Lecture 5: Genetic Drift (and Inbreeding) Flashcards
Natural Selection acts on __or __ __ in a population. without it, Natural Selection cannot occur
genetic or epigenetic variation
What is Natural Selection?
Process where organisms with advantageous traits are more likely to survive and reproduce, passing on those traits to future generations.
What generates genetic variation through random changes in the DNA sequence?
Mutation
What modifies gene expression without changing the DNA sequence itself?
Epigenetic modification
What causes fluctuations in allele frequencies and can reduce genetic variation in small populations?
Genetic drift
What is Migration in the context of genetics?
Movement of individuals between populations.
How can Migration act as a homogenizing force?
By reducing differences between populations through gene flow.
What can happen to a population with high migration rates regarding Hardy-Weinberg equilibrium?
It may go out of Hardy-Weinberg equilibrium.
What impact does Immigration (inward movement) have on genetic variation within a population?
It could introduce new genetic variation.
How does Emigration (outward movement) affect genetic variation within a population?
It could reduce genetic variation by removing individuals from the population.
Who was Sewall Wright and what did he study?
Sewall Wright studied agricultural stocks, such as cows, and was particularly interested in small, inbred populations.
What genetic mechanisms did Sewall Wright regard as particularly important?
Inbreeding and Genetic Drift.
What could Genetic Drift and Inbreeding generate according to Sewall Wright?
new gene interactions
What did Sewall Wright consider as the main substrate for selection?
New gene interactions, particularly those caused by epistasis through new recombinations.
What is Random Genetic Drift?
Changes in allele frequency from one generation to the next due to chance (sampling error).
Is Random Genetic Drift adaptive or non-adaptive?
Non-adaptive.
Did Charles Darwin recognize genetic drift as an evolutionary mechanism?
No.
- happens when populations are limited in size, violating HW assumption of infinite population size
- Changes in allele frequencies due to random chance events in small populations.
Genetic drift
Under what condition does Genetic Drift occur?
Limited population size, violating the Hardy-Weinberg assumption of infinite population size.
- During the Evolutionary Synthesis, Sewall Wright focused more on importance of __ __, whereas Fisher focused on __ __
- Genetic Drift
- Natural Selection
- Shortly after the Evolutionary Synthesis many focused on __ to the point of assuming that most phenotypes were the result of Natural Selection
- selection
- Emphasis on Genetic Drift resurged in the 1970s, 80s with __ “ __ __ “
Kimura’s “Neutral Theory”
Then in the 2000s and 2010s interest in Selection increased with the ability to detect signatures of Natural Selection in __ __ __
genome sequence data
How does population size influence the impact of chance events on allele frequencies?
In large populations, chance events cancel each other out; in small populations, random differences in reproductive success become more significant.
In Evolution, when we talk about population size, we mean
Effective population size
Who introduced the concept of effective population size (Ne)?
Sewall Wright
In which landmark papers did Sewall Wright introduce the concept of effective population size?
Wright 1931, 1938
The number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under random genetic drift or the same amount of inbreeding as the population under consideration.
Effective population size (Ne)
The size of an idealized Wright-Fisher population that would have the same increase in homozygosity and the same random drift in allele frequencies as in the actual population considered.
Effective population size (Ne)
Effective population size (Ne) - The number of __ __ in an idealized population that would show the same amount of __ of allele frequencies under random genetic drift or the same amount of __ as the population under consideration.
- breeding individuals
- dispersion
- inbreeding
Effective population size (Ne) - The size of an idealized __-__ __ that would have the same increase in __ and the same __ __ in allele frequencies as in the actual population considered.
- Wright-Fisher population
- homozygosity
- random drift
Is the effective population size (Ne) typically greater than or less than the census population size (N)?
Almost always either equal to or less than the census population size (N).
Why is the effective population size (Ne) usually smaller than the real census population size?
Because not everyone breeds and leaves offspring.
What factors can lead to an effective population size (Ne) that is smaller than the census size? (5)
- Unequal sex ratio
- variation in number of offspring
- overlapping generations
- fluctuations in population size
- nonrandom mating.
How does effective population size (Ne) depend on sex ratio? (equation)
Ne = 4 Nm Nf / (Nm + Nf)
Nm = number of males
Nf = number of females.
What effect does an unequal sex ratio have on effective population size (Ne) according to the equation?
An unequal sex ratio would lead to a lower Ne.
Why is effective population size (Ne) important?
Because Ne is the actual unit of evolution, rather than the census size (N).
Why is Ne considered more crucial for evolution?
Because only the alleles passed onto the next generation count in evolution, and individuals that do not mate or have offspring are evolutionary dead ends.
What happens to Ne if a population is completely inbred?
Ne = 1 or becomes extremely low, even if the census size is large.
What role do chance events play in genetic variation in populations without selection? (chance events influence what?)
Chance events influence:
- Which individuals leave offspring.
- The number of offspring produced.
- Which offspring survive, including which gametes and alleles are passed on.
What is an example of sampling error in genetic variation?
In a population with alleles for green (G, dominant) and blue (b, recessive) fur, if two individuals with genotype Gb mate, the next generation is expected to have a 3:1 ratio of green to blue fur (GG, Gb, bG, bb). However, due to chance, one family could get an unusual frequency, such as all bb offspring, leading to the accidental loss of the G allele.
How can chance events lead to the loss of a particular allele, even without any selective pressure?
Chance events can cause unusual frequencies in offspring, such as all bb individuals in a population with alleles for green (G) and blue (b) fur, resulting in the accidental loss of the G allele.
Why is genetic drift not considered selection?
Selection occurs when some individuals survive due to being better adapted to their environment. Genetic drift, on the other hand, is simply a numbers game where which gamete gets fertilized and which allele gets passed on is entirely random.
__ occurs when some individuals survive due to being better adapted to their environment. __ __, on the other hand, is simply a numbers game where which gamete gets fertilized and which allele gets passed on is entirely random.
- Selection
- Genetic drift
What are the consequences of genetic drift?
- Random fluctuations in allele frequency.
- If population size is reduced:
- At the Allelic level: Random fixation of alleles (loss of alleles).
- At the Genotypic level: Loss of heterozygosity due to fewer alleles.
What are the consequences of genetic drift at the allelic level if population size is reduced?
Random fixation of alleles (loss of alleles).
What are the consequences of genetic drift at the genotypic level if population size is reduced?
Loss of heterozygosity due to fewer alleles.
Why is the probability of loss of alleles greater in smaller populations?
Because smaller populations have fewer individuals, which increases the likelihood that certain alleles will not be passed on to the next generation.
Provide an example illustrating why the probability of loss of alleles is greater in smaller populations.
If a population contains 50 different alleles and a new population is founded by only 10 individuals, then the new population will likely be unable to capture all 50 alleles, resulting in the loss of many alleles.
Occurs when a population undergoes a drastic reduction in size, often due to a catastrophic event such as natural disasters, disease outbreaks, or human activities like habitat destruction. As a result, the surviving individuals represent only a small fraction of the original population’s genetic diversity.
Bottleneck effect
genetic drift becomes a dominant evolutionary force due to the small population size. Allele frequencies may change dramatically, and rare alleles may be lost entirely.
Bottleneck effect
occurs when an allele frequency becomes 100%, resulting in the loss of all other alleles by chance.
Fixation
- refers to the situation where a particular allele at a specific gene locus becomes the only variant present in a population
- it means that every individual in the population is homozygous for that allele at that locus.
Fixation
Fluctuations are much larger in __ populations
smaller
What is the probability of fixation of an allele if its starting frequency is 0.20, or 20%?
0.20, or 20%
This means there is a 20% chance that the allele will become fixed in the population over time.
What is the probability of fixation of an allele if its starting frequency is 0.60, or 60%?
0.60, or 60%
In this case, there is a higher chance, 60%, that the allele will become fixed in the population.
Probability of fixation of an allele = __ __ __ __
the allele’s starting frequency
What is the probability of fixation of an allele in a population and how is it calculated?
Probability of fixation = 1/2N for each allele in a population with 2N alleles. If there are X copies of an allele, the probability of fixation for that allele = X/2N
Where is the greatest allelic diversity of CYP genes found?
Africa
What do CYP genes help with?
Detoxification
What happens to allelic diversity as populations move away from Africa?
Loss
What happens to genetic diversity during genetic drift?
Decreases
How does genetic drift relate to human migrations?
It can cause loss of genetic diversity
What happens to the probability of fixation or extinction of alleles as populations get smaller?
It increases.
What happens to the probability of fixation or extinction of alleles in a large population?
It decreases.
Frequency of heterozygotes in a population (% of heterozygotes).
heterozygosity
Often used as an estimate of genetic variation in a population.
heterozygosity
What is the Hardy-Weinberg expected frequency of heterozygotes in a population?
2p(1-p) (at a locus with two alleles)
What happens to the frequency of heterozygotes as genetic drift drives alleles toward fixation or loss?
It goes down due to the reduction in the number of alleles.
What is the formula for the frequency of heterozygotes in the next generation following a population reduction?
Hg+1 = Hg (1 - 1/2Ne)
What does “2Ne” represent in the equation for the frequency of heterozygotes in the next generation following a population reduction?
Population size
How does heterozygosity in the next generation change as population size (2Ne) gets smaller?
It decreases.
Heterozygosity declines __ in smaller populations
faster
No Genetic Variation = __ __ __
No Natural Selection
Natural Selection is more efficient in __ populations, and less effective in __ populations
- larger
- smaller
How do you detect Genetic Drift?
Random fluctuations in allele frequencies, particularly in
- non-coding and non-functional genome regions
- correspond to changes in population demography, such as size.
How does genetic drift generally affect the genome? Are there exceptions?
Similarly across the entire genome, but with exceptions like mitochondrial DNA (mtDNA), which has a smaller effective population size and can experience different patterns of genetic drift.
Consequences of genetic drift at the allelic level if population size is reduced and Drift acts more intensely, we get:
Random fixation of Alleles (loss of alleles)
Consequences of genetic drift at the genotypic level if population size is reduced and Drift acts more intensely, we get:
Inbreeding, Reduction in Heterozygosity (because of fewer alleles)
What is the dilemma for conservationists regarding genetic variation?
It’s easier to remove genetic variation than to create it, especially adaptive variation. For example, despite a census size of 2000-3000 cheetahs in Namibia, the effective population size is much lower, posing challenges to maintaining genetic diversity.
__ is a consequence of genetic drift in small populations, resulting from the loss of alleles, leading to increased __.
- Inbreeding
- homozygosity
As effective population size declines, heterozygosity goes __ , homozygosity goes __
- down
- up
- Mating among genetic relatives, often due to small population size, resulting in alleles at a locus more likely becoming homozygous.
- It’s a consequence of the loss of alleles due to genetic drift, particularly the reduction in population size.
Inbreeding
coefficient of inbreeding (F) equation
Fx = ∑[(1/2)^n+1(1 + FA)]
Who developed the coefficient of inbreeding?
Sewall Wright
What does the coefficient of inbreeding (F) represent? What does it measure? What does F=1 indicate?
- F represents the fixation index
- measures homozygosity.
- A value of F=1 indicates that individuals are genetically identical.
__ population size → __ __ → loss of alleles → decrease in __ → increase in __ → increase of exposure of deleterious alleles
- genetic drift
- small
- heterozygosity
- homozygosity
What are the consequences of inbreeding? (3)
1) increase in homozygosity:
- exposing recessive alleles (that could be subjected to selection)
- Inbreeding Depression (reduction in survival and fitness)
2) Lower genotypic diversity (poor response to natural selection)
3) Deleterious recessive alleles are exposed as homozygotes (and less masked in the heterozygous state)
refers to a reduction in survival and fitness resulting from inbreeding.
Inbreeding depression
How does inbreeding affect genotypic diversity?
It lowers genotypic diversity, resulting in a poor response to natural selection.
In inbreeding, __ __ __ are exposed as homozygotes and are less masked in the heterozygous state.
Deleterious recessive alleles
How do deleterious recessive alleles get removed from inbred populations?
These alleles are removed more rapidly because they are exposed to natural selection due to their homozygous state.
a phenomenon that occurs when a new population is established by a small number of individuals, who carry only a fraction of the genetic diversity present in the larger parent population from which they originated.
Founder effect
What is the mechanism behind the expression of deleterious alleles?
Most deleterious alleles are recessive.
How many lethal recessive alleles does each individual typically carry on average?
3-5 lethal recessive alleles
In which individuals are deleterious alleles expressed?
inbred individuals
How are deleterious recessive alleles are in heterozygotes?
masked and not exposed to selection
__ __ is more likely to occur in closely related individuals, whereas __ __ is observed when genetically distinct individuals are crossed.
- Inbreeding depression
- hybrid vigor
__ __ can occur if individuals from highly divergent populations are crossed, leading to reduced __ in the hybrid offspring.
- Outbreeding depression
- fitness
How can inbreeding exacerbate the risk of extinction?
- reducing genetic diversity
- increasing the expression of deleterious alleles through inbreeding depression.
What strategies do conservationists employ to prevent extinctions?
- Habitat restoration
- captive breeding programs
- reintroduction efforts
- establishment of protected areas.
Why is maximizing genetic variance within a population crucial?
It maintains adaptive potential and resilience to environmental changes.
How do conservationists achieve maximizing genetic variance?
- increasing gene flow between populations
- reducing the negative effects of inbreeding
- maintaining connectivity among habitat patches.
What must conservation efforts prioritize?
Preserving genetic diversity within and among populations, as well as conserving key habitats and ecosystems that support biodiversity and ecosystem functioning.
What is the consequence of focusing excessively on a few breeds in agriculture?
The effective population sizes of many crops become incredibly small.
Inbreeding and Agriculture:
- How many breeds of livestock have disappeared?
- How many breeds are lost each year?
- How many breeds of livestock are considered at risk of extinction?
- 600 breeds of livestock
- ~78 breeds
- 1000-1500 breeds of livestock (30% of current breeds)
__ million Holstein cows are descendants of __ individuals.
- 8
- 37
What are some health issues associated with the limited genetic diversity of Holstein cows? (3)
- Neurological disorders
- autoimmune diseases
- fertility problems.
Inbreeding in humans: Research has shown that children born to parents who are __ __ have a higher risk of mortality compared to children born to non-related parents.
first cousins
Genetic Diseases due to Inbreeding have afflicted many Royal Families. What are some examples of those diseases? (3)
- Porphyria
- Acromegaly
- Hemophilia
- Accumulation of porphyrin precursors causing insanity.
- Dominant, but more intense in the
homozygous form
Porphyria
Overproduction of growth hormone by the pituitary gland. Recessive
acromegaly
Bleeding disorder, X-linked.
hemophilia
Who suffered from porphyria historically?
- Mary, Queen of Scots
- George III.
Who suffered from acromegaly historically?
Charles II, King of Spain
What is the historical significance of hemophilia?
Contributed to the downfall of the Russian monarchy.
What is the genetic background of the Amish population in Lancaster County, PA?
Descendants of ~200 Swiss who emigrated in the mid-1700s, closed genetic population for 12+ generations.
What are some characteristics of diseases that afflict the Amish population? (7)
1) High levels of infant mortality
2) high incidence of brain damage
3) large number of metabolic disorders (e.g., PKU)
4) Ellis-van Creveld (dwarfism) syndrome
5) Glutaric aciduria type I
6) high incidence of bipolar disorder
7) polydactyly (6 fingers).
occurs when a population is small, and leads to random loss of alleles due to sampling error
Genetic drift
an extreme consequence of Genetic Drift, results in increases in homozygosity at many loci, including recessive lethal
Inbreeding
Observations of __ __ changes that occurred during the divergence between species, show that molecular evolution (mutations) takes place at a roughly constant rate.
amino acid
molecular evolution is constant enough to provide a “__ __” of evolution, and the amount of molecular change between two species measures how long ago they shared a __ __.
- molecular clock
- common ancestor
From the observations about the molecular clock, Kimura concluded that most molecular evolution is “__” and
dominated by __ __ and __ __.
- neutral
- random mutations
- genetic drift
A theory that posits that the vast majority of evolutionary change at the molecular level is caused by random genetic drift rather than natural selection. Who proposed it?
The Neutral Theory of Molecular Evolution by Motoo Kimura
How does neutral theory relate to Darwin’s theory of evolution by natural selection?
Neutral theory is not incompatible with Darwin’s theory. It acknowledges adaptive changes but posits that they constitute a small minority of evolutionary shifts.
__ __ (negative selection) could still be acting to remove __ __ from the genome. But, the idea here is that most of the genetic variation within and between populations is due to __ __ (mutations and genetic drift).
- Purifying selection
- deleterious mutations
- neutral variation
What kind of evolutionary force is random genetic drift?
Non-adaptive evolutionary force
- When population is __ , chance events cancel each other out
- When population is __, random differences in reproductive success begin to matter much more
- large
- small
Natural Selection is less efficient when there is __ and __ __.
Natural Selection is more efficient in __ __.
- genetic drift
- smaller populations
- larger populations
