EB8 Flashcards
(28 cards)
what usually happens when there are two alelles for a gene in a population
what can happen
usually one allele becomes fixed.
ocassionally selection changes favouring one or another allele depending on the genetic and environmental background of the allele and/or the allele frequency. = balancing selection
what are the three processes leading to balancing selection with their examples.
- Heterozygote advantage: CF, SCA, Soay Sheep
- Negative frequency dependent selection: snails and parasites, elderflower orchids, prey switching
- Positive frequency dependence: driving on the wrong side of the road, Mullarian mimicry butterflies
what is favoured if it is advantageous to have both alleles present in a genome
a gene duplication will be favoured with each gene hosting both alleles = this tends to remove heterozygote adv.
when does heterozygote adv. occur
when the hetero is more fit than either of the homos preventing extcintion of one allele - heterozygote adv. is unlikely to be long lasting in evo time.
how does heterozygote adv. contain an element of frequency dependence
when mutation arises leading to favourable hetero, freq of mutation increases, when it becomes common it will occur in a homo state and be less favourable (freq dep)
What is CF caused by and what is the frequency of disease causing allele
loss of function of CFTR (deletion F508)
strong selection against the disease; little survival to reproductive age
freq: 2% in europeans (q=0.02)
what does modelling CF tell us
new mutations cannot explain the maintenace of CF at high freq of 0.02
how can CF be modelled to tell us that frequency is not due to mutation selection balance
Recessive and nearly lethal so assume h~0, s~1
Fitness of homozygote = q’ q’ =√(u⁄s) , u = 0.02^2 = 0.0004
Actual mutation rate is lower = 6.7*10-7 therefore mutation can’t explain frequency.
what are the two beta globin alleles and what are their frequencies and patterns
A and S beta globin alleles
S frequency = 15% in some parts of Africa yet homo is virtually lethal.
*prevalance of malaria correlates with HbS allele frequency
what did field data for malaria and SCA allele tell us
over 12000 ind. measured in Nigeria (malarai zone) freq A and S measured. showed not in HWE.
The inverse of the respective selection coefficent is equal to the ratio of allele frequency q/p = s/t
s= SC for AA, t= SC for SS
what could we expect to happen to S allele if malaria was eliminated
begin to be selected against in population
what do males of Soay sheep of St. kilda compete for and what is correlated with success
males compete for access to oestrous females
horn size postively correlated to male reprodutcive success
what gene is responsible for the genetic variation in horn morphology in Soay sheep
RXFP2 relaxin like receptor
two alleles
Ho+ for large horns
Hop = small horns
what did the long term study on 1700 sheep over 21 years show
Ho+/+ had highest reproductive success and lowest survival success vv for Hop/p.
heterozygote only marginally lower for both.
male fitness highest for hetero (Ho/p) and marginally higher for Ho+/+ than Hop/p
What is negative frequency dependence
fitness is a function of frequency
selection favours rare alleles (fitness is decreasing function of frequency of allele)
how does negative frequency dependence allow two alleles to exist in a population
As the frequency of one type increases its fitness declines
which freshwater snails and parasites are invovled in neg freq dep
freshwater snail P. antipodarum and trematode parasites Microphallus spp.
describe the negative frequency dependence of snails and parasites
parasites are best at infecting snails of the same lake (adaptation to local genotype) and best at infecting common clones of snails
*asexual snails, rare clones escape infection from locally adaptive parasites favouring rare genotypes.
clones become more common and are more likely to be infected
what are flowers show neg freq dep
Elderflower orchids: D. sambucina
how do elderflower orchids show negative frequency dependence
orchids pacakge pollen which is picked up by bumblebees visiting in search of nectar.
orchids may cheat bumbles by producing no nectar in some flowers leading to risk of bumbles learning to avoid empty flowers.
what was the experiment showing negative frequency dependence in elderflower orchids
10 plots of 50 flowers with varying Y and P flowers
** genetic polymorphism for flower colour; yellow and purple, yellow is cheat with no nectar.
when Y is rare more fit, more pollena nd bees and more fruit
when Y is common less fit than P, as bees presumably learned to avoid
what is prey switching behaviour
rare morphs of prey are fitter as predators concentrate on more frequent morphs`
what is positive frequency dependence
selection favours common alele which drives fixation and removes variation
give an example of positive frequency dependence
H. erato and H. mepomene butterflies differ widely in warning patterns across geographic locations, (both being distastful) but within one area they share the same patterns. this is because deviations from this pattern are likley to be eaten as they would not be recognised as being distasteful.
