Population Genetics 1 Flashcards
Main idea of fitness
Individuals show variation in phenotypic traits linked to survival and competitive ability
Main idea of natural selection
Individuals that are most likely to reproduce pass on the heritable traits to future generations
Adapative radiation
Diversification of a group of organisms into forms filling different niches
Main idea of speciation
Over time populations become divergently adapted to different environments ultimately resulting in speciation
Three parts of Darwin’s theory
Fitness
Natural selection
Speciation
What is the Hardy Weinberg principle?
Provides mathematical equation that can be used to calculate frequency of the alleles of a particular gene in a population
What does the hardy Weinberg equation assume?
The proportion of dominant and recessive alleles
of any gene in a population
remains the same
from one generation to the next
Hardy Weinberg equation
P^2 +2pq + q^2 = 1
p
Frequency of dominant allele
q
Frequency of recessive allele
What rule do we need to calculate probability of getting homozygotes? (AA/bb)
Multiplication rule
What is the multiplication rule
The probability of two independent events both occurring is
the production of the probability of each event occurring
What rule do we need to calculate the probability of getting heterozygotes (Ab/ aB)
The addition Rule
What is the addition rule
Probability of any one of a set of mutually exclusive events occurring = The sum of the probabilities of the individual events
Eg. Probability of rolling a 3 with 2 dice roles
1/6 (probability of rolling a 2)x1/6(probability of second role being a 1) + 1/6(probability do first role being a 1)x1/6(probability of second role being a 2) = 2/36
Probability of getting PP (green in the guys diagram)
Sperm = has p and q alleles Egg = has p and q alleles
Probability of getting two p alleles (two independent events occurring would be the zygote getting a p allele from speed and a p allele from egg) = product or probability of each event occurring (p x p)
Where 2PQ is derived from
Q x P
P x Q
Combined probability = the sum of probabilities of individual events (selection of a p and q allele = one event)
= 2pq
What does HW assume about mating and populations?
Mating is random
Populations are infinite in size
What does HW assume about mating and populations?
Mating is random
Populations are infinite in size
Genetic drift
Stochastic changes in allele frequency due to chance in small populations
Example of genetic drift
2 yellow peas, four green
Creature munches all four green
So just yellow peas left
Two yellow pease by chance produce all yellow peas
What does genetic drift do
Reduces genetic variation
If an allele becomes highly frequent because of genetic drift is that evolution?
Yes
Evolution is about changes in phenotypes over time (genetic drift can drive that)
Key thing about NS in comparison is it drives adaptation
Relationship between strength of genetic drift and population size
As population size increases, strength of genetic drift increases
Fitness
Relative reproductive rate of individual with a given genotype
Selection coefficient (s)
Change in fitness associated with the allele when its homozygous
When is an allele beneficial
When s>0
When is an allele deleterious
When s<0
What is dominance (h)
Extent to which the effect of the allele on fitness is masked when it’s heterozygous
When is an allele completely dominant
When h = 1
When is an allele completely recessive
When h = 0
What two things does change in allele frequency depend on
- Fitness of each genotype
2. Average fitness of population
How to calculate average fitness
Frequency of each genotype x it’s fitness
Define selective sweep
When an allele goes from a low frequency to a very high frequency in a population
(Sweeping out all other genetic variation at that locus in the population)
What is there to note about time taken for selective sweeps
It varies depending if it’s recessive, dominant or codominant
VEVOX
Why does dominance accelerate adaption?
Rare mutations are almost never homozygous
(Recessive alleles can only produce a benefit/effect if they are homozygous because otherwise they just get masked by dominant allele. But it’s just a thing that in random mating recessive alleles are rare in homozygous form)
Stickleback
Freshwater stickleback = loss of body armour
Marine sticklebacks = body armour
Freshwater lakes colonised by marine sticklebacks.
Introduced transgenic stickleback with recessive allele, found over time recessive allele increased in population
What is the marine stickleback thing an example of?
X
Define migration rate
Probability an allele copy arrives on an Island by migration
How does migration prevent divergence between populations
XX
If you allow migration then over time the allele frequency converges to average frequency
What does migration do?
Prevents genetic divergence between populations
Strong homogenising force in population genetics
Genetic diversity is shaped by interplay between
Migration = increases genetic diversity
Genetic drift = reduces diversity by chance
Selection = selective sweep (reduces diversity as high fitness allele sweeps low fitness allele out of population)
Alcohol dehydrogenase in drosophila example
Frequency of F allele decreases at lower latitudes (looks like it’s selection)
Overall trend for F allele at top but lots of variation around that (this is being shaped by migration, selection and drift)
Why does population genetics matter?
What two things does it help us understand and what does it fuel
Genetic diversity provides fuel for adaptation
- How genetic variation is organised
- How selection alter genetic diversity
