Lecture 8: Stabilizing Selection

Question 1

DEF: is any change in allele frequencies
in a population due to chance (known as
SAMPLING ERROR)

• It causes allele frequencies to drift up and down
  randomly over time

Answer 1

Genetic drift

Question 2

Drift occurs in every population, in every
generation
– It is especially prevalent in ____ populations

Answer 2

Small

Question 3

Flipping a coin can simulate which egg and which
sperm combine to produce offspring from two
heterozygous parents
* Can show change in allele frequency in population
over time
– Illustrates the random factor in mating
▪ Some alleles can be “___” and others can be “____”

Answer 3

Lucky, Unlucky

Question 4

A computer can simulate random combination of
gametes over time

• These simulations show that:
  – Genetic drift has a much larger effect on ____
  populations than on ____ populations
  – Over time, one allele will drift to fixation or be lost
• In some cases, drift may still be an important
  factor in large populations

Answer 4

Small, large

Question 5

Genetic drift is random with respect to f____
– Changes in allele frequency are not adaptive

• Genetic drift is most pronounced in s___ populations
  – This is a big concern for conservation biologists
• Over time, genetic drift can lead to the random
  l___ or f_____ of alleles
  – Results in a d____ in genetic variation in population

Answer 5

Fitness

Small

Loss, Fixation

Question 6

G_____ M____ are specific alleles that cause a
distinctive phenotype

Answer 6

Genetic Markers

Question 7

Kerr and Wright’s research on genetic drift in small
populations of fruit flies used a genetic marker for
morphology of leg bristles. This sex-linked gene
has two alleles:

1. Normal, or straight
   ▪ Dominant (AN)
2. “Forked,” or branched
   ▪ Recessive (AF)

T or F

Answer 7

True

Question 8

In the laboratory, genetic drift was found to
– D_____ genetic variation within populations
– I_____ genetic differences between populations

Answer 8

Decrease, Increase

Question 9

Genetic drift can be caused by any event or
process that involves s_____ error

• 2 examples are f_____ effects and
  b____

Answer 9

Sampling

Founder effects, bottlenecks

Question 10

DEF: occurs when a small group of
individuals establishes a new population in a new
area

Answer 10

A founder event

Question 11

A____ frequencies likely differ from the source
population if the new population is small enough

– This change in allele frequencies is called a FOUNDER EFFECT

Answer 11

Allele

Question 12

Founder Effects on the Green Iguanas of
Anguilla:

• common in the c_______ of isolated habitats like islands
• Each time a founder event occurs, a founder effect
  is likely to accompany it, changing allele
  frequencies through genetic drift

Answer 12

Colonization

Question 13

Ex.: Fishermen on a Caribbean island saw a raft
made of downed logs and debris float onto a
beach 2 weeks after two major hurricanes
* Green iguanas were seen on the raft and some on
shore
– None had been seen on this island before
– At least some were breeding two years later
– Thus, a new population had formed
– It was unlikely that the allele frequencies matched those from the source population

T or F

Answer 13

True

Question 14

DEF: A sudden decrease in population size in a large
population

Answer 14

Population bottleneck

Question 15

Population bottlenecks are commonly caused by: (2 points)

Answer 15

1. Disease outbreaks
2. Natural catastrophes

Question 16

Population bottlenecks lead to g______
b_______ —a sudden reduction in the number
of alleles in a population

Answer 16

Genetic bottlenecks

Question 17

Genetic drift occurs during genetic bottlenecks
and causes a change in allele f_______

Answer 17

Frequencies

Question 18

The presence or absence of gene flow has
important implications for conservation of
threatened or endangered species

-The decline of gene flow between isolated wild
populations is well documented
– Isolating events include h_____ f__________

• Gene flow between wild and captive populations is
  also well documented

Answer 18

Habitat Fragmentation

Question 19

What are the effects of accidental or purposeful
gene flow between captive-bred populations
and wild populations?

To answer this question, biologists studied
steelhead trout in the Hood River of Oregon
– Some of the trout are wild
– Some of the trout were raised in a hatchery and
released to supplement the diminishing population

T or F

Answer 19

True

Question 20

DNA tests were conducted to study the f____ (#
surviving adult offspring) of:
– Individuals with two wild parents
– Individuals with one wild and one captive-bred parent
– Individuals with two captive-bred parents

Answer 20

Fitness

Question 21

On average, compared to the fitness of fish with
two wild parents:
– Fish with one captive-bred parent had 16% lower
fitness
– Fish with two captive-bred parents had 38% lower
fitness

T or F

Answer 21

True

Question 22

Gene flow is occurring from captive-bred
to wild populations

In both cases, this r_____ the fitness of the wild
population
– Thus efforts to augment wild populations using captive bred fish may ultimately d_____ population size

Answer 22

Reduce, Decrease

Question 23

Gene flow DOES NOT ALWAYS reduce fitness:
- It can replenish alleles in a population that has lost
alleles due to genetic drift
– This should increase genetic diversity

Gene flow can increase or decrease fitness
depending on the situation
– However, movement of alleles between populations
always tends to reduce their genetic differences

T or F

Answer 23

True

Question 24

Evolutionary mechanisms like natural selection
and genetic drift r_____ genetic diversity over time

Answer 24

Reduce

Question 25

In contrast, m____ restores genetic diversity
– by creating new alleles,
– not only new combinations of alleles

Answer 25

Mutation

Question 26

DEF: change in a single base pair in DNA
- May result in a different amino acid in a polypeptide
- May change the regulation of the expression of other
genes
- Chromosome-level mutations
– Lateral gene transfer (aka horizontal gene transfer)

Answer 26

Point mutation

Question 27

DEF: a change in the number or composition of chromosomes

• Such as gene duplication, which can then diversify with subsequent point mutations and lose their function or create new alleles

Answer 27

Chromosome-level mutations

Question 28

DEF:
- Transfer of genes from one species to another
- Might be a more important source of heritable genetic
variation than previously thought

Answer 28

Horizontal gene transfer

Question 29

mutation is r_____ with respect to the fitness of the
individual

• Mutation does not occur in a way
  that tends to increase or decrease fitness

Answer 29

Random

Question 30

Most organisms are well adapted to their current
habitat – random changes in genes usually result
in products that do not work as well:

– most of them result in D____ alleles—alleles that
lower fitness
– Tend to be eliminated by P_____ selection

Answer 30

Deleterious alleles, Purifying

Question 31

Mutation produces b_____ alleles on rare
occasions
– Increases fitness of individuals
– Should increase in frequency in a population due to
natural selection

Answer 31

Beneficial

Question 32

Mutation can produce a n_____ allele with no
effect on fitness
– Occurs when a point mutation does not change the
amino acid sequence

Answer 32

Neutral

Question 33

Mutations can be a significant evolutionary
process in bacteria and archaea because they
have very short generation times
– Not usually the case in eukaryotes

T or F

Answer 33

True

Question 34

Mutation is slow compared with selection, genetic
drift, and gene flow
– But mutation can have a large effect when combined
with one of these mechanisms
– for every 10000 alleles produced on average 1 will
have a mutation at the gene in question

T or F

Answer 34

True

Question 35

Escherichia coli was used as a model to study
how mutation affects evolution

In one experiment, Richard Lenski and colleagues
set up 12 populations of E. coli and allowed them
to reproduce for 10,000 generations
* E. coli reproduces asexually, so mutation is its
only source of genetic variation
– No gene flow occurred

T or F

Answer 35

True

Question 36

The relative fitness of d_______ generations
increased dramatically over time
- The relative fitness of the populations i______
over time in jumps
- They hypothesized that this pattern resulted from
novel mutations arising and conferring a fitness
benefit under selection

Answer 36

Descendant, Increased

Question 37

After a b____ mutation occurred, the fitness of
the population stabilized for a time
– until another random mutation produced an increase in fitness
* These results demonstrate the combined effects
of m____ and n_____ s_____

Answer 37

Beneficial
Mutation, Natural selection

Question 38

Horizontal Gene Transfer
* Pea aphids are small insects that feed on plant sap
* There are two phenotypes in wild populations—red
and green
* Polymorphism is maintained by b_____ selection
– Ladybird beetles prey on red aphids, and parasitoid
wasps typically lay eggs on green aphids
* Colour is determined by carotenoid pigments, but it
does not come from their food—plant sap

Answer 38

Balancing