How heterozygote advantage and heterozygote disadvantage affect ...
Lecture 15: How heterozygote advantage and heterozygote disadvantage affect genetic variation: The heterozygotes advantage : --simUtext: heterozygotes have higher fitness than the other homozygotes.: Make the heterozygotes beneficial more than the other homozygotes. What happens is the freqencies of allele stabilize 50-50 ratio In genotypes we see that there are of heterozygots and a few of homozygotes. When a heterozygotes advantage at some allete, does the population experience selection? A- yes B- no C- at first yes, but only until allele frequencies have stabilized. -SOLUTION: yes, selection happen constantly all the way through. Selection happen when ever there is a fitness difference between the various genotypes. Heterozygotes advantage is a type of selection. 2- is it a HWE proportions? A- yes B- no SOLUTION: no, b.c. every time we have x and y alleles this must happen x=.5 and y=.5 But in this situation in pig there were more heterozygotes than we would expect under HWE in absence of selection. Then heterozygotes advantage is a kind of selection that affecy HWE
3- heterozygotes advantage, does the population evolve? A-yes B- no C- at first yes ,but only until allele frequencies have stabilized. SOLUTION: only at first and then stop stabilize. Once these alleles reach HWE, the allele frequencies stop changing, so evolution s defined an allele frequencies change from generation to the next. Selection does not always result in evolution b.c. selection is still happening but evolution is not happening and we can have evolution without being the result of selection! ------------ If heterozygotes (RS) have the highest fitness, but RR homozygotes are fitter than SS, what will eventually happen to alleles R and S? A-f(R) will reach 1, f(S) will reach 0 B-allele frequencies will stabilize at f(R)=f(S)=0.5 C-allele frequencies will stabilize at f(R > f(S) > 0 SOLUTION: C, it stabilize but it doesn‟t reach a 50-50 ratio 2- What if selection favour both homozygous genotypes (RR,SS) equally (heterozygote disadvantage)? A-the allele with the higher starting frequency will reach a frequency of 1, the other will reach 0 B-allele frequencies will stabilize at f(R)=f(S)=0.5 C-allele frequencies will fluctuate randomly -SOLUTION: it is A, the more starting allele went to fixation, so where as heterozygotes advantage is a type of selection that is fundamentally stabilizing type of selection that tries to maintain more than one allele in a population for long period of time,
heterozygotes disadvantage is exactly the opposite, it is a fundamentally unstable situation and very short lived in which any allele begins in low frequency in a population and in which any high frequency at starting have huge advantage over the other one . The reason of that is that the common allele is less likely to find itself in the body of a heterozygote This is not a very common situation in nature b.c. it is very short lived period. Whether selection always results in evolution: Selection can vary over time & space… Adaptations are environment-specific: -Selection vary overtime(temperature… wet dry …) -We end up in an adaptive trait that is always going to be environment specific. (ex. White and black mouse the color was selectively favoured because of environment “the different colored onse might be killed by preys ….” -Selection pressure could change as we move from place to plcace or from time to time -How do you fluctuating selection pressure affect genetic variation? The more that environment change over time and space, the more genetic variation tend to see maintained by selection
How positive and negative frequency-dependent selection affect genetic variation:
Negative frequency-dependent selection e.g. predators prey disproportionately on most common prey effect on genetic variation? phenotype; other examples? -There is a selective advantage of being rare ( we see this as a predictive prey interaction: if there is a predator it is easier to focus on the common type of preys (make a search image) “ black an grey squirrels which ever type of squirrel is common the predator is going to look for the common one” -the effect is that the allele and phenotype is maintained in the population b.c. there is a fitness advantage to being rare the rare will increase in frequency but as soon it increases to 50% of the population it no longer enjoys that rare phenotype advantage and the selective advantage flips to favour the other types, so it preserves high level of genetic variation.
Positive frequency-dependent selection E.g. predators learn to recognize prey warning coloration through bad experiences; tend to avoid common prey phenotypes -Negative frequency dependent selection is wide spread but positive is much more harer to come up with example -It is simply the opposite of negative frequency-dependent selection: -The individauls get the advantage of being common form in a population (ex. Prey and predator relationship: poison frogs that have developed these worning colors and predators have learned through bad expriences “I ate that colored frog and it tasted
terrible” so they learn to avoid these phenotype…. Then there an advantage of being the common phenotype in this situation b.c. the chances that a predator had a bad experience from another prey that looked the same is high -The effect: We would expect that when we start with positive frequency-dependent selection in a population with higher starting numbers it will reach to fixatioin eventually and the genetic variation will be lost! Effect of genetic drift on allele frequencies and genetic variation within a population: Genetic drift: random sampling error due to finite population size
-Unpredictable change in allele frequencies -Eventually, one allele „drifts‟ to fixation, other alleles lost (less variation) -Strongest in small populations, and populations suddenly becoming small Effect of genetic drift on variation (differences) between populations: Genetic drif REMOVES genetic variation from populations especially in small populations “see: bottleneck ex. Cheetahs & founder effect ex. Extra finger” The larger the population is the less likely for genetic drift to happen. And the differences still differ from one another So, genetic drift reduces genetic variation! Whether or not mutations are directed toward the needs of the
organism: Mutation creates new alleles (it is the only source of new alleles and genetic variation in a population so if there is no mutation in a population then there will be no variation for selection to act and genetic drift couldn‟t happen) Mutation is not “random”, but not directed towards the needs of the organism either -how do mutations affect fitness? It have little effect on fitness! -Of those that affect fitness, most are harmful! -mutation often opposes selection, but also provides raw material for adaptive evolution
General fitness effects of mutations: -Other evolutionary mechanisms that cause HWE to change: -One of non-adaptive evolutionary forces is mutation! Because it is the only source of new alleles. -mutation often opposes selection, but also provides raw material for adaptive evolution. However it is important for selection because selection require genetic variation and mutation is the only source of that raw material! Why most mutations that affect fitness are harmful: The general effects of mutation on fitness recall that most mutations have actually no effect on fitness or very little effect on fitness, but the ones have effect on fitness, they tend to be harmful, ex. When there is a certain kind of protein to be copied
and there is mutation, it wont be copied as good as if it was no mutation because the organisms genes are the product of millions and million of years of natural selection that are now well adapted to the environment (ex. Take the laptop and put it apart then try to put it together again, the odds are that it wont be as good as it was again!) thus, there are more odds on having something worse than the previous one with mutations. Effect of gene flow on genetic variation within a population: The effect of gene flow (migration) on genetic variation: it has a similar effect to mutation Effect of gene flow on variation (differences) between populations: Gene flow (migration) moves alleles between populations •
Like mutation, can introduce new alleles
•
Like mutation, often opposes selection (selection-migration balance)
How various evolutionary forces interact with (reinforce or oppose) one another: Selection (several kinds) Genetic drift Mutation Gene flow Which have similar effects on genetic variation within a population? On differences between populations?
Reasons why not all living things are perfectly adapted to their
environment: •
Many factors constrain (limit) selection (see kinds of selection) –
Examples?
•
Non-adaptive evolutionary forces often oppose effects of selection
•
But without these other evolutionary forces, adaptive evolution would stop
3- heterozygotes advantage, does the population evolve? A-yes B- no C- at first yes ,but only until allele frequencies have stabilized. SOLUTION: only at first and then stop stabilize. Once these alleles reach HWE, the allele frequencies stop changing, so evolution s defined an allele frequencies change from generation to the next. Selection does not always result in evolution b.c. selection is still happening but evolution is not happening and we can have evolution without being the result of selection! ------------ If heterozygotes (RS) have the highest fitness, but RR homozygotes are fitter than SS, what will eventually happen to alleles R and S? A-f(R) will reach 1, f(S) will reach 0 B-allele frequencies will stabilize at f(R)=f(S)=0.5 C-allele frequencies will stabilize at f(R > f(S) > 0 SOLUTION: C, it stabilize but it doesn‟t reach a 50-50 ratio 2- What if selection favour both homozygous genotypes (RR,SS) equally (heterozygote disadvantage)? A-the allele with the higher starting frequency will reach a frequency of 1, the other will reach 0 B-allele frequencies will stabilize at f(R)=f(S)=0.5 C-allele frequencies will fluctuate randomly -SOLUTION: it is A, the more starting allele went to fixation, so where as heterozygotes advantage is a type of selection that is fundamentally stabilizing type of selection that tries to maintain more than one allele in a population for long period of time,
heterozygotes disadvantage is exactly the opposite, it is a fundamentally unstable situation and very short lived in which any allele begins in low frequency in a population and in which any high frequency at starting have huge advantage over the other one . The reason of that is that the common allele is less likely to find itself in the body of a heterozygote This is not a very common situation in nature b.c. it is very short lived period. Whether selection always results in evolution: Selection can vary over time & space… Adaptations are environment-specific: -Selection vary overtime(temperature… wet dry …) -We end up in an adaptive trait that is always going to be environment specific. (ex. White and black mouse the color was selectively favoured because of environment “the different colored onse might be killed by preys ….” -Selection pressure could change as we move from place to plcace or from time to time -How do you fluctuating selection pressure affect genetic variation? The more that environment change over time and space, the more genetic variation tend to see maintained by selection
How positive and negative frequency-dependent selection affect genetic variation:
Negative frequency-dependent selection e.g. predators prey disproportionately on most common prey effect on genetic variation? phenotype; other examples? -There is a selective advantage of being rare ( we see this as a predictive prey interaction: if there is a predator it is easier to focus on the common type of preys (make a search image) “ black an grey squirrels which ever type of squirrel is common the predator is going to look for the common one” -the effect is that the allele and phenotype is maintained in the population b.c. there is a fitness advantage to being rare the rare will increase in frequency but as soon it increases to 50% of the population it no longer enjoys that rare phenotype advantage and the selective advantage flips to favour the other types, so it preserves high level of genetic variation.
Positive frequency-dependent selection E.g. predators learn to recognize prey warning coloration through bad experiences; tend to avoid common prey phenotypes -Negative frequency dependent selection is wide spread but positive is much more harer to come up with example -It is simply the opposite of negative frequency-dependent selection: -The individauls get the advantage of being common form in a population (ex. Prey and predator relationship: poison frogs that have developed these worning colors and predators have learned through bad expriences “I ate that colored frog and it tasted
terrible” so they learn to avoid these phenotype…. Then there an advantage of being the common phenotype in this situation b.c. the chances that a predator had a bad experience from another prey that looked the same is high -The effect: We would expect that when we start with positive frequency-dependent selection in a population with higher starting numbers it will reach to fixatioin eventually and the genetic variation will be lost! Effect of genetic drift on allele frequencies and genetic variation within a population: Genetic drift: random sampling error due to finite population size
-Unpredictable change in allele frequencies -Eventually, one allele „drifts‟ to fixation, other alleles lost (less variation) -Strongest in small populations, and populations suddenly becoming small Effect of genetic drift on variation (differences) between populations: Genetic drif REMOVES genetic variation from populations especially in small populations “see: bottleneck ex. Cheetahs & founder effect ex. Extra finger” The larger the population is the less likely for genetic drift to happen. And the differences still differ from one another So, genetic drift reduces genetic variation! Whether or not mutations are directed toward the needs of the
organism: Mutation creates new alleles (it is the only source of new alleles and genetic variation in a population so if there is no mutation in a population then there will be no variation for selection to act and genetic drift couldn‟t happen) Mutation is not “random”, but not directed towards the needs of the organism either -how do mutations affect fitness? It have little effect on fitness! -Of those that affect fitness, most are harmful! -mutation often opposes selection, but also provides raw material for adaptive evolution
General fitness effects of mutations: -Other evolutionary mechanisms that cause HWE to change: -One of non-adaptive evolutionary forces is mutation! Because it is the only source of new alleles. -mutation often opposes selection, but also provides raw material for adaptive evolution. However it is important for selection because selection require genetic variation and mutation is the only source of that raw material! Why most mutations that affect fitness are harmful: The general effects of mutation on fitness recall that most mutations have actually no effect on fitness or very little effect on fitness, but the ones have effect on fitness, they tend to be harmful, ex. When there is a certain kind of protein to be copied
and there is mutation, it wont be copied as good as if it was no mutation because the organisms genes are the product of millions and million of years of natural selection that are now well adapted to the environment (ex. Take the laptop and put it apart then try to put it together again, the odds are that it wont be as good as it was again!) thus, there are more odds on having something worse than the previous one with mutations. Effect of gene flow on genetic variation within a population: The effect of gene flow (migration) on genetic variation: it has a similar effect to mutation Effect of gene flow on variation (differences) between populations: Gene flow (migration) moves alleles between populations •
Like mutation, can introduce new alleles
•
Like mutation, often opposes selection (selection-migration balance)
How various evolutionary forces interact with (reinforce or oppose) one another: Selection (several kinds) Genetic drift Mutation Gene flow Which have similar effects on genetic variation within a population? On differences between populations?
Reasons why not all living things are perfectly adapted to their
environment: •
Many factors constrain (limit) selection (see kinds of selection) –
Examples?
•
Non-adaptive evolutionary forces often oppose effects of selection
•
But without these other evolutionary forces, adaptive evolution would stop
