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What two major factors contribute to phenotypic variation in populations?
- genetic variation: eg White and blue forms of snow goose caused by 2 alleles at same locus
- environmental variation: eg White (winter) and colored (summer) plumage in willow ptarmigan depends on season in which molt occurs
What are the two major reasons for why trait variation is usually distributed continuously in populations?
- ex- Human hair and skin color
- Multiple loci contribute to the continuous variation found in most phenotypic features (rather than multiple alleles at a single loci)
- Each contributing gene can have multiple alleles
What is a polygenic trait? What is pleiotropy?
- polygenic trait: traits which receive contribution from multiple genes
- *note- this allows for continuous variation
- pleiotropy: one gene affects multiple, seemingly unrelated, phenotypic traits
If p=0.2, what does q equal? What is the frequency of homozygous dominants in the population? Heterozygotes? Homozygous recessive? *no calculator
What is Hardy Weinberg equilibrium. What would a HW population look like?
- HW equilibrium describes genetic equilibrium of large sexually reproducing population (allele frequencies remain constant from one generation to the next unless acted on by outside forces)
- Mating is random (panmictic)
- Population size indefinitely large (removes effect of genetic drift)
- Genes are not added from outside the population (no gene flow)
- Genes do not mutate from one allelic state to another (no mutation)
- All individuals have equal probabilities of survival and reproduction (no natural selection)
What is the utility of knowing how allele frequencies behave under HW expectations?
You can contrast actual allele frequencies with HW expectations to determine if evolution is occurring in a population. You may also be able to determine the rate of evolution by contrasting the current frequencies to expected frequencies.
What are the 5 forces of evolution acting to change allele frequencies in populations?
- Mutation: random mutation in the genome
- Gene flow: exchange with other gene pools
- Genetic drift: Allele frequencies change randomly over time, alleles may be lost or fixed (increased intensity with smaller population size)
- Natural selection: Selective pressure against certain phenotypes inhibits reproductive success
- Non-random mating: mate choice, differential mating success, inbreeding
What is inbreeding? What is the classical signature of inbreeding in a population, with respect to HW expectations? What population-level problems does inbreeding cause? What are some examples of inbreeding depression?
- inbreeding: mating between individuals with greater than average degree of genetic relatedness (self-fertilization is most extreme form)
- Population inbreeding typically results in a decrease of heterozygosity
- population inbreeding occurs that genetic similarity occurs regardless of lineage
- Inbreeding often results in an increase to homozygosity of deleterious recessive genes
- inbreeding depression: occurs when inbreeding causes decline in components of fitness (# bad alleles in pop is same, but genetic load is increased)
- *note-introduction of members of other populations may increase survival rate (European Adders, Golden Lion Tamarin in captivity)
Are most loci polymorphic or fixed for a single allele in natural populations? Why?
- Phenotypic expression of many favorable traits leads to more offspring.
- NS acts on the sum of all traits, but each trait is effected differently.
- There is a graduation between those with all great traits and those with no great traits.
- A highly fit individual may still exhibit a bad trait.
What are linked genes? What is linkage disequilibrium? How can now alleles be formed in linked gene?
- If 2 genes are located on the same chromosome their alleles are packaged into gametes together
- Since their alleles are inherited as a single unit, they are "linked"
- Linkage disequilibrium: refers to the population-level effects of linkage
- New alleles formed during meiotic recombination (Dt/D0)
- LD will always decay to 0 in a population, and rate of decay is correlated with R (recombination)
- When R is .5 LD quickly approaches 0 (very few generations) - genes are far apart on chromosome
- When R is .001 LD very slowly (huge amount of generations) - genes are very close on chromosome
- *note-image on 238
What does VP=VG+VE mean?
- Variance in phenotype is the sum of genetic variance and environmental variance.
- *note- quantitative traits among individuals often fit a normal distribution; most useful statistical measure of variation is variance
What does heritability measure?
- The proprotion of phenotypic variance due to genetic variance of a trait
- h2 is equal to the slope of the regression line in a plot of mean value for all offspring vs Midparent mean
How can an artificial selection experiment allow us to assess the genetic component of a trait? How does the Dobzhansky & Spassky (1969) experiment demonstrate this?
- Artificial selection can only be used to detect genetic variation if a trait is genetically heritable
- EX: selection for movement in response to light in Drosophila
- flies introduced at bottom of maze moved up, making successive choices between light and dark, and ending in 1 of 6 tubes
- 25 flies of each sex that had extreme + or - score were saved to start the next generation (they boned once they got to the final tubes)
- The selected populations diverged radically
- conclusion- genetic variation and heritability give the ability to evolve rapidly
Are most natural populations sub-divided or panmictic? What types of subdivisions exist? What is a sub-species (EX)? What is a hybrid zone?
- Panmictic: total random (blender) of entire species
- Very rare (ex Devil's Hole pupfish are in single sinkhole, Eels from eastern NA and western Europe migrate to one are near Bermuda to breed)
- Vast majority have subdivided populations
- sympatric: distinct populations with overlapping ranges comes into contact
- allopatric: distinct populations with adjacent (not overlapping) ranges come into contact
- parapatric: distinct populations with separate ranges (never come into contact)
- Subspecies: "geographic races"- recognizable distinct population that occupies a different area than other populations of the same spp
- (EX- 2 subspp of Northern flicker differ in color of wings, crown face, and size. Have hybrid zone in midwest)
- (EX- 5 subspp of rat snake that differ dramatically in phenotype (color, pattern, etc). Rarely interbreed, speciation on horizon?)
What is a geographic cline? Example? What can such clines in genetic diversity tell us about the evolutionary past?
- Clinal variation: gradual change in a trait or allele frequency over geographic distance
- EX- body size in white-tailed deer (increase body size with increased lattitude over NA
- EX- Clines in frequency of F allele in alcohol dehydrogenase in fruit flies (decreased at lower latitudes in NA and Aus)
- Such clines illustrate that a trait is likely adaptive
What is an ecotype? What did the experiment on ecotypic variation in sticky cinquefoil in CA tell us about VG and VE?
- Ecotype: habitat-associated phenotypes
- very common in plants
- EX- ecotypic variation in sticky cinquefoil in CA
- plants cloned from several ecotypes grown in common gardens @ 3 altitudes
- flower color remained unchanged, regardless of altitude
- height diff. within each ecotype (at different elevations) reflects environmental effects
- height diff. among ecotypes (at same elevation) reflects genetic differences
- *image pg 247
What is Bergmann's rule? Allen's Rule? Gloger's Rule? What selective forces are acting to produce such patterns?
- Bergmann's rule: Birds and mammals are larger in colder climates (because SA is decreased relative to body mass over which heat is lost)
- Allen's rule: Birds and mammals in colder climates have shorter appendages (reduces SA and heat is lost more slowly)
- Gloger's rule: Animal populations in arid climate are paler (match pale soil and vegetation to avoid predation)
Describe character displacement and give an example
- character displacement: sympatric populations of 2 species differ more than allopatric populations in various traits
- EX- bill size in seed eating ground finches in Galapagos island
- when the two finches reside on the same island there is very little overlap in the beak size (each is "specialized") but looking at the same spp on 2 different islands the bill depth has much overlap
Does gene flow act to separate or homogenize populations? What is the typical isolation by distance pattern (draw a graph)?
- Gene flow: exchange of genes between populations
- carried by moving individuals (animals, seeds, spores) or gametes by wind, water or pollinators
- non-reproductive migration does not contribute
- Acts to homogenize populations (if there is gene flow among populations they will all eventually reach the overall average allele frequency)
- *graph is allele frequency (p) 0-1.0 vs # of generations. No matter what value p is at generation 0 they become .5 (looks like a broom)
- *pg 248