Transcript Genes and Their Evolution: Population Genetics

Genes and Their
Evolution: Population
Genetics
Chapter 4
Population
 A population is a group of individuals of the same
species who share a geographic area and usually mate
within the group
 The total genetic variation of that population is the
gene pool
 The number of times different genes or alleles occur is
the frequency
 And evolution is change in allele frequency over
generations
Species
 We only look at reproductive populations of
organisms. This is important because to be called a
species, organisms must be able to mate and have
fertile offspring
 Are these different species?
 Lion and tiger
 Horse and donkey
 Dog and wolf
Species
 When reproductive isolation occurs, this means that
two populations are kept from mating
 If enough time passes, these two groups will become
two different species
 This is allopatric speciation
 Example: two groups of beetles get separated by a
river. Over time, enough differences arise that they
become different species and would not be able to
mate again
Population Genetics
 This is the study of changes in genetic material
 More specifically, the change in allele frequency
 allele= different versions of genes
 Frequency= how often they occur
 Microevolution: small-scale; happens in a short
period of time
 Macroevolution: large-scale; occurs over many
generations; speciation
Different Views of
Evolution
 Darwin thought evolution was small changes
accumulating over long periods of time
 This is phyletic gradualism
 Gould and Eldredge said evolution could have long
periods of no or minor change, interrupted by sudden
change, such as speciation or extinction
 This is punctuated equilibrium
The Two Views
More Types of Evolution
 We know evolution shows common ancestry
 When two related species share phenotypic traits
because of common ancestry, this is parallel
evolution
 All primates have eyes close together and 5 fingers
 When distantly related species develop similar
adaptations to similar environments, this is convergent
evolution
 Crocodyles and cats have tails because they walk on 4
legs
 Chickens, bees, and bats all have wings to fly
Parallel Evolution
Convergent Evolution
Population Genetics
 We focus on the idea of change over time, especially in
the frequency of alleles
 Example: we are looking at a trait, which we will call
R
 The two alleles are R and r (dominant and recessive)
 Generation 1 has 50% R and 50% r
 When we come back and look at Generation 2, the
frequency has changed to 40% R and 60% r
 This shows evolution
Population Genetics
 We need evidence that evolution is occurring, and we
do this by looking at the frequencies of alleles in
populations
 If they do not change, there is no evolution
 If they do change, there is evolution
Intro to Hardy-Weinberg
 If the frequency never changed, the population would
be in equilibrium
 So, there is an equation to test for equilibrium
 If the numbers don’t change = equilibrium = no
evolution
 If they do change = no equilibrium = evolution
Hardy-Weinberg
 Godfrey Hardy and Wilhelm Weinberg developed a way to
test for equilibrium in allele frequency
 In order for equilibrium to exist, you must have:

No mutations

No natural selection

Completely random mating

An infinitely large population

Each organism having the exact same number of offspring
 Would this ever occur in nature?
Hardy-Weinberg
 Don’t let the equation intimidate you!
 Focus on what each part stands for and follow the
steps
 There will only be a couple of questions of this on the
midterm
Hardy-Weinberg
 Here is the equation:
p2 + 2pq + q2 = 1.00 (100% of population)
p2 = all individuals who are homozygous dominant
q2 = all individuals who are homozygous recessive
2pq = all individuals who are heterozygous
Also important: p + q = 1.00
p = the dominant allele
q = the recessive allele
Example
 A population of snails has a trait for either spotted or
striped shells. Striped shells are dominant. Out of 100
snails, 16 have spotted shells.
 Find the frequencies of SS, Ss, and ss
 The next year we return and find that 25 out of 100
have spotted shells. Why?
Hardy-Weinberg
 Please use the document titled “HW Explanation” on
BlackBoard to see details and how to solve a problem
Sources of Evolution
 So, evolution is change over time, but how do these
changes arise?
 4 sources:
 Mutation
 Natural Selection
 Gene Flow
 Genetic Drift
Mutation
 Small errors in DNA, especially during replication
 Most go unnoticed. Some can be harmful and some
beneficial
 They occur at random
 They are the only source of new genetic variation in a
population
Mutations
 Point mutation: a single base is changed

ATCGGTC  ATCGGTA
 Frameshift mutation: caused by a deletion or insertion of
genetic information; causes codons to be read incorrectly
 Parts of chromosomes can be mutated, or entire
chromosomes can be mutated

Kleinfelter’s Syndrom: males have extra sex chromosome:
XXY

Trisomy 21: person has extra 21st chromosome. AKA Down
Syndrome
Mutation Example
 A point mutation changes the hemoglobin blood cell in some
people
 This causes sickle-cell anemia. Their blood cells are deformed
and cannot carry enough oxygen
 It is often fatal
 Why do so many sub-Saharan Africans have this mutation?
 Because it protects against malaria, the #1 killer in Africa
 If a person is heterozygous for the sickle-cell trait, he/she will
have enough normal blood to carry oxygen and enough affected
blood to kill malaria parasites that enter body
 Watch: http://www.youtube.com/watch?v=1fN7rOwDyMQ
Malaria and Sickle-Cell
 The malaria/sickle-cell relationship is a balanced
polymorphism
 The heterozygous trait balances one negative trait with
another, giving the person a better chance of surviving
 This is selected for, and is also an example of natural
selection
 Pp. 317-320
Natural Selection
 Survival of those best adapted to current
environmental pressures
 Based on the idea of fitness: number of offspring
produced in a lifetime
 Has nothing to do with strength, speed, or intelligence
 Is just how good are you at surviving and making
babies
Natural Selection
 Read the box on p. 104
Who has better fitness?
Male, Harvard degree,
$500,000 salary, no kids
Male, high school drop
out, unemployed, 7 kids
Types of Natural
Selection
 Directional: selection
shifts in one direction
 Example: large beak
sizes in finches when
droughts leave only
hard food to eat
Types of Natural
Selection
 Stabilizing: selection
favors the average and is
against the extremes
 Example: birth
weight. Babies in the
normal range survive
more than premature
ones or obese ones
Types of Natural
Selection
 Disruptive: selection
favors the extremes, and
against the average.
Leads to speciation
 Example: beetles are
reproductively isolated
until 2 new species are
created.
Example of Natural
Selection
 In Great Britain, most moths were light colored to
blend in to the environment
 With the Industrial Revolution in the 1800s, the smog
and soot produced changed the environment to be
darker
 Did the light moths have the advantage still?
 No, they were eaten and darker moths survived. This
changed allele frequencies
Practical Example #2
(from lecture 2)
 Can you see viruses or bacteria?
 They are alive and, like everything else, they evolve
 Can evolve in a matter of hours
 This is why not taking antibiotics/medication correctly
leads to drug resistance
When people do not take medication correctly
Yellow = weak viruses
Purple = medium viruses
Red = strong viruses
When people do not take medication correctly
Yellow = weak viruses
Purple = medium viruses
Red = strong viruses
Genetic Drift
 This is an over-representation or an under-
representation of traits because of a small sample size
 Example: In a class of 25 people, I find that 20 have
Type B blood, 3 have Type O and 2 have Type AB
 Does this accurately reflect the frequency of blood
types in the entire human population?
 What about Type A?
Genetic Drift
 This shows why large populations are healthier…there
is more variation
 Endogamous groups only breed within their
population
 Exogamous groups breed with members outside their
population
 Which is better for variation and health?
2 Types of Genetic Drift
 1. Founder Effect: a small group breaks off from the
original population and forms its own group

Will that small group accurately reflect all the variation of
the original population?

Huntington’s Chorea and Tay Sachs: genetic defects that are
exaggerated due to founder effect and genetic drift
 2. Population Bottleneck: when a population is reduced
drastically, there is not enough variation to keep it going

Can cause extinction

This is what happens to endangered species
Genetic Drift: Founder
Effect
 A small group of original population creates new
population
 Some traits will be over-represented
 Some traits will be lost
Genetic Drift: Bottleneck
 Severe reduction in population
 Loss of variation
Gene Flow
 Movement of genes and mixture of them through
breeding
 Not only migration: have to mate as well, in order to
add variation
 So it is migration and nonrandom mating
 If there is no gene flow between 2 populations, they
could evolve into 2 different species
 With global travel and more open-mindedness in
cultural ideals, our human gene pool has had a large
increase in gene flow and variation
Gene Flow
 Variation is the key to success!
 Why is inbreeding so bad?
 It limits variation in the gene pool and can increase
harmful mutations
Discussion
 Read pp.113-115, including box on p. 114
 Why is biodiversity important?
 Why should we worry about our actions as humans?
 Why should we participate in conservation?
Review Questions
 What is the difference between microevolution and
macroevolution?
 How does the Hardy-Weinberg equation show that
evolution occurs?
 What are the sources of evolution? What are some
examples?