Selection Flashcards
Darwin’s Finches
an example of adaptive radiation/divergent adaption, where the variation was already present in the ancestral population but environmental influences (such as diet) dictated fitness in different descendent populations
Natural Selection
process where some of the inherited genetic variation results in differences in individual’s ability to produce healthy offspring
fitness
measure of an individual’s ability o survive and reproduce successfully
Positive (Darwin) Selection
process where an advantageous mutation increases in a population
Negative (purifying) Selection
process where a not-so-good mutation is removed from the population
Does selection cause evolution?
NO - Selection only works on exisiting diversity. Evolution does not cause a new mutation to occur, but once it is there, then selection can act on it.
Most common mutation?
neutral (no effect)
Fixation
when the allele frequency = 1 –> 2 copies present in every member of the poulation
What happens when the allele frequency equals 0?
the trait is lost from the population
Least common mutation?
positive
What can affect the rate that a trait rises to fixation?
dependent on the level of fitness advantage, initial allele frequency, and population size
3 Examples of Traits with positive selection
- Better ability to find/use food
- Disease resistance
- Improved fertility
Convergent Adaptation
two separate populations develop the same variant independent of one another
Heterozygote Advantage
where the heterozygotes have a higher fitness, and selection favors the maintenance of both alleles in a population via overdominant selection
Examples of Heterozygote Advantage
- Cystic fibrosis (1 copy resistant to typhus)
- Sickle Cell anemia (1 copy resistant to malaria)
Overdominant Selection
used in heterozygote advantage - think Africa and malaria
Do populations subject to overdominant selection have a higher frequency of recessive disorders?
Yes (but often can be homozygous lethal anyway)
How quickly are lethal mutations removed from the population?
Immediately. They’re dead, so
Genetic Drift
changes in frequency of neutral variants
occurs by chance –> RANDOM, no selective pressure
Can an allele reach fixation via genetic drift?
YES - bottlenecking, geographic isolation, stepping on a bug, and just time
Are allele frequency changes more prominent in small or large populations?
small –> easy to eliminate an allele
Genetic Drift vs. Selection
genetic drift is RANDOM
selection is NOT random and also directional in response to environmental pressures
Artificial Selection
aka humans messing with things
can be done via phenotype or genotype, depending on availability and/or industry standards
Pedigree Based Selection
Breed based on lineage, desirable traits should be passed down to offspring. Works well for simple traits but not anything more complex.
Estimated Breeding Value (EBV)
quantification of the likelihood of an indivdual passing on the desired trait to their offspring
Heritability (h^2)
proportion/percentage of a variation [of a trait] in the population due to genetic variation among individuals
(NOT percentage that is due to genetics)
Heritability of a Mendelian trait?
1 (100%)
Heritability of something caused entirely by environment?
0
EBV Calculation for one trait
EBV = h^2(individual trait value minus the average trait value of the population)
Index Merit/Score
Index = weighted sum of EBV for each objective trait (weight times EBV) added together
Where do you get the weight for each trait?
importance of each trait to the producer/breeder/client
Where do you get heritability?
many are published, but otherwise you have to make an educated guess
Where do you get the individual’s trait value?
the owner or vet calulates it
Correlated traits
selecting for one trait can often result in affecting another (can be desirable or undesirable)
ex: GSD and sloped back + hip dysplasia
Indirect selection
improving one trait by intentionally selecting another trait known to correlate with it
Inbreeding depression
a result of the increase of recessive diseases and generally associated with decreased performance
Does inbreeding help with selection?
NO - inbreeding decreases genetic diversity, butselection can only act on the diversity that is present in a population
Difference between marker-assisted selection (MAS) and GWAS?
just the number of markers - MAS selects for fewer quantitative trait loci (QTL), GWAS is the whole genome
Marker Assisted Selection
uses genotypes of a set of markers/SNPs most strongly associated with one or a few QTL
QTL (Quantitative Trait Loci)
GWAS done for a trait, looked at the regions of the genome associated with that trait, and selected a few markers/SNPs
