Chapter 20 (Final) Flashcards
Mendelian vs population genetics
Generations and relationships
M = known
P = unknown
Number of alleles analyzed
M = 2 (usually)
P = variable (one to thousands)
Influencing forces
M = known + controlled
P = unknown + inferred
Mode of reproduction
M = known
P = known or unknown
Population
a group of interbreeding organisms
Gene pool
the collection of genes and alleles found among members of a population
Population genetics
the study of allele frequencies and genotype frequencies within and between populations
Evolution
changes of allele frequency and genotype frequency over time
What is evolution influenced by (4 ex)
- mating patterns
- mutation rate
- genetic drift
- natural selection
etc
Hardy Weinberg equilibrium
- both independently concluded that random mating and absence of evolutionary change leads to stable allele frequencies in populations
model to calculate expected frequencies of alleles and genotypes of interest in large populations
H-W equilibrium - 6 assumptions
- Infinite population size
- Random mating within population
- No natural selection
- No migration/gene flow (no introduction of new alleles)
- No mutations (no introduction of new alleles)
- No genetic drift
H-W equilibrium - 4 predictions
- Allele frequencies remain stable over time
- Allele distribution into genotypes is predictable
- Stable equilibrium frequencies of alleles and genotypes are
maintained - Evolutionary and non-random mating effects are predictable
H-W equilibrium calculates expected genotype and allele
frequencies when evolution…
does NOT occur
Allele vs genotype frequency (calc)
Allele
p + q = 1
A1 + A2 = 1
Genotype
p^2 + 2pq + q^2 = 1
A1A1 + A1A2 + A2A2 = 1
Hardy-Weinberg Equilibrium for Two
Autosomal Alleles
When p = 1, one allele is fixed (no q)
When q = 1, other allele is fixed (no p)
Heterozygosity at highest frequency
when A1 = A2 = 0.5
2 methods to determine autosomal allele frequencies
- Gene-counting method
* Requires genotypes of all members to be identifiable
* Useful for codominant alleles - Square root method
* Used when gene has two alleles with a dominant-recessive relationship
* Simply take square root of q2, then p is calculated as 1-q
HW for 3 alleles
Allele frequency
p + q + r = 1
Genotype frequency
(p+q+r)^2 = 1
six possible genotypes
p^2+2pq+q^2+2qr+r^2 = 1
Natural selection
works through differential reproductive fitness and influences
genotype and allele frequencies of the next generation
Effect of natural selection
no longer HW equilibrium
Relative fitness (w) can quantify natural selection intensity
Selection coefficient (s)
Individuals that reproduce less have their fitness decreased by a proportion
called selection coefficient (s)
E.g, if individual A has a relative fitness of 1.0 and individual B has a relative
fitness of 0.8, the selection coefficient = 0.2
* Therefore, individual B reproduces 80% as well as individual A
Predicting genotypic and allelic frequency in the next generation
Predicting genotypic frequency in next
generation is equal to number of
individuals multiplied by relative fitness
Use gene-counting method to
calculate allele frequency in next
generation after selection
If strong selection intensity on an allele
directional selection
allele can become fixed over time
or v.v for lethal alleles
Selection favouring heterozygosity
balancing selection
balanced polymorphism frequency
Go over calculations for different selection models
missing some slides in the slideshow
Mutation (diversity)
the ultimate source of all new genetic variation in populations
Mutation rates
Forward mutation rate (μ): the rate of creating new alleles
Reverse mutation rate (v): rate of mutation to original allele
Gene flow + what it does
Occurs by movement of organisms and genes between
populations
Gene flow can:
* Introduce novel alleles
* Increase frequency of existing alleles
* Remove/reduce existing alleles
* Create admixed population
Genetic drift
Chance fluctuations of allele frequencies due to “sampling bias”
Occurs in all populations but is especially prominent in small populations
Brownian motion model
- random walk
- very small incremental changes
- changing over time
more significant in smaller pops
Inbreeding + issue
Inbreeding (consanguineous mating): mating between related individuals
increases homozygosity / decreases hetero
Inbreeding depression
the reduction in fitness of inbred organisms
Biological species concept
a group of organisms capable of interbreeding with each other but isolated from other species
“defines a species as a group of organisms that can reproduce with each other and produce fertile offspring”
Allopatric speciation
populations diverge due to physical barrier and thus new species develop in separate geographic locations
Sympatric speciation
populations share a territory, but are isolated by genetic, behavioural, temporal or other barriers that prevent gene flow
May coincide with allopatric speciation
* Populations first diverge when they geographically separate
* When they come back into contact they may further diverge in a sympatric fashion
Hybrid speciation
Formation of new species due to hybridization
between existing species
Hawaiian Drosophila species
- Phylogenetic relationships of
Hawaiian Drosophila species consistent with geological evidence
of island formation - Colonization of new islands =
allopatric speciation - This phenomenon often called
founder effect
Founder effect
- When a small population enters
into an isolated territory - Genetic drift + inbreeding can lead to changes in allele
frequencies
Mechanisms of reproductive isolation
Prezygotic (pre-fertilization) or postzygotic
Pre
- behavioural iso
- gametic iso
- geographic iso
- habitat iso
- mechanical iso
- temporal iso
Post
- hybrid breakdown
- hy inviability
- hy sterility
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