Lecture 7: Genetic Drift Flashcards
What is genetic drift?
The effects of sampling errors on allele frequencies due to chance events
What are chance events?
Events related to survival, reproduction, and inheritance of alleles
When are the effects of genetic drift especially important?
When HWE assumptions are false
What do you need to know to estimate genetic drift?
How many individuals in each population are contributing their alleles to succeeding generations
Basically, who breeds and how successful they are
What is the “poster child” for genetic drift?
Elephant seals
Do elephant seals have high or low genetic variance?
Low
What two issues have created the lack of genetic variability in elephant seals? What caused these issues?
1) Reduction in population size due to overhunting
2) Not much mating because males guard harems, so the effective population size is very low
How do allele frequencies change when discussing genetic drift?
Random sampling of the genes at the start of the next generation
Drift is truly ___ and ____.
random and unbiased
Drift is ___ in smaller populations and ___ in bigger populations
stronger, weaker
At peak variation, allele frequencies are __:__
50:50
More drift = __ variation
less
Drift causes populations to differentiate by fixation of alleles without the action of ______
natural selection
Genetic drift across many genetic loci causes populations to ____
diverge
In the simulation of genetic drift, after 32 generations, did most populations fix for one allele, or was there more of an even distribution of allele frequencies?
Populations fixed for one allele
If a gene is evolving neutrally…
there is no selection acting on it
Describe a diploid individual
two copies of every given gene, one from its mother and one from its father
What is a gene tree?
A genealogy of genes
What is coalescence?
Tracing the genealogy through time
When the lineages of two gene copies merge, we say that they ___
Coalesce
What is mitochondrial eve and what is it an example of?
The most recent common ancestor of human mitochondrial DNA. Example of coalescence
Is mitochondrial eve male or female?
Female
How many years ago did mitochondrial eve live?
~125,000 years ago
How was mitochondrial eve discovered?
By making a genealogy tracing all human mitochondrial DNA backward in time to the most recent common ancestor.
Is the ancestor of all Y chromosomes a male or female?
Male
True or false: Any two copies of gene share an ancestor at some past time.
True
Gene trees (do or don’t) always match phylogenetic trees. Why (2 reasons)?
Don’t.
Deep coalescence and incomplete lineage sorting
What does Ne stand for?
Effective population size
What is 2Ne?
The amount of generations it takes for two copies of a gene in a diploid organism that is evolving neutrally to get to the common ancestor
What is coalescence time?
The time to the most recent common ancestor
Does coalescence still apply if selection is acting? If so, what changes?
Yes, the time to common ancestor changes
What are deep coalescence events?
Events where the coalescence of living alleles traces further back in time before the speciation event
What is incomplete lineage sorting?
Incomplete fixation of gene lineages within species lineages
What is effective population size?
The number of individuals that would give the idealized population the same strength of drift as an actual population of interest
Is Ne always less than or greater than the actual population size?
Less than
What are 5 reasons why Ne < N?
1) Populations fluctuate in size
2) Uneven sex ratio
3) Juveniles and older individuals don’t reproduce
4) Reproductive success is variable among breeding age individuals (some have more offspring than others)
5) Limited dispersal from birthplace
Small Ne means _____ drift. Large Ne means ____ drift.
strong. weak.
Essentially, an infinitely large population would be ___ to genetic drift
immune
What are two events that cause massive fluctuations in population sizes?
Bottleneck, founder effect
What is a bottleneck?
Population reduced to small size for several generations
What is the founder effect? How does genetic drift play a role?
When a founder colonizes a new area. Genetic drift accompanies the start of a new founding population
Why are alleles under/overrepresented in founding populations?
Certain alleles may be missing in the founding population
Are small founding populations subject to slow or quick fixation/loss of alleles?
Quick
Do bottleneck and founder effect events reduce or increase genetic variation?
Reduce
What is a real-life example of a founder event?
Zebra finch
What is heterozygosity?
The chance that two chromosomes in population have different nucleotides at a given site
What is a measure of heterozygosity?
Nucleotide diversity (pi)
The Sunda founding population of finches was (smaller/larger) and (more/less) diverse than the original population
smaller, less
What are haplotypes?
DNA sequences that differ from homologous DNA sequences at one or more base pair sites
What does the plot of heterozygosity vs distance to Ethiopia show us?
As colonization occurs, this creates multiple founder events which reduce population size. As a result, there is less genetic variation further from the source
Which type of animals commonly experience uneven sex ratios?
Domestic animals
What are two reasons sex ratios may be uneven?
1) sex ratio at birth may be biased
2) females and males survive differently
Do fisheries have high or low effective population sizes?
Low
Does it take more generations for fixation to occur at Ne = 50 or Ne = 5?
Ne = 50
When p = ___, the time to fixation is greatest.
0.50
Is selection more effective in large or smaller Ne?
Large
Does self-fertilization reduce or increase Ne?
Reduce
Larger organisms tend to have (larger/smaller) Ne.
smaller
Which bacterium has an Ne much larger than most animals/plants?
E. coli
is polymorphism spread evenly or unevenly across the genome?
Unevenly
Where in the genome does polymorphism occur?
Introns
What is neutral mutation rate?
chance per generation that the locus mutates to another allele that does not change an organisms fitness
If Ne and μₙ go up, what happens to polymorphism?
Polymorphism increases
Deleterious alleles are weeded out by …
Purifying selection
In non-coding regions (introns), heterozygosity and polymorphism is ___
higher
What are selection sweeps?
Strong positive selection on a beneficial allele causes that allele to go to fixation
What happens to genetic variants/mutations near the beneficial allele?
They increase
Why is heterozygosity reduced near the center and ends of a chromosome?
Recombination is lower
Where are selective sweeps and background selection stronger?
Where there is less recombination
What is fecundity?
Offspring per day
What is heterozygosity related to? (3 things)
Population size
Fecundity
Propagule size
Propagule size and fecundity are ___ related.
Inversely
For species that have large Ne, is selection or drift more powerful?
Selection
What is codon bias?
Bias towards efficient codons
What did inbreeding in a population of Adder snakes result in?
Loss of non-deleterious alleles
How were the problems with the inbred Adder snake population fixed?
Males from different populations were introduced
