Selection and Drift Flashcards
what is the difference between continuous traits and discrete traits?
continuous traits are contributed to by many loci, each allele has a small effect. Discrete traits are determined by one or a few loci, there are large allelic effects and has a discontinuous distribution
what are 5 examples of single gene traits?
antibiotic resistance in bacteria, reisstance to pesticides, human resistance to malaria, human genetic diseases such as cystic fibrosis and huntingtons
what type of trait is the colour of the peppered moth determined by?
a single allele- it is a discontinuous trait
what are the two types of peppered moth?
typica and carbonaria
what is the peppered moth an example of?
selection at a single locus - discontinuous trait
how many alleles determine moth pigmentation?
two alleles at one locus
was the C allele for carbonaria recessive or dominant?
dominant
how can looking at the carbonara moths frequency over time be useful?
it is useful because once you study the change in allele frequency you can predict evolution in the future, calculate the force of selection and test biological scenarios- by following a few generations you can predict how the allele frequencies will change over time
how can looking at changes in allele frequency over time be used?
can be used to see if selection is occurring (does it follow hardy weinberg), can be used to predict evolution
what is neutral evolution?
when there is no selection occurring for an allele however allele frequencies occur due to genetic drift
why does stochasticity matter in small populations but not large populations?
in large populations random changes in allele frequency can occur outside of hardy weinberg- this can occur if animals randomly die or produce more or less young. however in large populations this is normally evened out. However, in small populations there is little room ofr adjustments to occur without perturbing from hardy weinberg- therefore the fitness of Wcc not always 1-s… can be greater
when does evolution but no adaptation occur?
when genetic drift can occur in smaller populations
what are the 2 consequences of drift?
loss of genetic diversity, genetic differentiation between populations
give an example of how genetic drift can occur in a single self fertilising individual?
this single self fertilising individual can produce an offspring that will either be CC or Cc or cc, if either homozygous occurs then heterozygosity will be lost forever and C will become fixed the probability of this occurring is 1/2over a single generation
what happens to allele frequency in smaller populations?
fixation and loss
what is an example of loss of diversity in an organism experiment?
drosophila melanogaster- Bw eye colour- 107 populations- populations mostly because fixed to bw or bw 75
when is drift stronger in terms of Ne?
when there is a low effective population size
what is lek polygyny and how does it contribute to genetic drift?
Lek polygyny involves females being driven in pursuit of males- males form aggregates in neutral locations and perform so that the females can compare and decide on the best male- when this occurs there is normally a great reproductive skew with one male copulating with most females- this is called reproductive skew and decreases the effective population size- increasing the change in allele frequency due to drift
what an ideal population what is the relationship between Ne and Nc?
the effective population size should equal the census population size
when is fixation time maximised?what happens when this isn’t the case?
when p=0.5, when this isn’t the case fixation of the dominant allele occurs a lot quicker
what are three ways that genetic diversity can be described?
within individuals, between individuals and between populations
what does alpha, theta and F all represent?
coancestry between populations, within populations and the inbreeding coefficient
how can Fis be used to make inferences about a populations mating system?
Fis shows whether heterozygosity is less or more than expected.
inbreeding or outbreeding
- if Fis is greater that 0 then homozygosity is greater than expected.
a negative value= outbreeding
what does Fst show?
- is Fst=0 then the two sub populations are homogenous however if Fst =1 then there has been complete differentiation. shows whether individuals are more similar within subpoplulatins that between subpopulations- if so then differentiation has occurred. can show whether a population is undergoing differentiation and has therefore undergone migration or isolation etc
what are SNPs?
single nucleotide polymorphisms- changes in DNA sequences associated with specific alleles of a gene in certain populations. the wild type has no changes but different alleles have different SNPs
what can be looked at by using SNPS in drift analysis?
- human migration
- dog breeds
what is the fundamental process behind using SNPs in drift analysis?
SNPs represent different alleles of a gene- they are what make up the allele itself. by measuring the presence of these alleles and their heterozygosity within subpopulations and populations- migration, drift, bottles necks, differentiation can be calculated
how can SNPs be used to study the expansion of the human population out of africa?
- as humans migrated from africa different groups migrated into different areas
- this resulted in a decreases in the population sizes and repeated bottlenecks/ founder events
- as a result of this allele frequencies changes and humans became more homozygous
- alleles began to be fixated in certain populations
- can measure Fst to measure how differentiated populations are- higher in later colonising areas- argentina
how can SNPS and the study of genetic drift be used when studying dog breeding?
you can find which genes in dog breed are involved in phenotypic traits- which have been selected for- can do this because the genes that are selected for in different breeds will have an excessively high Fst value due to large divergence in alleles
- the value for Fst will be even higher than just drift Fst changes
- the genes containing these SNPs will be the genes causing the phenotypic variation in species
- the HSA2 contributes to skin wrinkling
what does the HSA2 gene do?
causes skin wrinkling in sharp-peis
look in notepad for descriptions of the formula
…
what is deterministic evolution and why is it validity uncertain?
dynamics of allele frequency depend on the fitness effects of the alleles- favourable alleles go into fixation and deleterious alleles are eliminated- neither of these two things are certain in finite populations
why does deterministic evolution not apply to new mutations?
deterministic evolution states that changes in allele frequencies are determined by their fitness and deleterious mutations are lost. this can only be ensured in very large populations when the allele frequency is high- for new mutations the chance that they are lost is still high
what is the frequency of new mutations in haploids and diploids and what does this mean about their interaction with drift?
1/N in haploids and 1/2N in diploids- this means that there is a high probability that they will be lost due to random changes in allele frequency- the animal may be suddenly killed, or hit by disease or not breed
what is the probability of losing a neutral mutation in a haploid??
(1-1/N)^N because the probability of passing the new mutation on to the next generation is 1/N so 1-1/N is the probability of not picking it out… ^N because there are N chances of choosing it. tis equates to e-1 which is 0.3678
what is the likelihood of losing a new neutral mutation independent of population size?
0.3678
how do you calculate the probability of observing no mutants in the next generation given that it is selected for?
you use a poisson distribution in which lamda (the probability of the event happening) is 1+s and
k=0… so equals e-1 (1-s) - around 0.36 but decreases as s increases- less likely to be lost as s increases
how does the value of s effect the probability of immediate extinction?
as s increases the probability decreases but high s’s are often lost too
what is the difference between long term and short term fates of new mutations?
in the short term, no matter how high s is they can still be lost due to stochastic allele frequency changes but in the long term their fate is determined by is by is | 2NeS | >1
what is the probability of fixation for new mutations?
haploids = around 2s diplods= 2(1-h)s - the dominance influences
what is the difference between long term and short term fates of new mutations?
in the short term, no matter how high s is they can still be lost due to stochastic allele frequency changes but in the long term their fate is determined by is by is | 2NeS | >1
what is richard lenski do and why was it significant?
he grew 12 different strains of e.coli on separate dishes, all on the same experimental medium
- he froze the ancestors of each
- he repeatedly related each strain
- then then at different stages compared the fitness of each strain to the ancestor
- he found that some strains had increased in fitness greatly but others had increased in fines slightly- this shows that some strains had more beneficial mutations that others simply due to chance
- he then related the strains on different media to see how quickly each strain had specialised onto their experimental media
- some strains were unable to exploit most carbon sources due to the fact they had developed mutations that had shut down this pathway
- some still have the pathway available
- showed that identical strains develop beneficial mutations at different rates randomly
how can fixation of deleterious mutations occur?
when the effective population size is low and therefore genetic drift strength is high.. | 2NeS | isn’t higher than 1
how can fixation of deleterious mutations occur?
when the effective population size is low and therefore genetic drift strength is high.. | 2NeS | isn’t higher than 1
how do you calculate the intensity of drift?
1/2Ne
what is the interaction between the value of S and fixation?
if negative and causes 2NeS to be less than -1 then deleterious will be lost or over 1 will be fixed
how do you calculate selection relative to drift
2NeS
what does it mean when 2NeS =0?
selection= drift = 1/2Ne
what do transposable elements tell us about the relationship between effective population size and the ability for selection to act?
in humans there is a large about of transposable elements because homo sapiens have such a small effective population. compared to c elegans and yeast who’s Ne is much larger how low effective populations so getting rid of stuff which is bad for us so genome is full of things which are slightly deleterious
how can the relative effects of drift and selection be used?
- predicting extinction risk
- looking at effect of mutations
- conservation
how can the relationship between drift and selection actively be seen in nature?
glanville fritillary butterfly populations were studied- these populations were very small- saw whether these populations persisted- the populations that disappear tend to be the small one where drift is strong and can’t get rid of deleterious mutations
how can you use the interaction of selection and drift to study the effect of mutations?
- want to observe the effect of strong deleterious mutation which would normally be removed in selection- in a culture make drift so strong that we can view these mutations without them being selected- mutation accumulation
how can the relationship between drift and selection be applied in conservation?
- endangered species are normally small and isolated so that selection is strong and inefficient
- accumulate deleterious alleles
- have trouble fixing advantageous mutations
- get even smaller populations
- the smaller the Ne the more quickly a population becomes unviable
what is the relationship between large and small population sizes and the value of s in terms of increasing population size?
want big s in large populations so that deleterious mutations can be removed quickly but in small population sizes want small s so that delterious mutations aren’t that harmful when they can’t be removed
what is inbreeding depression
create homozygosity of harmful recessive alleles which decreases fitness
what is inbreeding depression?
create homozygosity of harmful recessive alleles which decreases fitness
how does inbreeding depression effect endangered species?
as smaller Ne the liklihood of inbreeding increases expression of deleterious mutations - especially when put in challenging environment
how are swiss villages relevant to drift and selection?
remote swiss villages often participate in inbreeding so have low fecundity- this highlights that in small populations you get inbreeding depression which further exacerbates the population
how are swiss villages relevant to drift and selection?
remote swiss villages have small population size so genetic drift can cause fixation of deleterious mutations causing further decrease- often participate in inbreeding so have low fecundity- this highlights that in small populations you get inbreeding depression which further exacerbates the population
in a population, what is the probability of inbreeding?
q^2n
why does inbreeding increase homozygosity?
- when you think that each loci have many many alleles, in a large population the probabillty of an individual mating with another which has the same allele is quite low. however when inbreeding occurs between a smaller population the chances of mating with someone with the same allele as you - due to co ancestry- is a lot higher so homozygosity increases. homozygosity by descent
what are the 4 example you should talk about when referring to the interaction between dirt and selection can be applied?
- mutation accumulation experiments
- glanville fritillary butterfly (small and genetically poor population)
- conservation and inbreeding
- droso melan and the increase in 5 of populations extinct due to high F values
when can natural selection only have an effect on evolution?
when the phenotype that is being selected differs in genotype
what are the relative rates of gene flow and genetic drift?
the same
what are 3 things that the consequences of natural selection depend on?
the relationship between phenotype and fitness, the relationship between phenotype and genotype, the relationship between fitness and genotype
what are three forms of quantitative selection?
direcional, stabilising and diversifying
what is stabilising selection?
the maintenance of heterozygosity- heterozygous advantage
when does diversifying selection occur?
when there is heterozygous disadvantage
what is the absolute fitness and how can it be calculated?
the number of offspring that a genotype produces = O/E
how do you calculate relative fitness?
take the highest absolute fitness value as 1 and then the others are a proportion of that- divide it by the highest number
with additive variance, what will the influence of an advantageous mutation be?
it will always increase in number under selection alone
what is an estimation for the amount of time that fixation takes in a diploid population?
4N
what is a mutations probability of fixation in a diploid population?
1/2n (its frequency)
give an example of directional selection in nature?
the use of warfarin poison was used to prevent coagulation in rats through preventing the regeneration of vitamin K, this meant rats lead to death, a single mutation occurred with produced an enzyme that was less efficient at regenerating it K but was not effected by warfarin, the frequency of this allele sharply increased.
what is the equation for calculating the equilibrium frequency of mutation rate?
q frequency= sqrt of mutation rate / selection coefficient
when there is heterozygous disadvantage, what is the equilibrium frequency like?
unstable because as soon as it shifts towards one, that allele will go into fixation
why is it hard to move between peaks when there is het disadv in small populations?
because drift can cause fixation of one peak
what is an example in nature of heterozygous advantage?
sickle cell anaemia- homo rec gives you anaemia and you die, homo dom means you are less resistant to malaria, so het is supported via antagonistic selective factors
what is spatial and temporal fluctuation and what do they do?
they result in high heterozygosity. spatial involves certain genotypes being selected for in certain microenvironments or when resources are scarce. temporal is when changing times favour different phenotypes and this changes a lot
give an example for spatial and temporal selection
temporal- darwin finches on an island, their beaks changed from deep thick bills to thinner small bills depending on whether there was a drought- the S value for different genotypes fluctuated.
- temporal is the black bellied seed cracker- individuals within a population either have thick or thin weeks depending on which seed they eat- multiple niche polymorphism
what is the name of the black bellied seed crackers beak variations?
multiple niche polymorphism
give an example in terms of flowers of negative frequency dependents
there are self incompatibility loci
seen in many flowers- for example there are 3 alleles: S1,S2,S3. a pollen can not fertilise a plant that has the same allele frequency. a pollen of type S1 can’t fert S1S2 or S1S3. this means that S1 allele is good to be low frequency as more likely to fertilise. when it reaches 1/k (k number of alleles) frequency it decreases in fitness
what is an example in fish of negative frequency dependent selection?
chichlid fish each other fish via attacking them from one side. they either have mouths on the left or right and the attack from the opposite side of their mouth. therefore, when of mouth side is more common fish become vary of that side but not of the other, this is why is it good to have a low frequency
what is the equilibrium stability of negative frequency?
stable because as one moves up, it will soon get knocked back down
what is the sentence that explains sex ratio in terms of negative frequency dependent selection?
Because the average per capita reproductive success of the minority sex is greater than that of the majority sex, selection favors genotypes whose individual sex ratios are biased toward that sex that is in the minority in the population as a whole
what is mullerian mimicry?
the act of mimicking another species to benefit from their selective advantage
when an drift be good in terms of under dominance?
it can move one fitness peak to another higher fitness peak, during episodes of very low population size, allelel freuqcnies may fluctuate so far by genetic drift that the alllele frequency crosses the fitness valley, then selection can increase it to the peak.
what is background selection?
when the neutral linked genes to an deleterious mutation are removed from a population too- causing a decrease in diversity of haplotypes the effective population size for these loci is reduced to the number of gametes that is free from delterious mutation - reduction in heterozygosity
what is the probability of fixation due to selection for diploids and haploid?
haploids= 2s diploids= 1(1-h)s
what is the probability of fixation of an allele due to drift?
its frequency
