Population Genetics Flashcards
1.Natural selection acts upon the ____.
2.Evolution is measured at the ____ level.
-individual
-population
The mating of two purebred individuals having a trait that is caused by 2 alleles within a population yields a…
F1 generation of heterozygous individuals
The crossing of hybrids (aka an F1 generation from a purebred cross) yields an F2 generation of individuals that have —- # of distinct phenotypes, and —– # of distinct genotypes.
-2
-3
yields 1 homozygous dominant (RR), one homozygous recessive (rr) and 2 heterozygous (Rr)
What equation represents the resulting genotypes of individuals within a population if alleles are randomly selected in pairs from a pool of independently occurring alleles within the population?
(this is saying the alleles did not come from a single organism)
(RxR) + (rxR)+ (rxR) + (rxr)=1.0
combined to R^2 + 2Rr + r^2
We can also say that the alleles R and r represent…
the frequencies at which the alleles R and r occur in our population
Why are the allele frequencies multiplied together (ex. RxR/rxr) in the HW equation?
Because the alleles are independently passed from one generation to the next, and represent the probability that those particular alleles will be paired up in our random draw and subsequent mating.
Why do the total accumulation of the genotypes in HW equation equal one?
Because they represent the entire (100% of) the population.
The HW equation is a representation of…?
The frequency of the different genotypes within the generation created by the hybrid (Rr) parents.
P (variable p) represents? Q (variable q) represents?
Each represent the frequency of one allele in the gene pool. P (p) represents the frequency of the dominant allele in the gene pool. Q (q) represents the frequency of the recessive allele in the gene pool.
Why must p+q=1?
Since there are only two alleles at this locus in the pool, the sum of their frequencies must equal 100% of the alleles.
p+q=1, but, what can their combined genotypic frequencies be expressed as?
The Hardy-Weinberg Equilibrium equation (p^2 + 2pq + q^2)
If more than 2 alleles are present in a population at the same locus, their combination is still…
a product of the sums of the alleles.
Ex. if there are 4 alleles (p ,q ,w , z) then p+q+w+z=1.0; the resulting genotypic frequency would be p^2 + 2pq + 2pw + 2pz + q^2 + 2qw + 2qz + w^2 + 2wz+ z^2
Obviously, the genotype represented by p, q, w, and z cannot exist in a single organism where only two alleles are allowed (they are diploid!), but the combination of the 4 alleles and their frequencies can be assembled…
in an imaginary individual that represents an average individual (of sorts) in the population that has each of the alleles at the given frequencies
Before H-W we thought…
- Dominant traits would eventually go to fixation in a population and recessive traits (unless they were selected for) would go awat
- Or, that alleles would get distibuted evenly (50/50 in a 2 allele set)
- And/or the only way alleles change frequency was through sexual reproduction
H-W says:
- p is always the same generation after generation
- q is always the same generation after generation
Assumptions of the H-W model
- random mating
- no mutation
- no migration
- infinite population size
- no natural selection
Notes about the HW assumptions
- Assumptions are with respect to particular alleles at the locus in question
- If any of these assumptions are violated, then H-W does not apply
- If the allelic frequency does not match from one generation to the next, then something is happening to the population… points to one or more of the assumptions as the culprit
H-W as a null hypothesis
- The H-W equilibrium hypothesis acts as a null hypothesis for the sake of determining whether genetic changes are taking place at a particular locus within a population
What is a null hypothesis?
A standard, caused by randomness, that we compare our actual data against to determine if change has occurred.
-If the change in our data from one generation to the next is greater than what randomness would allow for (the HW model) then we say that the null (HW) is rejected, and that the population has evolved (there has been a change in the allele frequencies over generations)
If the change in data is within what randomness would predict, then the population has not changed, and evolution has not taken place.
If you detect change in a population (HW says u shouldn’t) you must assume that…
HW does not fit your population (you reject HW) and that something is causing the changes (likely a violation of one of the assumptions).
Then your job is to find out what it is that is causing the change (this is what population ecologists can do)
If you detect change in a population (HW says u shouldn’t) you must assume that…
HW does not fit your population (you reject HW) and that something is causing the changes (likely a violation of one of the assumptions).
Then your job is to find out what it is that is causing the change (this is what population ecologists can do)
