Download Population genetics as a means to explore

CHARLES DARWIN
(1809-1882)
ALFRED WALLACE
(1823-1913)
POPULATION GENETICS
as a means to explore
EVOLUTION
A. Theory of Evolution

Around world, many people held view that each species
came about by divine (godly) creation
Buddhism: 1. who cares?; 2. spiritual beings take many forms
including man in each cycle
 Hinduism: 1. primal man Purusa sacrificed to become universe head became heavens, arms became warriors, legs the
commoners, feet the serfs; 2. Vishnu commanded servant Brahma
to create world which he did out of lotus flower
 Islam: Allah (God) created heaven and earth in 6 days (Quran
7:54)
 Judeo-Christian: God created each creature including man in 6
days (Genesis I:20-25)

During 19th century, many fossils discovered of
species not currently in existence
 Question of origin spawned many hypotheses
 1831 voyage of amateur naturalist Charles
Darwin along with 1854 exploration of
naturalist Alfred Wallace gave enough evidence
for both to propose theory of evolution of
populations of organisms via natural selection

Poor Mr. Wallace
B. Natural Selection


Process that results in survival and propagation of
organisms with favorable mutations that are better
suited to adapt to environment than those without
Nature selects adaptations of organisms best suited to
succeed in environment (“survival of the fittest”)
 Acts on phenotype of individual – whatever physical
appearance or expression of trait to help it to
survive and reproduce (homozygous or heterozygous
genotype is not important)
 Best phenotype of population will dominate
What makes one mutation
good vs. bad? Nature!
Beneficial
Mutation
No longer referred
to as “mutations” but
rather “adaptations”
Detrimental
Mutation
Mutations will always
happen. Some are
good, some are bad,
most are neutral.
Darwin’s Finch Evolution

Two types of adaptations:
Structural – physical changes in
appearance
Physiological – change in how organism
functions or metabolizes (making venom,
digesting cow milk)
Structural Adaptations
Thorns made it harder for
predators to eat rosebuds
White coat helps it blend in,
making it easier to stalk prey
Structural Adaptations
Pouch enables joey to be protected
but lets mom still roam around to
find food
.
Opposable thumbs makes it easier
to grasp small objects with one
hand while leaving other hand
free for defense.

Evidence of natural selection is of peppered moths
in industrial age England via “industrial melanism”
 Prior
to industrial revolution, 98% of peppered moths
were light colored to blend in with light colored tree
bark
98%
2%
carbonaria
typica

After industry & soot from factories turned tree
trunks black, those dark peppered moths had
an advantage and soon became most common
98%
2%
 3 different types of natural
selection that act on structural
variations:
1.Stabilizing selection
2.Directional selection
3.Disruptive selection
STABILIZING SELECTION
1. Stabilizing Selection: natural selection that
favors AVERAGE individuals in a population
 In
spider population, average size is a survival
advantage. Why?
• Smaller spider may find it hard to find food (too
weak or food is too big)
• Bigger spider may not find enough food to survive,
or predators like birds, can see them now and eat
them
In this case, the average spider is at an
advantage so gets “selected for”

Shape of bell curve shows selection of the
middle or average type of spiders
• In humans, nature favors medium heights
(polygenic inheritance)
Selection for average size for
particular spider species
400
Number
of
300
Spiders
Normal variation (for
particular spider species)
200
100
0.5
cm
1.5
cm
2.5
cm
3.5
cm
4.5
cm
Size of Spiders
5.5
cm
6.5
cm
7.5
cm
DIRECTIONAL SELECTION
2. Directional selection: natural selection
favors ONE of the EXTREME variations of
a trait
 In hummingbird population, suppose that
particular flower has lots of nectar for
the taking, but the flower is very long ,
so only hummingbirds with very long
beaks will be able to get at it
• those hummingbirds with EXTREMELY long
beaks will be favored in this region where
these flowers grow and will be “selected
for” by nature
• average-sized or small beaks will be at a
disadvantage since their beaks can’t reach
the nectar
Directional Selection
Normal variation
Number of
Hummingbirds
Selection for
longer beaks
1
2
3
4
5
6
7
8
9
10
11
Beak Size (cm)
• Shape of this curve shows the selection for
the EXTREME length (~8 cm)
DISRUPTIVE SELECTION
3.
Disruptive Selection: natural selection favors
BOTH EXTREME variations for a particular trait
In the snail-like marine organism called a
limpet, shell colors range from white, to tan, to
dark brown
• adults attach onto rocks which are either
light or dark colored – there is no medium
(intermediate) colored rocks in this
environment
• birds eat limpets – if they see them
 white limpets will have advantage on
light rocks due to camouflage
 dark limpets will have advantage on
dark rocks or the same reason
 tan or medium colored limpets will be
at a disadvantage since there are no
rocks to camouflage them
 Shape of this curve shows selection for both
of the extreme colors
Number of
limpets
Intermediate (tan) variation
Extreme (white) variation
Extreme (dark) variation
C. Changes in Gene Pool


All alleles in population are considered gene pool
 Only these alleles can make new organisms
 Ex: If mom & dad are homozygous dominant for
brown eyes, blue eyes just aren’t in your gene pool
Numerical count of alleles in specific
population called allelic frequency

What is allelic frequency of B & b
in this gene pool?
TOTAL: 14 alleles (7 pigs)
B: 7/14 = 50%
b: 7/14 = 50%
CHANGE in ALLELIC FREQUENCY =
EVOLUTION!
G = 9/12
B = 3/12
G = 17/24
B = 7/24

If gene pool has large variety of alleles, it’s
considered genetically diverse
 High
biodiversity is important in health &
survival of population against:
 Disease
 Natural
disasters
 Loss of habitat
 Irish
Potato Famine (1845-1852) is direct result
of lack of genetically diverse potatoes
succumbing to mold infestation
Irish Potato Famine 1845-1852



Potatoes originated from Andes Mountains in S.
America – many different varieties of potatoes
 Only few varieties made it to Ireland, where poor
farmers would grow it as mainstay of food
“Potato blight” fungal disease of potato crops carried
on ships landed in Ireland in 1844 destroying entire
crops
Mass starvation, disease, emigration led to 20-25%
drop in population of Ireland

How does an organism acquire diversity?
 Mutations!
Beneficial - improved survival
Neutral – doesn’t improve or harm survival
Lethal – would kill organism

If allele is lethal (as in t in Tay-Sachs), will
only be passed down if it doesn’t affect
heterozygote
 Tt
is completely healthy, will live to reproduce
 tt will die before reproducing

Exceptions are H dominant allele in
Huntington’s where individual reproduces
before lethality of allele manifests

Are we in danger of famine due to lack of
diversity?
 Corn
 Chickens
 Soybeans
A new study (2008) has found that commercial birds raised for eggs and
meat are missing more
than
half of thevarieties
genetic diversity
found in native
Many
different
of soy,
chickens, possibly although
increasing awe
vulnerability
to new
only grow
onediseases and raising
serious questions about the sustainability of the poultry industry.

Without diversity, organism is vulnerable to disease
and at risk of extinction
Passenger
Pigeon
N. America
1914
Dodo
Mauritania, 1681
Golden Toad Costa Rica 1989
Hardy-Weinberg Principle


To see if evolution is occurring, there must be a change
in allele frequency
An equation called the Hardy-Weinberg equation is a
mathematical way to determine if change is happening
 p2
+ 2pq +q2 = 1.0
p
= numbers of dominant alleles
 q = numbers of recessive alleles
 As long as equation = 1.0, no evolution
 If equation  1.0, change is happening = evolution
H-W Sample Problem

Ex: Within a population of butterflies, the color
brown (B) is dominant over the color white (b). And,
40% of all butterflies are white. Given this simple
information calculate the following:
 A.
percentage of butterflies in the population that are
heterozygous (Bb).
 B. frequency of homozygous dominant individuals (BB).
 Remember,
p = B, q = b
 What is 2pq?
 What is p2?
 Equation:
 White
p2 +2pq +q2 = 1.0; p + q = 1.0
(recessive) = bb = 40%
 then bb = q2 = 0.4
 q = 0.4 = 0.632 (i.e. 0.632 x 0.632 = 0.4)
 q = 0.63
 Since p + q = 1.0, p = 1.0 - 0.63 = 0.37
 p = 0.37
 A. heterozygous?
 2pq = 2 [(0.37)(0.63)] =
 B. homozygous dominant?
 BB = p2 or (0.37)2 =
D. Evidence for Evolution


Direct evidence for evolution (via drug-resistant bacteria
& different plant species) makes it easy to see evolution
exists in lower-order species
Evidence for evolution in the higher-order species
(animals & humans) is more indirect:
1.
2.
3.
4.
Fossils: bone structures are similar but act differently
Anatomy: looking at bones of different species tells us
certain bones do identical things
Embryology: species that are way different as adults
(humans vs. chickens) look identical when embryos
Biochemistry: DNA or RNA differences between species is
good comparison (chimps & humans 98% identical)
1. Fossils

Important pieces of evidence for evolution
because it provides record of early life and
evolutionary history
 Paleontologists gathered evidence of whale’s
ancestors
and believe they were landWhale still has
dwelling,
dog-like animals
mammalian
HIP BONES!
2. Anatomy

Can compare two or more different species and see
they have structural features in common which are
called homologous structures
3. Embryology

Evidence of same structure in embryonic stage tells us
that all have common, albeit distant, ancestor
 Embryo: earliest stage of growth and development
of both plants and animals
 All fish, reptile, bird, and mammal embryos have
tails and gill slits (no air in uterus)
4. Biochemistry

Looking at DNA and RNA of individuals
and species gives us biochemical
evidence for evolution
look at sequence of mitochondrial DNA
and RNA to see who’s related to whom
End!
Quiz on this section
Monday, April 8