Transcript Populations and Gene Pools

Evolution and
Populations
Chapter 17
Genetics and Molecular Biology

Darwin had no idea how heredity worked, and he was
worried that this lack of knowledge might prove fatal to his
theory.

As it happens, some of the strongest evidence supporting
evolutionary theory comes from genetics. A long series of
discoveries, from Mendel to Watson and Crick to
genomics, helps explain how evolution works.

Also, we now understand how mutation and the
reshuffling of genes during sexual reproduction produce
the heritable variation on which natural selection operates.
Life’s Common Genetic Code


All living cells use information coded in DNA and RNA
to carry information from one generation to the next and
to direct protein synthesis.
This genetic code is nearly identical in almost all
organisms, including bacteria, yeasts, plants, fungi, and
animals.
A Testable Hypothesis

Darwin hypothesized that the Galápagos finches he
observed had descended from a common ancestor.

He noted that several finch species have beaks of very
different sizes and shapes. Each species uses its beak like
a specialized tool to pick up and handle its food.
Different types of foods are most easily handled with
beaks of different sizes and shapes.

Darwin proposed that natural selection had shaped the
beaks of different bird populations as they became
adapted to eat different foods.
Genetics Joins Evolutionary Theory
 In
genetic terms, evolution is any change in the
relative frequency of alleles in the gene pool of a
population over time.
 Researchers
discovered that heritable traits are
controlled by genes.
 Changes
in genes and chromosomes generate
variation.
Genotype and Phenotype in Evolution
 An
organism’s genotype is the particular
combination of alleles it carries.
 An
individual’s genotype, together with
environmental conditions, produces its phenotype.
 Phenotype
includes all physical, physiological, and
behavioral characteristics of an organism.
Genotype and Phenotype in Evolution
 Natural
selection acts directly on phenotype, not
genotype.
 Some
individuals have phenotypes that are better
suited to their environment than others. These
individuals produce more offspring and pass on
more copies of their genes to the next generation.
Populations and Gene Pools

A population is a group of individuals of the same species
that share a geographical area, mate and produce
offspring.


Smallest unit in which evolution occurs
A gene pool consists of all the genes, including all the
different alleles for each gene that are present in a
population.

Combined genetic information of all the members of a
particular population
BbbbBbbbBb
bbbbbBbbbb
Populations and Gene Pools
 Researchers
study gene pools by examining the
relative frequency of an allele. The relative frequency
of an allele is the number of times a particular allele
occurs in a gene pool, compared with the number of
times other alleles for the same gene occur.
 Allele Frequency = # of a certain allele
total # of alleles of all types
in the population
Red Allele - 9/36 = ¼ = 0.25
White Allele - 27/36 = ¾ = 0.75
Populations and Gene Pools
 For
example, this diagram shows the gene pool for
fur color in a population of mice.
Populations and Gene Pools
• Evolution, in genetic terms, involves a change in
the frequency of alleles in a population over
time.
• Note: Although natural selection acts on
individuals it is the population that evolves, not
individuals
Allele Frequency Example
 Calculate
the frequency of the dominant and
recessive alleles in the gene pool below.
AaaAaAaaAa
A=
a=
4/10 = 0.4
6/10 = 0.6
Alleles Frequencies & the Gene Pool:
 Phenotype
Frequency =
# of individuals with a particular phenotype
total #of individuals in population
Determining Phenotype and Allele Frequencies
using Japanese four o’clock flowers:
 1st
Generation:
RR
RR RW RW
 Phenotype Frequency:
White - 0
Pink 4/8 = 0.50
Red 4/8 = 0.50
RR
RW RW RR
Allele Frequency:
R = 12/16 = 0.75
W = 4/16 = 0.25
Predicting Genotypes & Phenotypes of
Second Generation:

According to the laws of probability, the chance of an R gamete meeting with another
R gamete is the product of the allele frequencies in the gene pool.

Red (RR)

White (WW) = W x W
= 0.25 x 0.25
= R x R = RR
= 0.75 x 0.75 = 0.5625
= WW
= 0.0625
The frequency of all types expected in the second generation
must add up to 1.0
1.0 - RR - WW = RW
1.0 - 0.5625 - 0.0625 = 0.375
Single-Gene Traits
A
single-gene trait is a trait controlled by only one
gene. Single-gene traits may have just two or three
distinct phenotypes.
 Dominance of an allele for a single-gene trait does
not necessarily mean that the dominant phenotype
will always appear with greater frequency in a given
population.
No widow’s peak is
a recessive trait
Natural selection on single gene traits
• Evolution does not act on genes. Instead it acts on
phenotype frequencies by changing allele frequencies!
• Evolution = any change in the relative frequencies of
alleles in a population’s gene pool
Initial Population
Generation 10
Generation 20
Generation 30
90%
80%
70%
40%
10%
20%
30%
60%
Polygenic Traits
 Polygenic
traits are traits controlled by two or more
genes.
 Each gene of a polygenic trait often has two or more
alleles.
 A single polygenic trait often has many possible
genotypes and even more different phenotypes.
How Natural Selection Works
 Evolutionary
fitness is the success in passing genes to
the next generation.
 Evolutionary adaptation is any genetically controlled
trait that increases an individual’s ability to pass along
its alleles.
Natural Selection on Single-Gene Traits
 Natural
selection for a single-gene trait can lead to
changes in allele frequencies and then to evolution.
 For example, a mutation in one gene that determines
body color in lizards can affect their lifespan. So if the
normal color for lizards is brown, a mutation may
produce red and black forms.
Natural Selection on Single-Gene Traits
 If
red lizards are more visible to predators, they might
be less likely to survive and reproduce. Therefore the
allele for red coloring might not become common.
 Black lizards might be able to absorb sunlight. Higher
body temperatures may allow the lizards to move
faster, escape predators, and reproduce.
Natural Selection on Polygenic Traits
 Polygenic
traits have a range of phenotypes that often
form a bell curve.
 The fitness of individuals may vary from one end of
the curve to the other.
 Natural selection can affect the range of phenotypes
and hence the shape of the bell curve.
3 Types of Selection
Natural selection on polygenic traits
Natural selection can affect the relative fitness of
phenotypes involving polygenic traits in any of 3 ways:
1. Stabilizing Selection -average form of a trait is favored.
2. Directional Selection -one extreme form of a trait is
favored.
3. Disruptive Selection - either/both extremes of a trait are
favored over an average form of a trait.
Types of Natural Selection (2:21)
Directional selection
Stabilizing selection
Disruptive selection
Directional Selection
 Directional
selection occurs when individuals at one
end of the curve have higher fitness than individuals
in the middle or at the other end. The range of
phenotypes shifts because some individuals are more
successful at surviving and reproducing than others.
Directional Selection:
Anteaters feed by breaking open termite nests (extend
their sticky tongues into the nests).
 New species of termites that build very deep nests.
 Anteaters with long tongues more effective than those
with average or short tongues
Directional Selection
Directional Selection
A
drought on the Galápagos island of Daphne Major
in 1977 reduced the number of small seeds available
to finches, causing many of the small-beaked finches
to die. This caused an increase in the finches’ average
beak size between 1976 and 1978.
Stabilizing Selection
 Stabilizing
selection occurs when individuals near
the center of the curve have higher fitness than
individuals at either end. This situation keeps the
center of the curve at its current position, but it
narrows the overall graph.
Stabilizing Selection:
Most common kind of selection
A – small lizards may not be able to run fast enough to
escape
B – large lizards may be more easily spotted, captured,
& eaten by predators
o
Stabilizing Selection
Disruptive Selection
 Disruptive
selection occurs when individuals at the
upper and lower ends of the curve have higher
fitness than individuals near the middle. Disruptive
selection acts against individuals of an intermediate
type and can create two distinct phenotypes.
Disruptive Selection:
Limpets are marine organisms that have shells that vary in
color from white to dark brown. Limpets live their adult
life attached to rocks.



On light colored rock, white shelled limpets are at an
advantage because the birds that prey upon them have a
difficult time locating them.
On dark-colored rock, dark-shelled limpets are well
camouflaged.
On the other hand, tan-colored limpets are easily spotted on
either light or dark rocks.
Genetic Drift
 Genetic
drift occurs in small populations when an
allele becomes more or less common simply by
chance. Genetic drift is a random change in allele
frequency.
 The smaller the population, the more susceptible it is
to such random changes.
Evolution vs. Genetic Equilibrium



The Hardy-Weinberg principle states that allele
frequencies in a population should remain constant unless
one or more factors cause those frequencies to change.
The Hardy-Weinberg principle makes predictions about
certain genotype frequencies.
According to the Hardy-Weinberg principle, five
conditions are required to maintain genetic equilibrium:
(1) The population must be very large
(2) There can be no mutations
(3) There must be random mating
(4) There can be no movement into or out of the population
(5) No natural selection
Evolution vs. Genetic Equilibrium
A
population is in genetic equilibrium if allele
frequencies in the population remain the same. If allele
frequencies don’t change, the population will not
evolve.
 Real populations rarely exist under the rigid conditions
of the Hardy-Weinberg Equilibrium.
The Hardy-Weinberg principle predicts that 5
conditions can disturb genetic equilibrium and
cause evolution to occur:
Nonrandom mating – individuals select mates based on
heritable traits
2. Small population size – evolutionary change due to
genetic drift happens more easily in small populations
3. Immigration or emigration – movement of individuals
into (immigration) or out of (emigration) may introduce
new alleles or remove alleles from the gene pool
4. Mutations – introduce new alleles changing allele
frequencies
5. Natural Selection – different genotypes have different
fitness
One or more of these conditions usually holds for real
populations = evolution happens most of the time
1.
 Nonrandom
mating example – female peacocks
choose mates on the basis of physical characteristics
such as brightly patterned tail feathers
Isolating Mechanisms
 When
populations become reproductively isolated,
they can evolve into two separate species.
Reproductive isolation can develop in a variety of
ways, including behavioral isolation, geographic
isolation, and temporal isolation.
 Speciation is the formation of a new species. A
species is a population whose members can
interbreed and produce fertile offspring.
Isolating Mechanisms
 Reproductive
isolation occurs when a population
splits into two groups and the two populations no
longer interbreed.
 When populations become reproductively isolated,
they can evolve into two separate species.
Behavioral Isolation
 Behavioral
isolation occurs when two populations
that are capable of interbreeding develop differences in
courtship rituals or other behaviors. Blue Footed Booby Courtship
watch to 1:30 min
Blue footed boobies perform an elaborate courtship display that involves the male
‘skypointing’ (pointing the head and beak upwards and spreading the wings), alternately
lifting each blue foot, with the tail held cocked, and often emitting a whistling call.
Both members of the pair may then skypoint, touch beaks, lift the feet, or pick up twigs
or stones and place them on the ground
Behavioral Isolation
 Eastern
and western meadowlarks are difficult to
distinguish between based on size, shape, and color,
however, their calls are quite distinct.

Presumably this difference serves to distinguish mates
from the different species.
 Behavioral
isolation may also prevent different firefly
populations from mating because different species
have their own pattern of light pulses. Other
examples include mating dances and various
courtship rituals.
Synchronous Firefly Display
Western
Meadowlark Call
Eastern
Meadowlark Call
Geographic Isolation



Geographic isolation occurs when two
populations are separated by geographic
barriers such as rivers, mountains, or bodies of
water.
Once completely separated, the two
populations possess variations of some genes,
resulting in two "species" that differ in
appearance (color, size, etc.) and behavior.
For example, the Kaibab squirrel is a
subspecies of the Abert’s squirrel that formed
when a small population became isolated on
the north rim of the Grand Canyon. Separate
gene pools formed, and genetic changes in one
group were not passed on to the other.
Geographical Isolation


The Kaibab squirrel
(Sciurus aberti
kaibabensis, left) became
geographically isolated
from the common
ancestor with its closest
relative, the Abert
squirrel (Sciurus aberti
aberti, right) in the North
Rim of the Grand
Canyon about 10,000
years ago.
Since then, several
distinguishing features,
such as the black belly
and forelimbs have
gradually evolved.
Temporal Isolation
 Temporal
isolation happens when two or more
species reproduce at different times.
 For example, three species of orchid live in the same
rain forest. Each species has flowers that last only
one day and must be pollinated on that day to
produce seeds. Because the species bloom on
different days, they cannot pollinate each other.
Temporal Isolation
 The
Red-legged Frog (Rana aurora, left) breeding
season lasts from January to March.
The closely related Yellow-legged Frog (Rana boylii,
right) breeds from late March through May.
Temporal Isolation
 Drosophila
persimilis breeds in early morning,
while closely related Drosophila pseudoobscura
breeds in the afternoon
Founders Arrive
 Many years ago, a few finches from South America—
species M—arrived on one of the Galápagos islands,
as shown in the figure.
Geographic Isolation
 Because of the founder
effect, the allele
frequencies of this founding finch population
could have differed from those in the South
American population.
Changes in Gene Pools
 Over
time, populations on each island adapted to
local environments.
Behavioral Isolation
 Natural selection could have caused two distinct
populations to evolve (A and B), each characterized by
a new phenotype.
Competition and Continued Evolution
 Birds that are most different
from each other have the
highest fitness. More specialized birds have less
competition for food. Over time, species evolve in a way
that increases the differences between them, and new
species may evolve (C, D, and E).
Speciation in Darwin’s Finches
 Speciation in Galápagos
1.
2.
3.
4.
5.
finches occurred by:
founding of a new
population
geographic isolation
changes in the new
population’s gene pool
behavioral isolation
ecological competition
Gradualism
 Gradualism
involves a
slow, steady change in a
particular line of descent.
 The
fossil record shows that
many organisms have
indeed changed gradually
over time.

The pattern of slow, steady change does not always hold.

Horseshoe crabs, for example, have changed little in
structure from the time they first appeared in the fossil
record.

This species is said to be in a state of equilibrium, which
means that the crab’s structure has not changed much
over a very long stretch of time.
Punctuated Equilibrium


Punctuated equilibrium is
the term used to describe
equilibrium that is
interrupted by brief periods
of more rapid change.
This is a proposed theory to
explain the gaps in the fossil
record.
Rapid Evolution After Equilibrium
 Rapid
evolution may occur after a small
population becomes isolated from the main
population. This small population can evolve faster
than the larger one because genetic changes spread
more quickly among fewer individuals.
 Rapid evolution may also occur when a small
group of organisms migrates to a new
environment. That’s what happened with the
Galápagos finches.
Genetic Drift
A
change in allele frequencies that occurs due
to chance events rather than differences in
fitness
 Genetic drift can cause big losses of genetic
variation for small populations.


Population Bottlenecks
Founder Effect
http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml
Population Bottlenecks
 Population
bottlenecks occur when a population’s size
is reduced to just a few individuals, often as a result of
some form of disaster such as disease


The smaller population has less genetic variation than the
original, larger one.
Even when population increases, genetic variation
remains low.
Cheetahs have extremely low genetic
variation due to their population
having gone through a population
bottleneck about 10,000 years ago - at
the end of the last ice age when a
remarkable extinction of large
vertebrates occurred on several
continents.
Population Bottleneck

Because genetic drift acts more quickly to reduce genetic
variation in small populations, undergoing a bottleneck
can reduce a population’s genetic variation by a lot, even if
the bottleneck doesn’t last for very many generations.

This is illustrated by the bags of marbles shown below,
where, in generation 2, an unusually small draw creates a
bottleneck.
Bottleneck Example

Northern elephant seals have reduced genetic variation probably
because of a population bottleneck humans inflicted on them in
the 1890s. Hunting reduced their population size to as few as 20
individuals at the end of the 19th century. Their population has
since rebounded to over 30,000—but their genes still carry the
marks of this bottleneck: they have much less genetic variation
than a population of southern elephant seals that was not so
intensely hunted.
http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml
Reduced genetic variation means that the population
may not be able to adapt to new selection pressures,
such as climatic change or a shift in available resources,
because the genetic variation that selection would act on
may have already drifted out of the population.
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:


Decreased genetic variation from the original
population by reducing the number of alleles.
a non-random sample of the genes in the original
population.
 Alleles
are regularly lost from populations. However,
the process of mutation produces new alleles.
http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml
Founder Effect
Founder Effect Example
 The
Afrikaner population of Dutch settlers in
South Africa is descended mainly from a few
colonists.
 Today, the Afrikaner population has an unusually
high frequency of the gene that causes
Huntington’s disease (nerve cells in the brain
degenerate)

those original Dutch colonists just happened to
carry that gene with unusually high frequency.
Founder Effect Example
Pennsylvania Amish
 In the 1700s, a small group of Europeans settled in eastern Pennsylvania.
 Among this small group was an individual who carried an allele for Ellis-van
Creveld syndrome.





Ellis-van Creveld syndrome is a very rare form of dwarfism causing short stature,
extra fingers (polydactyly), abnormal teeth and nails, and heart defects.
The allele for Ellis-van Creveld syndrome is found at a frequency of 7% in the
Pennsylvania Amish in comparison to only 0.1% in the general population.
The low allelic frequency of 0.1% was also the allelic frequency of the original
European population from which the Amish migrated.
While the Amish live in close proximity to large, diverse human populations
that would be capable of breeding, the culture of the Amish restricts marriage
outside of the group.
This results in genetic isolation and group interbreeding that allows the
frequency of the allele for Ellis-van Creveld syndrome to not only persist but
increase over time.