BIOL 2500 - Population Genetics (Topic 8)

tools that can detect genetic variation

• single nucleotide polymorphisms (SNPs)

• microsatellites

• haplotype network analysis

single nucleotide polymorphisms (SNPs)

difference in nucleotide sequence at a specific locus 

microsatellites

short, repeating sequences; # repeats differs in each allele

population genetics

study of populations and the distribution and amount of genetic variation

• can be traced to early 1900s

gene pool

sum total of all alleles in the breeding members of a population at a given time

genotypic frequency and allelic frequency must all at up to _____

1

assumptions of the Hardy-Weinberg (H-W) equilibrium

• random mating

• no natural selection

• no mutation

• no migration

• no genetic drift 

• population size is infinite (assumption you kinda ignore)

*if no assumptions are violated, you can use H-W to determine allelic frequencies

allelic frequencies are stable at _________ for two alleles

• = copies of one allele / sum of all alleles

p + q = 1

genotypic frequencies are distributed according to _________

• = number of progeny of one genotype/total number of progeny

p²+2pq+q² = 1

population

group of interbreeding organisms (same species)

nonrandom mating assumption - violated

• only affects genes associated with mate choice (sexual selection)

• positive assortative mating

• negative assortative mating

positive assortative mating

• “like with like", increases homozygosity, decreases genetic variation (e.g. female lion wanting to mate with male who has biggest mane)

negative assortative mating

“opposites attract”

• decreases homozygosity, maintains genetic variation

• also called disassortative mating

isolation by distance - H-W assumption violated

individuals closer together will tend to mate more than individuals farther apart

inbreeding

form of non-random mating

• mating between related ind.s and affects the entire genome

• increases homozygous genotypes and reduced heterozygosity (can lead to inbreeding depression)

• DOES NOT change allelic frequencies… just redistributes the alleles into different genotypes (no longer 1:2:10

self fertilization (“selfing”)

most extreme case of inbreeding

• common in plants (sometimes in animals)

• proportion of heterozygotes is halved after each generation as homozygosity increases

• not necessarily bad

inbreeding coefficient

probability that two alleles in an ind. trace back to the same copy in a common ancestor

identical by descent (IBD)

when two alleles came form a common ancestor

what increases when smaller populations have an increased likelihood of mating with close relatives (3)

• increases inbreeding depression

• increases inbreeding coefficient 

• increases identical by descent

*we see an increase in genetic diseases in populations with more relative mattings

inbreeding depression

increases the homozygosity within a population which in small pop.s can reduce the overall fitness of the populations/species

• seen often in captive breeding programs

• e.g. cheetahs

conservative genetics

looks to design, conduct, and manage captive breeding programs to increase genetic diversity

what’s the ultimate source of genetic variation

mutation

• changes in nucleotides change amino acids sequences, which change gene expression

• by itself, a slow evolutionary process since its effect on alleles in a population is small and gradual

why are mutations slow

because they affect a allele in 2 directions

forward mutation rate (u)

creates new A₂ alleles by mutating A₁

• (A₁  ————> A₂ )

reverse mutation rate (v)

changes A₂  alleles by a mutation to A₁

• ( A₂ ———→ A₁)

• can create a balanced equilibrium in the absence of other factors

mutation-selection balance

natural selection removes the recessive trait, but mutation keeps it in the population

gene flow (migration)

moves alleles into and out of populations

• introduction of novel alleles can increase allelic frequencies already present

• indi.s moving out can reduce the allelic frequency

admixed populations

addition of new organisms into an existing population

allele frequency changes due to drift are ________

random

• allele frequency after one generation does affect gamete availability in the next

T/F: in absence of other evolutionary forces, drift will cause alleles to become fixed in a population and eliminate other alleles

TRUE

founder effect

a new, small population branches off a larger one

when does a genetic bottleneck occur

when a large population is drastically reduced to a small population

• catastrophes and natural disasters

• independent of natural selection

• survivors have low genetic diversity due to the huge loss of alleles from the gene pool

what is the driving mechanism for evolution over time

natural selection

adaptations

features in an organism’s form or physiology that allow it to cope to varying envr.s

• arise due to natural selection

• indi.s with certain heritable features are able to survive better and reproduced, passing their trait (alleles) to the next gen.

conditions that natural selection require

• varying phenotypes

• genetic variation is heritable

• more offspring are born than will survive to maturity → “struggle for existence”

• some genetic variants produce more offspring than others

darwinian evolution

“survival of the fittest”

darwinian fitness

ability to survive and reproduce

• considers viability and fecundity 

absolute fitness (W)

number of offspring an ind. has

• results from differential reproductive success of ind.s in a pop. (i.e. no longer random mating)

• increases and decreases the frequency of certain alleles

relative fitness (w)

quantifies the reproductive success of a genotype compared to the most favored genotype in a population

• not measured on ind.s

• genotypes with the greatest fitness have w = 1

• genotypes less favored w<1

directional natural section

shifts the phenotypes in the pop. to the homozygous phenotype

• have higher relative fitness than the other genotype

• positive selection

• purifying selection

positive selection

increases the allelic frequency of the favored allele

purifying selection

fixation or decreases the frequency of the unfavored allele until loss

balance polymorphism

alleles reach an equilibrium

• selective pressure favors maintaining heterozygote but selects against homozygous recessive