Changes In Allele Frequencies Over Time Flashcards
What is genetic drift?
stochastic changes in allelic frequency caused by the chance disappearance of alleles when individuals die or fail to reproduce.
What is genetic drift?
changes in allele frequencies that are caused directly by changes in population size
what is the peppered moth a good example of?
the changes in allele frequency due to traits being determined by a single gene
what happened to the peppered moth?
the typical moth was white so that it could be camouflage on silver birches however during the industrial revolution things went black so a single allele which caused black colourings called carbonaria was selected for instead, the numbers of black greatly and quickly increased while the white ones dropped. after. when the clean air act was enforced the white went up again - industrial melanism
what was the process that caused moths to go black?
industrial melanism
how is the example of the peppered significant?
it shows how allele frequency can be calculated based on the selective advantage that the mutated allele gives
what is assumed to be the life cycle when calculating the change of frequency of a single allele?
birth occurs, survives, becomes an adult, reproduces (cycle begins again), dies. the parents don’t out live their young
what does the hardy weinberg frequencies show when calculating the change in allele frequencies?
the expected population- the numbers before selection has occurred- at birth how many are present before some have been selected against
the survival of offspring depends on what?
their genotype and its relative fitness
what would be the relative fitness of the genotypes if CC and Cc were equally likely to survive but cc was selected against?
1:1:1-s
what is the definition of relative fitness?
probability of survival relative to reference genotype
what does W(genotype) / the mean of W represent?
the probability of survival relative to population average
what is the equation for calculating average fitness?
the sum of each genotype frequency multiplied by its relative fitness (relative fitness = fitness divided by average fitness of the population
what is the definition of fitness?
how successfully a genotype contributes to the gene pool of the next generation RELATIVE TO THE REFERENCE GENOTYPE (want to know how good you are in the current population)
what would a selection coefficient of -1 mean?
all the genotypes with that s die
what would a selection coefficient of 0 mean?
it isn’t selected against
what is the dominance coefficient?
how dominant one allele is over the other in a heterozygote genotype
what is the dominance coefficient? what would values of 0 and 1 mean?
how dominant one allele is over the other in a heterozygote genotype- 0 would mean that A is completely dominant however 1 would mean that takes the full cost of selection as a
what is the overall change in allele frequency equation and what does it tell us about the different values relative to rate of change?
the position of s shows us that a change in allele frequency is strongest when s is large (there is a big relative selection pressure)
when the allele frequencies are intermediate and (p x q gives greatest answer when 0.5 x 0.5) therefore when the population is as genetically diverse as possible
for recessive alleles, (h=0) change is greatest when p is large
for dominant alleles (h= 0) then changes occurs mist when p is small - however eventually the curve will plateau because all of the a alleles will be hidden so it is hard to get rid of them
under or over dominance - the het is either better or worse than the two homozygotes- stabilise at very easily because both A and a will be present at an intermediate
what are the assumptions in the change of one allele frequency over time model?
this model assumes that
A single population
(can change if Frequencies can change due to migration)
Deterministic dynamics (survival is only influenced by fitness
(can change if Stochastic effects in finite populations)
No mutation
(Frequencies can change due to mutation)
Random fusion of gametes
(Mate choice affects genotype frequencies)
Selection acts on juvenile survival
(Genotypes can affect reproductive success)
what does stochasticity mean?
the random perturbations from the predicted allele values due to randomness- such as some individuals not mating, some having more offspring that others, some being randomly killed
when does stochasticity have its greatest effect?
in smaller populations when random events are unlikely to be counteracted in other individuals- so large changes in allele frequencies can occur
how can genetic drift cause fixation?
random changes in allele frequencies in small populations causes random alleles to become dominant and then fixed
what are 2 causes of allele fixation and loss?
genetic drift and selection for advantageous alleles
