Quest 4 Flashcards
Law of large numbers
as the size of a random sample increases, realized frequencies will be close to expected frequencies
genetic drift
random fluctuation in allele frequencies due to sampling effects in small populations
Wright-Fisher Model
Like HWE but relaxes the infinite population size assumption
Wright-Fisher Model assumptions
- NS is not acting on traits in question
- Mating is random
- No mutation
- No migration
selectively neutral
alleles with no fitness difference between them, NS does not act on the frequencies of these alleles
observed heterozygosity
fraction of individuals in the population that are heterozygous at the given locus
expected heterozygosity
fraction of heterozygotes expected under HWE model given the allele frequencies in a population
Ne
effective population size
Effective population size
The number of individuals in a population that are actively breeding
population bottle neck
brief period of small population size
founder effect
change in allele frequencies from sampling effects when a small number if individuals from a large population colonize a new area
leading edge expansion
form of drift: founder effects cause reduced genetic diversity in a newly colonized area
He
heterozygosity
neutral theory of molecular evolution
biologists look at DNA, RNA and amino acid sequences and how they change over time as opposed to phenotype
assumptions of neutral theory of mol evolution
- most variation in a pop. is selectively neutral.
- most change in DNA and amino acid sequence are selectively neutral
substitution
-new allele arises from mutation and is subsequently fixed in the population
-MOST SUBTITUTIONS ARE NEUTRAL NOT MOST MUTATIONS
mutation
a change in the genes DNA sequence to produce something different
Pseudogenes
nonfunctional, untranslated segment of DNA that arise from previously functional segment and show evolutionary history as they are derived from known homologous genes
positive selection
promotes the spread of beneficial alleles, beneficial allele will be favored in the population
purifying selection
minimizes the spread of deleterious alleles, deleterious alleles will be selected against in a poulation
Molecular clock
DNA and protein sequences that evolve constantly over time among different organisms
equidistant principle
if molecular changes are constant, members of a clade should be equidistant from the outgroup
Coalescent time
number of generations to go back for a population to be reduced to two parental lineages. RECENCY OF COMMON ANCESTRY
polygenic trait
trait that is affected by many genes simultaneously
additive genetic effects
alleles of single genes combine to equal the sum of their individual effects
Latent variation
-So much variation within populations that not all possible genotypes could be represented
-selection shifts allele frequencies even in the absence of mutations
epistasis
effect of a gene mutation is dependent on presence or absence of mutations in one or more other genes
haplotype
Set of gene copies that tend to be inherited together
Physical Linkage
two genes are located on the same chromosome
linkage disequilibrium
-non-random association of alleles at different loci
-loci in LD when frequency of association is higher or lower than what would be expected.
genetic hitchhiking
when an allele changes frequency because it is near another gene that is undergoing a selective sweep and that is on the same DNA chain
background selection
loss of surrounding alleles when a deleterious mutation is selected against
selective sweep
advantageous mutation reduces variation in linked sites as it increases in frequency in the population
3 consequences of genetic drift
- allele frequencies fluctuate over time even in the absence of NS
- some alleles are fixed and some lost, the fraction of heterozygotes in pop decrease over time
- separate populations diverge allele frequencies
New Zealand Snappers
small effective population size. Have a population size of around 3 million fish but only around 200 of them can reproduce, so their Ne is 200
divergence between populations of Lava Lizards
population subdivision: limited or no gene flow between subpopulations of a larger population
Manx cats
founder effect: prevalence of the M allele on the Isle
Northern Elephant Seal
population bottleneck: causing low levels of heterozygosity, skewed reproductive success
Spruce trees in leading edge expansion
two hallmarks for genetic drift: low diversity within, high diversity between
selection and drift interactions
selection and drift are not mutually exclusive, selection must be strong to reach fixation due to drift on the population.
Neutralist and Selectionist debate
dispute on weather drift or selection is the primary driver of evolutionary change.
Do molecular clocks work?
useful when comparing a single locus over a short time for closely related organisms
effects of population size on coalescence
coalescence occurs more quickly in smaller populations compared to larger populations
effects of selection on coalescence
selection drives the alleles to fixation quickly. When under positive selection coalescence occurs more quickly.
antibiotic resistance and compensatory mutations
mutation at one lotus confers antibiotic resistance and a compensatory mutation at a second locus reduces the fitness cost of resistance
Most variation seems continuous because…
polygenetic traits are continuous
Ways LD can happen
-selection
- rate of genetic recombination
- genetic drift
- system of mating
- population structure
- genetic linkage
How does loss of linkage disequilibrium happen?
meiosis with crossing over and outbreeding (asexual reproduction)
