Lecture 8: Stabilizing Selection Flashcards
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
Genetic drift
Drift occurs in every population, in every
generation
– It is especially prevalent in ____ populations
Small
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 “____”
Lucky, Unlucky
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
Small, large
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
Fitness
Small
Loss, Fixation
G_____ M____ are specific alleles that cause a
distinctive phenotype
Genetic Markers
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:
- Normal, or straight
▪ Dominant (AN) - “Forked,” or branched
▪ Recessive (AF)
T or F
True
In the laboratory, genetic drift was found to
– D_____ genetic variation within populations
– I_____ genetic differences between populations
Decrease, Increase
Genetic drift can be caused by any event or
process that involves s_____ error
- 2 examples are f_____ effects and
b____
Sampling
Founder effects, bottlenecks
DEF: occurs when a small group of
individuals establishes a new population in a new
area
A founder event
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
Allele
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
Colonization
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
True
DEF: A sudden decrease in population size in a large
population
Population bottleneck
Population bottlenecks are commonly caused by: (2 points)
- Disease outbreaks
- Natural catastrophes
Population bottlenecks lead to g______
b_______ —a sudden reduction in the number
of alleles in a population
Genetic bottlenecks
Genetic drift occurs during genetic bottlenecks
and causes a change in allele f_______
Frequencies
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
Habitat Fragmentation
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
True
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
Fitness
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
True
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
Reduce, Decrease
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
True
Evolutionary mechanisms like natural selection
and genetic drift r_____ genetic diversity over time
Reduce
In contrast, m____ restores genetic diversity
– by creating new alleles,
– not only new combinations of alleles
Mutation
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)
Point mutation
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
Chromosome-level mutations
DEF:
- Transfer of genes from one species to another
- Might be a more important source of heritable genetic
variation than previously thought
Horizontal gene transfer
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
Random
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
Deleterious alleles, Purifying
Mutation produces b_____ alleles on rare
occasions
– Increases fitness of individuals
– Should increase in frequency in a population due to
natural selection
Beneficial
Mutation can produce a n_____ allele with no
effect on fitness
– Occurs when a point mutation does not change the
amino acid sequence
Neutral
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
True
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
True
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
True
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
Descendant, Increased
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_____
Beneficial
Mutation, Natural selection
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
Balancing
