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Evolution - Test 2


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Conover and VanVoorhees (1990)  
  • populations of silverside fish with environmental sex determination
  • placed biased ratio of fish at constant temperatures
  • sex ration evolved to 50:50 in 7-8 generations
Consequences of Genetic Drift
  1. Alleles drift to either fixation or loss
  2. Frequency of heterozygotes declines
  3. Every population follows it's own unique evolutionary path
  4. occurs more rapidly in smaller populations
  5. given enough time
Consequences of heterozygote Inferiority 
  1. leads to a loss of genetic diversity within a population
  2. leads to the maintenance of genetic diversity across populations
Did inbreeding occur in the sea otter?
Yes, because there are far fewer SF heterozygotes than expected based on the Hardy-Weinberg equation.  Lidicker and McCollum ruled out other explanations.
Equilibrium depends on...
  1. mutation rate of deterious alleles
  2. strength of selection against alleles
a discipline that seeks to improve the human race through selective breeding
Fisherian sex ration evolution  
sex ratios should evolve to 1:1 through frequency dependant selection for rarer sex
Formula to figure out probablity a given allele will be fixed through genetic drift

Population has 2N alleles:

     p= 1/2n

If there are x copies of the allele:

    p= x/2n (allele frequency) 

Founder Effect
  • a population is usually very small when first established by migrants
  • the founders may not have been representative of the allele frequency of the source population
General Effects of Migration
  • Homogenizing force
  • Tends to prevent divergence among populations
Genetic drift

Change in allele frequency resulting from:

  • sampling error in drawing gameets from the gene pool
  • chance variation in the survival and/or reproductive sucess of individualsd
The more samples made per genera
Gigord, Macnair, Smithson (2001)
  • noticed elderflowers come in two colors in every population
  • placed pots of orchids in varying color frequency
  • measured amount of pollen removed
Hardy-Weinberg Principle
Heterozygote Inferiority
  • aka underdominance
  • AA and BB have higher fitness than AB
  • One of the two alleles tends to die out (same allele does not die out in every population)
Heterozygote Superiority  
  • aka overdominance
  • natural selection maintains 2 alleles in the population at a stable equilibrium
  • causes genotype frequencies to change across generations
  • does not cause allele frequencies to change
  • evolution has not occured
Inbreeding Depression
  • reduced fitness of individuals or populations that results from matings among kin
  • inbreeding causes more alleles to be expressed in homozygous state
  • deleterious recessive genes get exposed
Lake Erie Water Snakes
  • Island rocks make plain individuals camoflaged better
  • Expect to see allele for plain color increase, but it does nt
  • Migration from mainland keeps brining banded alleles into island gene pool
Lord William Thomson Kelvin
  • assumed Earth was cooling from a molten state
  • calculated age of earth based on this assumption
  • estimate in conflict with theories by Lyell and Dawin
  • short, repeating sequences of noncoding DNA
  • tremendous variation in alleles at a single locus

The movement of alleles across population

  • movement of actual individuals
  • movement of seeds or pollen
  • gene flow
  • can take allele frequencies away from h-W equilibrium 
Mukai and Burdick (1959)
  • V and L alleles at one loci in Drosophilia
  • VV and VL are viable
  • LL is lethal
  • established populations of 100% heterozygotes
  • Heterozygotes had higher fitness than homozygotes
  • <
Mutation - selection balance 
rate at which they are eliminated is equal to the rate that they appear through mutation (an equilibrium state)
Pinegal Island
  • colorblindness and sensitivity to light = achromatopsia
  • all people descended from 20 individuals in 1775
  • One of those individuals was heterozygous for the gene
RA Fisher


continuos variation in a population can be explained by many independent genes for the same trait 

Richard Lenski et al.
  • Bacteria (E. Coli) encapable of conjugating
  • Genetic mutation = only source of variation
  • survival and cell size increased
  • fitness increased in jumps

Sampling Error
  • random discrepancy between theoretical expectations and actual results
  • the error decreases as population size reaches infinity
Selection Sweeps
Advantageous mutation sweeps through population and reaches fixation through natural selection
Self fertilization in snails leads to...
a loss of heterozygosity across 3 generations
Sonya Clegg et al.

analyzed 6 microsat loci and found

  • diversity decreased at each island step
  • founders for each colony had only a subset of the orginal allele 
TH Morgan

rediscovered Mendel's principles using fruit flies

theory that genes resided on chromosomes 

The Modern Synthesis


Incorporated a genetic explanation with Darwin theory of national selection


The question that stumped Darwin  
Why do domestic animals almost always produce males and females in equal amounts?
The role of mutations in genetics  
  • mutation is the source of all new alleles and genes
  • weak force of evolution(occurs at a very small rate on their own)
Theodosius Dobzhansky

russian, immigrated to US

student TH Morgan

Population genetics and speciation in natural fruit fly populations 

True or False: Nonrandom mating causes evolution  
Under an infinite amount of time...  
  • the beneficialallele frequency will eventually reach 1
  • the allele is fixed in the population
  • the deterious allele frequency will reach zero
What happened to the IL Greater Prarie Chicken?
  1. Population size went down
  2. Habitat fragmentation
  • creates many small, isolated populations
  • these increased role of genetic drift, inbreeding depression and founder effect
Introducing migratio
Why eugenics wont work
  • most disease is recessive
  • flawed methods of detecting genetic diseases
  • disease alleles are often introduced by random mutation
  • possible heterozygote superiority

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