The Evolution of Populations
I Introduction
A. recall that evolution is a main theme of biology
-recall the Intro Lecture
B. individual organisms DO NOT evolve
1. natural selection acts on individuals
a. each individual organism either survives
and reproduces or it does not
survive and reproduce
b. each individual organism in a
population is a survival experiment
-each individual organism may or
may not have an advantage or
a disadvantage that improves or
hinders its CHANCE of survival and
reproduction
C. populations of individuals DO evolve
1. natural selection, chance, mutation etc
act on individuals in a population
2. relative numbers of individuals with
certain genes changes with time
-change in rel. numbers due to
natural selection, chance, mutation, etc
3. evolution = the change in the
genetic makeup of a population over time
- this is a more generic definition
4. evolution = the change in the allele
frequencies of a population over time
-this is a more specific definition
II Introduction to population genetics
A. recall the Intro Lecture
and Darwin's two key observations
B. "members of a population VARY in form and
behavior."
1. existing variation in populations is the "raw material"
of evolution and natural selection
2. no pre-existing genetic variation => no natural
selection & no evolution
a. Consider:
(1) California Condor
(a) several years ago there were 36 individuals
(all in zoos)
(b) all were closely related
(c) How much genetic variability is left?
(d) What are the chances that this
species can undergo natural selection
and evolution in the future?
(2) the Hawaiin Puaiohi
(a) 1995 were only about 200 individuals in the world
(b) most live on one mountain in a rainforest
(c) rest live in a captive breeding program
(d) essentially no genetic variation exists
(e) What are the chances that this
species undergo natural selection
and evolution if the environement changes?
C. Genetic variation
1. where does genetic variation between
individuals come from?
a. independent assortment of homologous
chromosomes in anaphase I
-homologous pairs of chromosomes (one from each parent)
separate
-get new COMBINATIONS of existing alleles
-alleles from one parent mixed with alleles from other parent
-many phenotypes (intelligence, creativity,
athletic ability, etc) are due to combinations
of many alleles
-What happens when all individuals
in a population are very closely related?
-less chance of producing genetic
variation during independent assortment
b. crossing over during prophase I
-new COMBINATIONS of existing alleles again
-see above
-What happens when all individuals
in a population are very closely related?
-less chance of producing genetic
variation during crossing over
-occasionally crossing over occurs
within a gene
-can produce new alleles
(new DNA sequences that didn't
previously exist!)
-So, crossing over CAN produce NEW alleles
-new alleles may or may not be advantageous
c. gene mutation
-changes in DNA sequence of
one or more alleles
-NEW alleles
-BUT: Most, but not all, mutations make
the organism less fit
d. chromosomal mutation
-split one chromosome and make two
-new COMBINATIONS of existing alleles
-get reproductive isolation from very closely
related species
-eg: humans and chimpanzees
closely related, BUT
-chimps have 48 chromosomes
-humans have 46 chromosomes
-different chromosome number
produces reproductive isolation
e. errors in DNA replication
-a mechanism for producing mutation
-get changes in DNA sequence
-NEW alleles
f. viral infection
-virus infects cells
-viral DNA synthesized in cytoplasm
-are mechanisms for moving cytoplasmic
DNA into nucleus and incorporating into
chromosome
-NET EFFECT: insert NEW alleles into
chromosome and thus, into populations
-eg: swine flu (pig -> human)
-eg: HIV (green monkey -> human)
-eg: SARS (civet -> human)
-eg: herpes virus
-new alleles
-possible medical applications for viral infections
-use to cure cystic fibrosis
-use to cure juvenile diabetes
-use to cure genetic disorders
g. bacterial infection
-some bacteria enter cytoplasm of cells
and start to control nucleus of host cells
with bacterial DNA
-eg: agrobacterium causes
crown galls in many dicot plants
-agrobacterium inserts NEW alleles into
host plant's chromosome
-agrobacterium widely used in genetic
engineering of plants
h. genetic engineering
-insert new alleles into organism
-NEW alleles
-RoundUpReady corn and soybeans
-Bt corn
-FlavorSaver tomatos
-Golden Rice
-inserting NEW alleles in organisms has become
one of the most powerful scientific tools
in most branches of biology
2. genetic variation within a population
a. population = a group of individuals of the same
species occupying a given area at a given time
b. each individual has many genetic "traits"
(1) height
(2) weight
(3) skin color
(4) enzyme A levels
(5) enzyme B levels
(6) metabolic control mechanisms
(7) 10,000's more
c. each "trait" is an expression of at least one, but
probably many proteins (and thus, genes!)
d. when a given trait is measured in all individuals
within a population, one observes a range of
variation for the trait
(1) consider the # of melanin molecules (a
brown protein) per cell of the epidermis
(a) melanin absorbs uv radiation so that uv light
does less damage to skin cell genes
(b) melanin helps prevent UV from
breaking down folic acid
-folic acid is critical in embryo development
(c) BUT, melanin tends to block light
required for vitamin D synthesis
(2) # of melanin molecules per cell varies
greatly in humans
(a) # of melanin molecules in cells
is controlled by genes
(3) amount of light varies with latitude and weather
(4) skin color can be a selective
advantage or disadvantage
3. natural selection acts on the variation
a. people use fluorocarbon propelants
b. fluorocarbons build up in upper
atmosphere and destroy ozone layer
c. ozone normally absorbs almost all uv
radiation coming from sun
d. Thus, amount uv radiation greatly
increases in absence of ozone
NOTE: this is an environmental change,
the high uv environment IS NOT the
environment to which we are adapted
e. Which population, those with more melanin
per cell or those with less melanin per cell,
will have selective advantage under the new,
high uv conditions?
f. Which population will have greatest
CHANCE of survival (and thus, reproduction)
under the new conditions?
III. Measuring Change in Variation
A. gene pool = all genes of a given population
B. allele = one of two or more alternate forms
of a given gene at a given locus
C. allele pool = all alleles of a given population
D. one allele might be more common or less
common than other alleles in allele pool
E. allele frequencies = the relative abundance
of different alleles
within a population
1. alleles are found at a given locus on a chromosome
2. allele frequencies range from 0 to 1
3. for a given locus, the sum of all allele
frequencies must = 1
1 = fa1 + fa2 + fa3 + fa4 + fa5 + ...........
where fa1 = frequency of allele 1 at locus A
where fa2 = frequency of allele 2 at locus A
and where fa1 = # of allele 1 at locus A divided
by the total # of alleles for locus A in the population
F. Why measure allele frequency?
1. we can measure evolution as it occurs
2. when allele frequencies change,
a population is evolving!
3. evolution IS changing allele frequencies
in a population
4. IF there is no evolution, THEN there can't be
changes in allele frequency
G. Measuring allele frequency in class
1. assumptions:
| Phenotype | Genotype |
| blue eyes | bb |
| dark eyes | Bb |
| dark eyes | BB |
further, assume Bb/BB = 2/1
Spreadsheet for data
**********************************
Class Survey on Eye Color
Data Collected Monday, 29 November, 1999
Blank Table for Use In Class
| Phenotype | Number of Students | Genotype | # of B alleles | # of b alleles |
| blue | 45 | bb | 0 | 90 |
| dark | 23 | Bb | 23 | 23 |
| dark | 8 | BB | 16 | 0 |
| Total Students = 76 | | Total "B" = 39 | Total "b" = 113 |
| Total Alleles = 152 | | | |
Calculating Allele Frequencies
| frequency of allele "b" = | # of "b" alleles
--------------------
total # of "B" + "b" alleles | fb = 113/152 = 0.74OOOOO |
| frequency of allele "B" = | # of "B" alleles
--------------------
total # of "B" + "b" alleles | fB =39/152 = 0.26 OOOOO |
V. Factors that can change allele frequencies
A. mutation = a heritable change in
the DNA base sequence
1. mutation that produces a NEW allele
SUCH THAT the allele can be
inherited
-example of a mutation that cannot be inherited
-example of a mutation that can be inherited
2. as soon as the mutation occurs it is
part of the allele pool and the allele
frequencies all change slightly
2. new allele MIGHT be more adaptive
than original allele
-new allele MIGHT increase the
LIKELIHOOD of producing more offspring
-new allele MIGHT start increasing
in allele frequency
3. MORE LIKELY that the new allele
is less adaptive
B. genetic drift = random fluctuations in allele
frequencies over time due to chance alone
1. critically important in small populations
2. by pure chance, one allele (a) could
disappear in one generation
a. eg: two allele system A & a, two individuals,
both heterozygous (Aa), produce
several offspring, all of which
(by pure chance) are AA
C. gene flow = a change in allele frequency due to:
1. immigration (new individuals enter population)
-eg: consider human gene pool of
North America prior to 1492
-What happened to gene pool as
a result of Europeans coming to North America?
2. emigration (some individuals leave the population)
-Would more adventurous individuals be
more likely to leave Europe to come to North America?
-Would there be a decrease in "adventure seeking" genes in Europe?
D. natural selection = differential survival and
reproduction of genotypes within a population
E. genetic engineering = transfer of alleles from
one population into another population by
recombinant DNA technology
E. relative of importance of factors above for
most species in nature:
natural selection >> gene flow > mutation or genetic drift > genetic engineering
-but, many questions about relative
importance of genetic engineering
End Coverage for Exam 5 and Final Exam for Fall 2009
VI. Types of Natural Selection
A. read "Natural Selection Can Follow........" in
Brooker et al.
B. stabilizing selection
1. decreases the frequency of alleles which
give rise to extreme forms of the trait
-see figure 24-4 in Brooker et al.
2. intermediate forms (already well adapted)
are favored
3. eg: human birth weight
a. see figure
b. stabilizing selection favors individuals with
birth weights around 8 lbs
-What happens when babies are very small?
-What happens when babies are very large?
c. Thought Question: Using modern medical
techniques it is now possible to save
babies at both extremes. What will happen
to the shape of the mortality curve? Is this
good? What would happen if humans
had suddenly go without modern medical
technology?
C. directional selection
1. moves frequency of distribution of alleles
in a steady, consistent direction
-see figure 24-3 in Brooker et al.
2. phenotypic character of the population shifts
as a whole, either as a response to changing
environment or to a new environment
3. example: peppered moth in England
(see Lab on Population Genetics in the Symbiosis Lab Manual)
4. cuckoos and magpies in Spain
-cuckoos and cowbirds lay their eggs
in nests of other birds (eg: magpies)
-other birds raise the young cuckoos
-young cuckoos push biological offspring
out of nest
-reproductive success of parasitized
magpies is greatly reduced
-But, some magpies eject eggs that
are not just right
-helps if parasitized
-but reduces reproductive success
slightly if dirty eggs are ejected
-scientists study % magpies that
eject eggs in two areas
- area A: magpies and cuckoos has
both been present for 100s of years
-78% of magpies eject cuckoo eggs
- area B: cuckoos were introduced in
the 1960's
-only 14% of magpies eject cuckoo eggs
-scientists watched the evolution of magpie
ejection of cuckoo eggs in Japan where
populations have only recently overlapped
-Thought Questions:
-Is behavior influenced by genes?
-Which of your behaviors are
influenced by your genes?
-Nature, not nuture
-Which of your behaviors are not
influenced by your genes?
-Nuture, not nature
-How would you test your hypothesis?
-One way
D. disruptive = diversifying selection
1. increases the frequency of two or more alleles
that give rise to extreme forms of a trait
-see figure 24-5 in Brooker et al.
2. intermediate forms are selected against
3. eg: African Swallowtail butterflies
a. females have one of three different
morphologies
(1) one extreme--females mimic a species
of butterfly that is unpalatable to birds
-the mimics are not eaten
-mimics would be palatable to birds,
but are protected by their
resemblance to unpalatable species
(2) other females look like males, bright
yellow swallowtails (easy to see and
palatable to birds)
(3) another extreme--females mimic a
completely different species of butterfly
that isn't palatable to birds
-again, the mimics are not eaten
-mimics would be palatable to birds,
but are protected by their
resemblance to unpalatable species
b. birds eat more of the yellow form =>
reduced CHANCE of reproduction
for the yellow form relative to the two
mimic forms
c. BUT, ALL females ONLY MATE with bright yellow males,
so it is to the males advantage to remain
brightly colored (but very vulnerable)
d. since females of all morphs keep choosing
yellow males, interbreeding continues and
the population remains a single species
e. What MIGHT happen if the populations
got isolated? (Hint: see next chapter)
VII Natural selection cannot produce perfect
organisms
A organisms are bound by historical constraints
1. organisms always change from
what they were
2. organisms don't start over with
all new alleles
3. organisms change as new alleles or
new combinations of alleles appear,
have advantage and are selected for
4. eg: common back problems in humans
are due to fact that humans are not yet
optimized for walking upright, our anatomy
is still related to our four legged ancestors
B adaptations are often compromises
1. seals have flippers that are great in water
but barely adequate on land
-need to come to land to give birth
-limits how far evolution of flipper can go
2. in becoming fully marine, whales have
superb flippers that are totally useless
on land
3. humans have highly flexible and articulate
limbs that provide great agility at the
expense of many strained ligaments
and tendons
C not all evolution is adaptive
1. chance plays a very significant role
a. eg: strong storm blows insects to
island where they didn't live before
b. new small population begins
c. new population not necessarily
well adapted to island life
(1) bottleneck effect in small
population MIGHT lead to loss
of some adaptive alleles
(2) founder effect MIGHT produce
a population with very
different allele pool
2. thought question: Is modern medicine
and technology in general influencing the
allele frequencies in the human population?
Is this change making the human population
better adapted to the current environment?
Is the allele pool larger or smaller as a result
of medicine? Is this good or bad?
D selection (natural or artificial) can only
edit existing genetic variation
1. just because an allele WOULD BE
adaptive to organism doesn't mean
that this allele WILL EVER BE part of that
organisms gene pool
a. if cats could see infrared radiation,
then they could use this sense to
better hunt mice in areas with
no visible light
b. BUT this would require
new alleles which MIGHT NEVER
become available
2. changes in alleles happen by CHANCE
3. changes MAY or MAY NOT occur
4. BUT, humans can now.......
++++++++++++
Identical twins raised apart
See website of leading researcher at UofM.
