Genetic Drift Flashcards
What is genetic drift/random sampling?
changes in relative allele frequencies due to random events (ex. disasters like hurricanes)
Is genetic drift more influential to variation in small or large populations? why?
small - there’s a higher chance of losing alleles (genetic diversity) if there’s fewer individuals
T or F: genetic drift is an important driver of evolution
true
T or F: direction of genetic drift is unpredictable
true
Why is the direction of genetic drift unpredictable?
genetic drift is a result of RANDOM events, so whether or not, and which alleles will be lost or retained and in what frequencies is totally random
How does genetic drift effect variation within a population (increase, decrease, maintain)?
it reduces variation within a population because alleles can be lost
how does genetic drift reduce variation within a population?
results in loss of alleles –> increases homozygosity
What phenotypes did the 2018 (first) study by Donihue and Losos look at?
they measured toe pad size against body size of lizards before and after (surviving) a major hurricane (genetic drift event) to determine whether natural selection is acting on, and selecting for, larger toe pad size
Why would increased toe pad size and shortened femurs in the island lizards result in increased fitness?
larger toe pads and shorter femurs compared to body size results in a better ability to grip onto branches during a hurricane = increased survival
What did the 2018 (first) study Donihue and Losos find?
their figure showed a clear difference between the average toe pad size (compared to body size) of lizards before and after the two hurricanes. they found that afterwards, the lizards had larger average toe pads
How does the 2018 (first) Donihue and Losos study provide evidence for natural selection?
their study shows a fitness difference between phenotypes (larger vs smaller toe pad size) by finding the average toe pad size after a hurricane event was larger than before the event (those with larger had higher fitness)
Why does the 2018 (first) Donihue and Losos study not provide evidence for evolution?
even though there’s a phenotypic difference, there’s no evidence that the larger toe pad size is genetic/heritable and for evolution to occur, the phenotype with increase fitness must be heritable
What could Donihue and Losos do to determine the heritability of the toe pad phenotype?
- study the same experiment in the next generation for those traits
- analyze information on the genetic basis of toe pad size
What did Donihue and Losos do in their follow-up study to gain more evidence for evolution of toe pad size?
they looked at the next generation in the same lizard population and found the same results: after the hurricane in 2019, the toe pad sizes were larger
this provides evidence that there’s a connection between survival and the next generation (heritability)
What was the overall major finding of the work by Donihue and Losos?
they provided strong evidence supporting that hurricanes are causing the evolution of toe pad sizes in these island lizards
= on islands where hurricanes were more frequent, the lizards had larger toe pads (across different species and in different locations)
How does genetic drift affect the relative frequencies of genotypes?
it increases homozygosity and decreases heterozygosity because it causes the loss of alleles
What is the expected result in terms of genetic drift from a pop G simulation if the population size is large (10,000 individuals), both alleles are at 0.5 frequency, over 100 generations and no mutations, migrations, or natural selection?
the allele frequencies exist about the same over time and hover around the zero genetic drift line, P(A) = 0.5
How can we use pop G to simulate the effects of genetic drift?
by lowering the population size, keeping allele frequency at 50%, no introduced mutations, migrations, and no natural selection (relative fitness = 1 for all) to compare to the zero genetic drift line
What happens in pop G when the population size is decreased?
the frequency of A and a fluctuate more and more as the population size decreases = more genetic drift occurring
some of the populations even become fixed for A or a (ie., one of the alleles is lost) = reduction in variation
T or F: the reduction in variation (fluctuation of A and a frequency) in small populations is due to fitness differences between the alleles
false = when genetic drift is a strong influence (when pops are small), the loss of alleles or change in allele frequency is due to chance
What is the evolutionary result of genetic drift (ie., across multiple populations)?
divergent evolution of populations (populations of the same species that have different allele frequencies)
What is a relevant example of genetic drift in humans?
microcephalin - this was not due to fitness differences (no natural selection), just random chance
Explain how bowling can be a metaphor for genetic drift
the width of the bowling lane can be considered the population size = the narrower the lane, the easier it is for the ball to drop into the gutter = an allele to be lost
the proximity to the edge of the lane can be considered the allele frequency = the closer the ball is to the gutter, the more likely it is to go into the gutter = the lower the frequency of an allele, the more likely it is to be lost
What is a genetic bottleneck?
a reduction in population size to a small size