4B Flashcards
What is a population?
What is a population?
* Interbreeding group of
individuals that belong to the
same species and live within
a restricted geographical are
The frequency of an allele in a population is often ___ ___ ____ as the ratio in a ____ ____
* Population Genetics: consequences of ________ _________ in a population
* shift from individual to population-level
thinking
* What do we expect
The frequency of an allele in a population is
often not the same as the ratio in a single cross
* Population Genetics: consequences of
Mendelian Genetics in a population
* shift from individual to population-level
thinking
*The frequency of an allele in a population is often
not the same as the ratio in a single cross
What is our working definition for evolution?
A change in allele frequencies in a population over
generations
Darwin’s Four Conditions for
Evolution by Natural Selection
1.Individuals within a species VARY
2.Some variation is passed to offspring (heritable)
3.More offspring are produced than can survive and/or reproduce
4.Survival and reproduction is not random, but related to phenotypic
variation
Evolution by Natural Selection will not occur when
-no variation, or
* variation is NOT heritable, or
* variation is heritable but has NO fitness consequence
What happens to genetic variation in the absence
of Evolution
Phenotype- >(no migration)->(no natural selection)->reproductive success ->genotype -> (no mutation)-> phenotype (*triangle thingy again)
How do we test hypotheses?
- Need to establish that there is something to explain at all (i.e. are
allele frequencies in a population changing; is evolution
happening?) - establish the default condition, what we expect if any observed
differences are from chance alone, called the null hypothesis
if evolution is change in allele frequencies over time, then
null hypothesis should be ___ _____ in allele frequencies over time
if evolution is change in allele frequencies over time, then
null hypothesis should be no change in allele frequencies
over time *soully cause of chance
Question: What will happen to single
trait, at a single genetic locus, that is
encoded by two alleles, in the absence
of evolution?
In 1908, GH Hardy and W. Weinberg used
probability theory to solve this problem for
2 alleles at a single locus. the theory was that if we dont have any evolutionary chnages to a trait that trait should stay stable forever
Genetic Diversity in the absence of Evolution
The assumptions of their model considered a “population” with the
following characteristics:
Population 1
No new alleles arise in the population
1)A single locus with two alleles does not change state between generations, i.e. no mutation!
2) Alleles are not added to the population,
No gene flow from other populations
(no migration in and no migration out)
3)3) The population is very big (infinite in theory)
4) This assumption eliminates the effect of random
processes that could change allele frequencies
(ie, no genetic drift)
Drift : allele frequencies can change by chance alone given the number of
matings that can occur when you have a low number of individuals
compared to a large number of individuals
4) Natural selection does not affect the alleles considered
Regardless of their genotype at the locus of interest,
all diploid individuals have the same fitness
Probability of surviving to breed is the same
Mating and fertilizing ability is the same
5)5) Random mating
Regardless of their genotype at the locus of interest, all diploid
individuals have the same fitness
HWE conditions:
Hardy-Weinberg equilibrium
* Population allele frequencies do not change if:
–There is no mutation
–There is no migration (gene flow)
–Population is infinitely large
–Genotypes do not differ in fitness (no selection)
–Mating is rando
Under large population size, no _____, no _______ ______, no ____ ____, and random mating – allele frequencies will be _______ between
generations _______
Under large population size, no mutation, no natural selection, no gene
flow, and random mating – allele frequencies will be constant between
generation forever
-random mating - gametes _____ _______
* ∴ haplotype of egg independent of haplotype of sperm as it fertilizes that
egg to form a _____
* Probability theory - probability of two independent events occurring
together is the product of their individual __________
Thus, P(event 1 and event 2) = P(event 1) x P(event 2), if events 1 and 2 are
independent
random mating à gametes unite randomly
* ∴ haplotype of egg independent of haplotype of sperm as it fertilizes that
egg to form a zygote
* Probability theory à probability of two independent events occurring
together is the product of their individual probabilities
Thus, P(event 1 and event 2) = P(event 1) x P(event 2), if events 1 and 2 are
independent
Given HW assumptions, result occurs _____________ of _________ in the previous generation
* In _________ of changes in allele frequencies, this combination of
genotypes persists _________ = HW
Given HW assumptions, result occurs regardless of genotypic
frequencies in the previous generation
* In absence of changes in allele frequencies, this combination of
genotypes persists indefinitely = HW
Because the gametes are ______, and mix ________, if there is no
selection at the genotype level, all gametes goes back into gene pool
* even if a population is not in HWE, one round of _________ mating with
the other conditions met (__ _______ or _______ and a ___ ________) will return the population to equilibrium
* This result can be generalized to any number of alleles at a ______ ______: i.e. ___________
Because the gametes are haploid, and mix RANDOMLY, if there is no
selection at the genotype level, all gametes goes back into gene pool
* even if a population is not in HWE, one round of random mating with
the other conditions met (no evolution or migration and a big
population) will return the population to equilibrium
* This result can be generalized to any number of alleles at a single
locus: i.e. (p + q +…0=1)
These genotype frequencies are reached after a single generation of random mating
from any ______ _______ frequencies.
These genotype frequencies are reached after a single generation of random mating
from any initial allele frequencies.
Population allele frequencies do not change if:
- Population allele frequencies do not change if:
–There is no mutation
–There is no migration (gene flow)
–Population is infinitely large
–Genotypes do not differ in fitness (no selection)
–Mating is random
HWE tells us when evolution __ ___ ______
– ______ model or_________ in evolution
* Departure from equilibrium suggests that one or
more of the assumptions has been ______
* Can also use HWE to predict ________ __________ when we have less information
about a _________
* Note: even if a population is not in HWE, this does not prove that it is _________ - we need to
see a ________ _______ in allele frequencies
HWE tells us when Evolution is NOT occurring
– null model or hypothesis in evolution
* Departure from equilibrium suggests that one or
more of the assumptions has been violated
* Can also use HWE to predict genotype
frequencies when we have less information
about a population
* Note: even if a population is not in HWE, this
does not prove that it is evolving - we need to
see a sustained change in allele frequencies
In general, in a sample of n individuals, the
frequency of an allele is
the number of occurrences of the allele divided by twice the number
of individuals in the sample (2n)
Calculate allele frequencies, p and q
use : freq(p)=p^2=p^2+0.5(2pq)
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