating mechanisms are always part of
the process
Reproductive isolation
 The end of gene exchange between populations
 Beginning of speciation
FOUR BUTTERFLIES, TWO SPECIES
REPRODUCTIVE ISOLATING MECHANISMS
 Reproductive isolating mechanisms prevent
interbreeding among groups.
 Heritable aspects of body form, function, or behavior that
arise as populations diverge… they start to change because
of the different selection pressures of the different
ecosystem and the traits available the founding population.
 What are the isolating mechanisms?
1.
2.
3.
4.
5.
6.
Mechanical
Behavioral
Hybridization
Allopatric
Sympatric/ Polyploidy (genetic)
Parapatric
1. MECHANICAL ISOLATION
2. BEHAVIORAL ISOLATION
3. HYBRIDIZATION
 Reduced hybrid viability (ligers, tigons)
 Extra or missing genes
 Reduced hybrid fertility (mules)
 Robust but sterile offspring
 Hybrid breakdown
 Lower fitness with successive generations
4. ALLOPATRIC SPECIATION
Geographic Isolation
= “Allopatric speciation”
A physical barrier arises and ends gene flow
between populations
Genetic divergence results in speciation
Example: llamas, vicunas, and camels
ALLOPATRIC SPECIATION
THE INVITING ARCHIPELAGOS
Winds or ocean currents carry a few
individuals of mainland species to remote,
isolated islands chains (archipelagos) such as
Hawaii
Habitats and selection pressures that differ
within and between the islands foster
divergences that result in allopatric speciation
ALLOPATRIC SPECIATION
ON AN ISOLATED ARCHIPELAGO
A A few individuals of a
mainland species reach
isolated island 1. In the new
habitat, populations of their
descendants diverge, and
speciation occurs.
B Later, a few
individuals of a new
species colonize
nearby island 2.
Speciation follows
genetic divergence
in the new habitat.
C Genetically different
descendants of the
ancestral species may
colonize islands 3 and 4
or even invade island 1.
Genetic divergence and
speciation may follow.
Fig. 18-21a, p. 293
OTHER SPECIATION MODELS
Populations sometimes speciate even without
a physical barrier that blocks gene flow.
 Sympatric speciation
 Parapatric speciation
5. SYMPATRIC SPECIATION
In sympatric speciation, new species form
within a home range of an existing species, in
the absence of a physical barrier
Polyploidy An example when there is a change
in chromosome number that can cause instant
speciation .
On Lord Howe Island, species of palms are
reproductively isolated because of polyploidy
SYMPATRIC SPECIATION IN PALMS
6. PARAPATRIC SPECIATION
In parapatric speciation, populations in
contact along a common border evolve into
distinct species
Hybrids in the contact zone are less fit than
individuals on either side
PARAPATRIC SPECIATION
T. barretti
hybrid zone
T. anophthalmus
Fig. 18-24c, p. 295
DIFFERENT SPECIATION MODELS
KEY CONCEPTS
HOW SPECIES ARISE
 We have looked at microevolution, small allele frequency
changes.
 Microevolutionary events that occur independently lead
to genetic divergences, which are reinforced as
reproductive isolation mechanisms evolve
 Now let’s look at macro, large-scale changes.
 Macroevolution refers to the appearance of new species
over time
 New species evolve mainly because of DIVERGENCE.
 These show how relationships influence the creation of
new species.
 Speciation, the formation of new species due to
evolution, links the two.
VII. MACROEVOLUTION
Macroevolution
 Large-scale patterns of evolutionary change
 Includes patterns of change such as one species giving rise to
multiple species, the origin of major groups, and major
extinction events
 Coevolution
 Stasis
 Exaptation
 Adaptive radiation
 Key innovation
 Extinctions
MACROEVOLUTION…THE PATTERNS OF
EVOLUTION
Patterns of Macroevolution: that affect species.
 Co-evolution: Organisms are part of one other’s environment,
so they can affect one another’s evolution. Species that live
in close contact often have clear adaptations to one another’s
existence.
 Adaptive Radiation: Over time, species may split into two or
more lines of descendants, or lineages. As this splitting
repeats, one species can give rise to many new species. The
process tends to speed up when a new species enters an
environment that contains few other species (meaning less
competition). Helps explain finches.
 Extinction: If all members of a lineage die off or simply fail to
reproduce, the lineage is said to be extinct. The fossil record
shows that many lineages have arisen and radiated, but only
a few of their descendants survived and evolved into the
species present today. Scientist estimate that 99% of all
species that have lived, went extinct.
COEVOLUTION
Two species in close ecological contact act as
agents of selection on each other
(coevolution)
 Predator and prey
 Host and parasite
 Pollinator and flower
Over time, the two species may come to
depend on each other.
Coevolution
Hawk moth & Orchid
Coevolved species. The orchid
Angraecum sesquipedale, discovered
in Madagascar in 1852, stores its
nectar at the base of a floral tube 30centimeters (12 inches) long. Charles
Darwin predicted that someone would
eventually discover an insect in
Madagascar with a proboscis long
enough to reach the nectar and
pollinate the flower. Decades later, the
hawkmoth Xanthopan morgani
praedicta was discovered in
Madagascar. Its proboscis is 30–35
cm long.
proboscis
nectar tube
1018-25,
cmp. 296
Fig.
BEE & THE ORCHID
ECOLOGICAL
NICHES
 A niche is the entire
collective habitat
where a population
lives.
 When populations
inhabit different
niches, divergence
often
follows…because
there are different
selection pressures.
 Adaptive radiation
 A burst of speciation that occurs
when a lineage encounters a new
set of niches because of a Key
Innovation: A structural or
functional adaptation that allows
individuals to exploit their habitat
in a new way
 Again, caused by variation and
natural selection
rabbits
rodents
primates
horses, other
perissodactyls
dolphins
deer, other
artiodactyls
whales,
carnivores
bats
shrews, other
insectivores;
armadillos
anteaters
manatees
elephants, other
proboscideans
kangaroos, other
marsupials
platypus, other
monotremes
Eomaia scansoria
Cenozoic
Evolutionary tree diagram showing the adaptive radiation of mammals following
the K–T extinction
Mesozoic
event. Branch widths indicate the range of biodiversity in each group at different times. We show only a
sample of modern mammals. The entire mammalian lineage includes more than 4,000 modern
species. The photograph shows a fossil of Eomaia scansoria (Greek for ancient mother climber),
complete with the imprint of its fur. About 125 million years ago, this mouse-sized insect-eater crawled
on low branches. It is thought to be an offshoot of the lineage that led to mammals.
Fig. 18-26a, p. 297
PATTERNS OF MACROEVOLUTION
How do Species Evolve?
There are two main schools of though…
1. Gradualism
2. Punctuated Equilibrium
MACROEVOLUTION
Gradualism : In Darwin’s day, the idea of slow,
gradual change was new to geology as well as
biology. Darwin had argued that large scale
changes, such as the formation of new
species, must require many small changes to
build up gradually over a long period of time.
This model is called gradualism.
MACROEVOLUTION
Gradualism conflicts with another more
modern theory that suggests evolution can
happen in bursts over a very fast period of
time.
Punctuated Equilibrium: Some biologists
argue that species do not always evolve
gradually. Species may remain stable for long
periods until environmental changes
drastically and suddenly create new
pressures. Then, many new species may
“suddenly” appear. This model is called
punctuated equilibrium.
Which way is correct?
Probably both but there is a lot of
evidence that PE is the more
correct.
Gradualism
Punctuated Equilibrium
Small evolutionary changes
accumulate over long periods
of time. These result from
mutations present in alleles
present in population.
Catastrophic events, like
volcanoes or meteor strikes,
suddenly change the
environment, creating new
selection pressures. The fittest
survive.
VII. HOW DOES EVOLUTION STOP?
Eliminated Evolution…
Stasis
Exaptation
Extinction
STASIS AND EXAPTATION
 Stasis
 A lineage exists for millions of years with little or no change
(e.g. coelacanth)
 Exaptation (preadaptation)
 Some complex traits in modern species held different adaptive
value in ancestral lineages (e.g. feathers in birds and
dinosaurs)
EXTINCTION
 Extinction
 The irrevocable loss of a species from Earth
 Mass extinctions
 Extinctions of many lineages, followed by adaptive radiations
 Five catastrophic events in which the majority of species on Earth
disappeared
 K–T extinction, abbreviation of Cretaceous –Tertiary extinction,
also called K–Pg extinction or Cretaceous–Paleogene extinction
a global extinction event responsible for eliminating
approximately 80 percent of all species of animals at or very
close to the boundary between the Cretaceous and Paleogene
periods, about 65.5 million years ago.
http://www.britannica.com/EBchecked/topic/1314796/K-T-extinction
SO…
Exhausted Yet?
We’ve covered a great deal today
- The Parameters of Genetic Equilibrium
- The Mechanisms of Microevolution
- The Methods of Genetic Change
- The Major Patterns of Macroevolution
Do you have to remember EVERYTHING?
- Pretty much.
Should you be aware of these terms for later investigations?
- Yep.
Review the list on the board and be prepared to use these next
week.
CLOSURE
Questions?
Reflect on what you’ve learned…
Go to the website & download the ppt to make
up anything you missed.
Block periods: Natural Selection Lab
Friday: Bacterial lab: Last Check