Download Lecture 15

How Populations Evolve and
Introduction to Population
Genetics
05-23-2016
Chapter 19 sections 1, 2, and 3
Learning Goals for Today:
• Explain the connection between
populations, genes, and evolution
• Predict how populations may change over
time
How Are Populations, Genes, and
Evolution Related?
• Evolutionary change occurs over time to
populations
• A population is a group that includes all
members of a species living in a given area
• Individuals live/die, reproduce/not
…depending on their traits
Population genetics - concepts
• A gene is a segment of DNA located at a
particular place on a chromosome
• Different members of a species may have slightly
different nucleotide sequences for the same
gene, these are called alleles
– Different alleles code for slightly different versions of
the same protein
Population genetics - concepts
• In a population, there are usually two or more
alleles of each gene
• If the population is diploid,
– an individual with the same two alleles of a gene is
homozygous
– an individual with two different alleles of a gene is
heterozygous
Genes and the environment
interact to determine traits
• Example: human height
– Genes
– Childhood nutrition
– Growth hormones
Example of Tracking Alleles in a
Population
• Hamster coat color
• The dominant allele encodes for an enzyme
that catalyzes black pigment formation (B)
• The recessive allele encodes for an enzyme
that catalyzes brown pigment (b)
• How many alleles will 1 hamster have?
• What color is a Bb hamster?
Alleles, Genotype, and Phenotype in
Individuals
Coat-color allele B is
dominant, so heterozygous
hamsters have black coats
Each chromosome
has one allele of the
coat-color gene
phenotype
genotype
BB
B
chromosomes
Bb
B
B
bb
b
b
b
Gene Pools
• Sum of the genes in a population
– Population genetics deals with the
frequency, distribution, and inheritance of
alleles in populations
– A gene pool consists of all the alleles of all
the genes in all individuals of a population
Gene Pools
• The number of copies of each allele in a
gene pool is equal to:
1. The number of heterozygous individuals that
carry one copy of the allele
2. Twice the number of homozygous
individuals for that allele since they carry
two copies of the allele
Gene Pools
• Sum of the genes in a population
– If we counted all of the alleles present for one
gene in a population we could determine
allele frequency for each different allele
– Allele frequency the proportion of one
allele relative to all the alleles of that gene
How Are Populations, Genes, and
Evolution Related?
• Determining allele frequency
• A population of 25 hamsters contains 50 alleles of
the coat color gene
• If 20 of those 50 alleles code for black coats (are
B), then the frequency of the black allele (B) is
• 20/50 = 0.4 = 40%
• What is the frequency of the b allele?
A Gene Pool
Population: 25 individuals
Gene pool: 50 alleles
The gene pool for the
coat-color gene contains
20 copies of allele B and
30 copies for allele b
BBBBBBBB
BB
BB
BB
BB
BBBB b b b b
Bb
Bb
Bb
Bb
BBBB b b b b
Bb
Bb
Bb
Bb
BBBB b b b b
Bb
Bb
Bb
Bb
bbbbbbbb
bb
bb
bb
bb
bbbbbbbb
bb
bb
bb
bb
bb
bb
Fig. 15-2
Example: calculating allele
frequencies
Population of 50 hamsters (25 BB, 25 bb)
Frequency of B allele? Of b allele?
Population of 50 hamsters (all Bb)
Frequency of B allele? Of b allele?
What do the allele frequencies always add up
to?
The Big Picture
• Evolution is the change of allele
frequencies within a population
– Frequencies change (over generations) =
evolution
– Frequencies stable (over generations) = no
evolution
How can we tell if a population is evolving?
• The Hardy-Weinberg principle: a mathematical
model for population biology
• Evolution: change in allele frequencies
• Mathematician Godfrey H. Hardy and physician
Wilhelm Weinberg came up with a way to track
allele frequencies
• No change = no evolution
– Said to be in “equilibrium”
Conditions required to have no allele
change (no evolution occurring)
Conditions
1. No mutations must occur in the population
2. There must be no gene flow between
populations
•
No movement of alleles in or out of the population
•
All genotypes have equal reproductive success
3. The population must be very large
4. Mating must be completely random
5. There can be no selection
Under these conditions, allele frequencies in a
population will remain the same indefinitely
Hardy-Weinberg Principle
•
Without evolution, alleles in a population will
quickly reach equilibrium
•
Once the frequency of alleles in a population is
known, we can use the Hardy-Weinberg equation
•
p = frequency of the dominant allele in the population
•
q = frequency of the recessive allele in the population
Where does it come from?
• Assume that 2 heterozygous individuals
produce offspring
p
q
p
pp
(p2)
pq
q
pq
qq
(q2)
What does it mean?
•
We can calculate what proportion of individuals in
the next generation will have a given genotype and
phenotype
•
You will do this in Lab 9
Example
• Two alleles exist for color in a certain type of
beetle. Red (R) is dominant to blue (r).
• In a specific population of beetles, 51% are red
and 49% are blue.
• Assuming the population is in H-W equilibrium,
what are the frequencies of the red and blue
alleles in the gene pool?
Example
= RR and Rr
p2 and pq
= rr only
q2 only
q2 = 0.49
q = 0.7
p+q=1
1 – 0.7 = p
p = 0.3
Example
• Cystic fibrosis is an autosomal recessive disorder
that affects 1/2500 (0.0004) Caucasians.
• Assuming the population is in H-W equilibrium,
what percentage of Caucasians are carriers?
Example
= CC and Cc
p2 and pq
= cc only
q2 only
q2 = 0.0004
q = 0.02
p+q=1
1 – 0.02 = p
p = 0.98
Example
= CC and Cc
p2 and pq
= cc only
q2 only
Frequency of heterozygotes = 2pq
2(0.98)(0.02) = 2pq
0.0392 or 3.92%
Check Your Work!
p2 + 2pq + q2 = 1
p = 0.98
q = 0.02
(0.98)2 + 2(0.98)(0.02) + (0.02)2 = ?
1
Practice calculating allele
frequencies from phenotypes
Population of 100 hamsters
Year 1: 25 brown, 75 black
Year 2: 36 brown 64 black
Did the allele frequencies change?
Evolutionary forces
•
•
•
•
Mutations
Gene flow
Genetic drift
Non-random mating
Mutations
• Changes in DNA sequence
– Usually occur during DNA replication
– Rare
• A new mutation will appear in an allele
– 1 of every 100,000 human gametes
– 1 of every 50,000 human babies
– Causes very small changes in the frequency of any allele
• But we have
– 20 to 25,000 genes
– 50,000 alleles
– Most newborns have one or two mutations
– New alleles  new variation  evolution
Mutations
• Changes in DNA sequence
– Usually occur during DNA replication
– Rare
– Source of new alleles
– Spontaneous
– Can be neutral, harmful, beneficial
– Provides a potential for evolutionary change
Mutations
• Changes in DNA sequence
– Usually occur during DNA replication
– Rare
– Source of new alleles
– Spontaneous
– Can be neutral, harmful, beneficial
– Can be inherited (IF gametes carry the mutation)
Gene Flow
• Movement of alleles from one population
to another
– Individuals move around
– Gametes (like pollen) can also move
– Baboons live in troops,
juvenile males move from
troop to troop to rise in social
status
– Changes the alleles of the
destination population to be
more like the source
population
Fig. 15-4
Gene Flow
•
Examples:
•
A migrating bird changes flocks
•
Grolar bears (gene flow between species)
Genetic Drift
• Random change in allele frequencies over time brought
about by chance alone
– Only makes a difference in small populations
• Examples of bad luck:
– Seeds can fall into a pond or parking lot and never
sprout
– Animals can be killed by floods, fires, volcanic
eruptions
Genetic Drift
• Imagine you have a population of 20 hamsters
• B = 0.5 and b = 0.5
• If all animals are allowed to breed randomly and yield 20
new hamsters, the frequencies would not change
• But if you chose two randomly and let only them breed
and yield 20 new hamsters, allele frequencies would
change dramatically
– Two causes that lead to:
• Population bottleneck
• Founder effect
Alleles are more
likely to disappear
due to random
chance in small
populations
Genetic Drift by Population Bottlenecks
Natural catastrophes,
over-hunting
Some examples:
Elephant seals
Cheetahs
California condors
Limited genetic variation
leaves populations
vulnerable to extinction
Fig. 15-7
IMPORTANT!!
•
A bottleneck event is random
•
A plague that kills off individuals lacking a
particular allele is natural selection
•
Natural selection kills individuals due to genetic
make-up
•
Bottlenecking kills indiscriminately
Genetic Drift by the Founder Effect
• Small number of individuals leave a large
population and establish a new isolated population
• By chance, not all alleles are present in this new
smaller population
http://beacon-center.org/blog/2012/01/03/beaconresearchers-at-work-how-the-cricket-lost-its-song/
Genetic Drift by the Founder Effect
•
Example:
• All Amish in Lancaster, PA
descended from ~200
mostly Swiss individuals
that migrated in 1744
• Ellis-Van Creveld
syndrome is common
among old order Amish
•
•
Amish: 1 or 2 in 200
General population: 1 in
1000
Non-Random Mating
• Mating within a population is almost never random
– Lack of mobility
• Many organisms inbreed (self-fertility is common in plants)
– Can increase frequency of homozygous recessive individuals
• Mates have preferences
• Snow geese mate with partners of
the same color
– Assortative mating
• In large population, neither
inbreeding nor assortative mating
will alter allele frequencies
Fig. 15-9
Evolutionary forces
– Mutation
– Gene flow (between populations)
– Small population size (genetic drift)
– Non-random mating
– Selection occurs
Natural selection
• When an allele provides “some little
superiority”, the individuals with that allele
are favored by natural selection
• Natural selection favors traits that increase
an individual’s survival only to the extent that
the individual’s survival leads to
improved reproduction
• Reproductive success determines the fate
of the alleles
Natural Selection and Antibiotic
Resistance
• Penicillin: antibiotic, widely-used starting
around WWII
• Killed most bacteria
• Some individual bacteria were naturally
resistant (rare resistance allele)
Before penicillin
treatment
Penicillin
treatment
After penicillin
treatment
Natural Selection and Antibiotic
Resistance
• Penicillin killed most bacteria
• Some individual bacteria were naturally resistant
(rare resistance allele)
• Natural selection:
– Did not cause genetic changes in individuals
– Acted on individuals (killed them)
– Caused the population to evolve/change allele
frequency
• Evolution by natural selection is not progressive;
it does not make organisms “better”
Patterns in evolutionary change
• Directional selection
• Stabilizing selection
• Disruptive selection
How selection can alter population
structure
Directional
Stabilizing
Disruptive
Directional selection
• Occurs when natural selection favors one
extreme of continuous variation
• Over time, the favored extreme will
become more common and the other
extreme will be less common or lost
Dark-peppered moths were
prevalent in the early 19th century
Directional Selection
•
Historical case in England: peppered moth
Pre-Industrial
revolution
Post Industrial
revolution
Stabilizing selection
• Removes individuals from both ends of a
phenotypic distribution, thus maintaining
the same distribution mean
• Occurs when natural selection favors the
intermediate states of continuous
variation
– Common when there are opposing forces
acting on a trait
Goldilocks
Human Birth Weights: an Example
of Stabilizing Selection
Disruptive or diversifying selection
• Removes individuals from the center of a
phenotypic distribution and thus causes
the distribution to become bimodal
• Occurs when natural selection favors both
extremes of continuous variation
• Disruptive selection can lead to two
new species
Balanced polymorphism
• Often occurs when environmental conditions favor
heterozygotes
• Normal and sickle-cell hemoglobin alleles coexist in
malaria-prone regions of Africa
Types of selection
• Natural
– Kin selection
– Sexual selection
– Frequency-dependent selection
• Artificial
Kin selection
• A type of selection that involves altruistic
behavior, e.g., the protection of offspring
• Occurs when natural selection favors a
trait that benefits related members of a
group
Kin selection
• Worker bees exhibit altruistic behavior
• In terms of simple fitness, the worker bee
does not reproduce
• However, all of the bees in the hive are close
relatives, a worker bee's genes will be passed
to the next generation indirectly
Young bee-eaters remain with parents
when breeding opportunities are low
• Habitat is saturated or no more suitable
nesting sites
• More beneficial to assist family with
siblings
Sexual Selection
•
In many species, males and females look very
different
•
Sexual dimorphism
Sexual selection
• A type of selection in which the forces
determined by mate choice act to cause
one genotype to mate more frequently
than another genotype
Sexual selection in Long-tailed
widowbird
• The evolutionary fitness of an organism not only
depends upon its ability to survive but also its ability
to reproduce
• To reproduce, an individual must obtain a mate and
produce viable offspring
• Natural selection favors traits that maximize the
ability of an individual to compete for and attract
mates, and/or the ability to produce offspring
Sexual Selection
•
Some of these differences give a competitive
advantage when competing with each other for
mates
•
Intrasexual selection
Sexual Selection
•
Some of these differences give a competitive
advantage when attempting to attract mates
•
Intersexual selection
Sexual Selection: one
explanation for traits that appear
disadvantageous
Fig. 14-14
Frequency-dependent selection
• Individuals with either the common (positive
frequency-dependent selection) or the rare (negative
frequency-dependent selection) phenotype are selected
for
• Negative frequency-dependent selection serves to
increase the population’s genetic variance
• Positive frequency-dependent selection usually decreases
genetic variance
Lizards in the PNW show negative
frequency-dependent selection
• The common side-blotched lizard
• Alternate male strategies leads to
frequency-dependent selection
Artificial Selection
People have bred many varieties of animals and
plants
Selective breeding of organisms by humans is
artificial selection
These organisms have been changed to look very
different from the original parent stock
Types of Selection:
• Natural
• Occur in nature (no
human intervention)
– Directional selection
– Stabilizing selection
– Disruptive selection
• Artificial
• Organisms bred by
humans for specific
traits
– Directional
– Stabilizing
– Disruptive
Crop breeding: artificial selection
in action
Teosinte: wild ancestor
of corn
What traits were selected for
in the selective breeding of
corn?
http://the10000yearexplosion.com/pictur
es/evolutionary-business/1720176
Brassica oleracea: many different
forms from one ancestor
A cultivar is a plant or grouping of plants selected for desirable
characteristics that can be maintained by sexual or vegetative
propagation.
Varieties of Canis familiaris
Ancestor
to all dogs
Canis lupus
http://www.blueberrybasket.com/catalog/home/home_KDOG.htm
Domestic cat breeds: recent
example of artificial selection
Closest relative
http://en.wikipedia.org/wiki/List_of_cat_b
What happens when artificially
selected organisms return to
nature?
Crop and animal breeding
• Genetic variance is the diversity of alleles and genotypes within a
population
• Heritability is the fraction of phenotype variation that can be
attributed to genetic differences, or genetic variance, among
individuals in a population
• Breeders attempt to increase a population’s genetic variance to
preserve as much of the phenotypic diversity as possible and to
reduce inbreeding
• Inbreeding depression is the reduced biological fitness in a given
population as a result of inbreeding
Upcoming lab:
• Lab 9, population genetics
• Hardy-Weinberg
• Equation for calculating allele frequencies
from phenotypes