Hardy-Weinberg Equilibrium: Genetic Drift and Neutral Theory Flashcards
what is drift
- chance events
- Random events that cause allele frequencies to change
Does drift cause changes in allele frequencies?
yes
Does drift adapt animals to environment?
no because its completely random
does drift violates the HW assumption 4 that population sizes are infinitely large
yes it does bc it changes allele frequencies
Does drift increase or decrease variation?
- Decrease bc some alleles go into fixation by chance
- Randomly leads to some alleles disappearing
what are 3 patterns about genetic drift
- unique evolutionary path
- effect in small populations
- changes in large populations
3 patterns about genetic drift - unique evolutionary path
Because fluctuations in allele frequencies are caused by random sampling error, every population follows a unique evolutionary path
3 patterns about genetic drift - effect in small populations
Genetic drift has more rapid and dramatic effect on allele frequencies in small populations that in large populations
3 patterns about genetic drift - changes in large populations
Given enough time, genetic drift can produce substantial changes in allele frequencies even in populations that are fairly large
if genetic drift is the only evolutionary mechanism at work, what is the outcome of a locus with 2 alleles?
Eventually one allele will drift to a frequency of 1 (fixation) and all others will be lost
What is the probability that a given allele (A vs a) will be the one that is fixed?
It is equal to its initial frequency
If frequency of A1 in a population is 0.1 (10%), then its probability of fixation is _____
- 0.1 (10%)
- 90% chance of becoming lost
Wandering of allele frequencies produces what two important effects
- Random Fixation of Alleles
- Loss of Heterozygosity
two important effects - loss of heterozygosity
As allele frequencies in a finite population drift toward fixation or loss, the frequencies of heterozygotes decreases
loss of heterozygosity - why does this happen
- Heterozygotes are at their highest frequency when A1 = 0.5 and A2 = 0.5
- As one allele or the other drifts to fixation, you get farther from this value
example of random fixation and loss of heterozygosity in natural populations
Templetonn et al. Ozark
collared lizards
random fixation and loss of heterozygosity - Templetonn et al. prediction
- glade population of lizards is small in size and had genetic isolation
- bc of this, they predicted that the lizards would bear a strong imprint of genetic drift
Templetonn et al. experiment predictions - loci fixation within populations
within populations, most loci should be fixed for a single allele, and genetic variation should be very low
Templetonn et al. experiment predictions - allele fixation
Which allele became fixed in any particular population should be a matter of chance, however, so there should be considerable genetic diversity among populations.
Templetonn et al. experiment - result
- most glade populations were fixed for a single multilocus genotype, and different genotypes were fixed in different glades
- seven distinct multilocus genotypes
Important points about drift
- causes allele frequencies to change at random
- does not adapt animals to environment
- impacts small populations more than large ones
- causes loss of genetic variation within populations
- can cause harmful alleles to become fixed BY CHANCE
important points about drift - causes loss of genetic variation within populations
alleles become fixed AND heterozygosity is lost
important points about drift - can cause harmful alleles to become fixed BY CHANCE
Because it is a completely random process
what causes small genetic population sizes?
- founder
- bottleneck
what causes small genetic population sizes? - founder
- Few individuals found the population and did not bring many genes with them
- Not a lot of genetic diversity if the population size grows to become large
what causes small population sizes? - bottleneck
- Massive population gets squeezed so the population is lowered
- Even if population, recovers, genetically it cannot recover bc it got so small
what was neutral theory used to explain
- the discovery that amino acids sequences change constantly like a clock
- there is one fixation every 2 years
neutral theory - beneficial mutations
Beneficial mutations are rare, start at low frequency, and are often lost—uncommon—can be fixed via positive selection
neutral theory - deleterious mutations
Deleterious mutations are removed by purifying selection (so don’t contribute to evolution)
neutral theory - neutral mutations
Neutral mutations rise and fall b/c drift
what are neutral theory ideas
- most of the mutations going to fixation must be neutral
- drift dominates sequence evolution
- rate of molecular evolution is equal to the mutation rate
- asks if these neutral mutations cause adaptations
what are neutral theory predictions
- non-coding regions of the genome (pseudogenes) should be neutral - no effect on fitness
- synonymous substitutions should be more frequent than nonsynonymous
neutral theory predictions - how to test that pseudogenes should be neutral
they should have the fastest mutation rates
neutral theory predictions - why would synonymous substitutions > nonsynonymous
- Synonymous/silent changes are not exposed to selection so they operate under drift.
- Nonsynonymous should be mostly eliminated via negative or purifying selection
how can you test for positive selection
- when polymorphism is present
- you can compare ratio of nonsynonymous vs synonymous substations
- allows us to measure the signature of positive selection
what is polymorphism
different individuals in population have different versions/alleles
test for positive selection - nonsynonymous-synonymous ratio < 1
- deleterious
- When sequences evolve by drift and negative selection, synonymous substitutions outnumber replacement substitutions.
test for positive selection - nonsynonymous-synonymous ratio = 1
neutral
test for positive selection - nonsynonymous-synonymous ratio > 1
- positive
- When sequences evolve by drift and positive selection, replacement substitutions outnumber synonymous substitutions.
