MENDELIAN GENETICS in POPULATIONS II:
GENETIC DRIFT and NONRANDOM MATING

Chapter 7 in the 4th edition, Chapter 6 in the 3rd.

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review questions

Introduction

• The Greater Prairie Chicken
  1. mating display [Fig. 6.1]
  2. Habitat Destruction [Fig. 6.2]
  3. population in danger of extinction [Fig. 6.3]

6.1    MIGRATION

Adding migration to the H-W analysis: Migration as an evolutionary force

• migration: movement of alleles between populations = gene flow
• One-island model [Fig. 6.4]
  • most gene flow is from the continent to the island.
  • Migration into a small island population can change both genotypic and allelic frequencies [Fig. 6.5]

Empirical research on Migration and Allele frequencies: Migration-selection balance

• color pattern variation in Lake Erie water snakes [Figs. 6.6, 6.7]
  1. snakes on the mainland tend to be banded; on islands, unbanded to intermediate [Fig. 6.7]
  2. selection favors unbanded snakes on islands
  3. migration maintains banded individuals

Migration as a Homogenizing Evolutionary Force across Populations

• homogenizing effect of gene flow on allele frequencies of red bladder campion on islands of different ages [Fig. 6.9]
  1. younger populations have more variation (between islands) as a result of founder effect
  2. migration homogenizes variation in intermediate age populations
  3. remnant older populations have more variation as a result of drift.

 6.2     GENETIC DRIFT

A Model of Genetic Drift

bag-of-beans analogy

1. start with 100 beans: A₁ = 0.6, A₂ = 0.4
2. pick 20 at random to produce 10 individuals of the next generation 
3. A₁ = 0.7, A₂ = 0.3 [Fig. 6.10]

range of possible outcome for 10 zygotes produced at random [Fig. 6.11]

Genetic Drift and Sample Size

1. Random fluctuations in gene frequency due to sampling error in finite populations. 
   • probability of bias decreases as sample size increases
   • Random biases are expected when sample sizes are small.
   • A₁ = 0.6, A₂ = 0.4: initially allele frequencies fluctuate wildly, but approach Hardy-Weinberg value for a large number of zygotes [Fig. 6.12]
2. sampling error does not lead to adaptation, but leads to changes in allele frequency 
   • genetic drift: deviation of a population from Hardy-Weinberg expectations due to sampling error
   • Biases due to sampling error are purely random
3.  Several processes in natural populations result in sampling error
   1. genetic drift: during random mating (of small populations), alleles are "sampled" to form progeny
   2. founder events: sampling small number of genotypes from a source population
   3. population bottlenecks: population crashes to low numbers then regrows. 

Sampling Processes and the Founder Effect

Founder effect depends on allele frequencies in source population and number of founders (N_f)

• three males and two females of large ground finches remained on Daphne Major in 1982 and bred there. The population established there by 1993 had larger and wider bills than nonbreeding visitors
• High frequencies of disease alleles in some isolated human populations.
  • Ellis-van Creveld Syndrome (dwarfism) in Amish populations (F&H p 168)

Random Fixation of Alleles

1. Variation in small populations is reduced by genetic drift
   1. alleles can be lost or fixed [Fig. 6.13]
      1. rate of loss or fixation depends on sample size
      2. even for larger sample sizes, substantial variation in allele frequency between populations can be created [Fig. 6.13c]
      3. If genetic drift is the only evolutionary force at work, eventually one allele will drift to a frequency of 1.
   2. Probability of an allele being lost through drift is inversely proportional to its initial frequency.
      1. experiment with fruit flies shows that nonselected allele initially present at 0.5 has an equal probability of being lost or fixed [Fig. 6.14]
      2. drift reduces heterozygosity [Fig. 6.15].
2. Drift in Ozark collared lizards (Tempelton et al., 1990)
   1. Collared lizards populations are currently limited to scattered, small exposed rocky outcrops (glades), that are islands in the forested habitat
   2. populations on glades are small, therefore they should show evidence of drift [fig. 6.16]
      1. single genotype within populations (because drift fixes one allele at each locus)
      2. variation in genotypes among populations (because which allele is fixed is random)
3. Population size and genetic diversity in four plant taxa [Fig. 6.17]
   1. Genetic diversity increases with population size

The Rate of Evolution by Genetic Drift

When mutation, genetic drift, and selection interact, three processes occur

1. Deleterious alleles appear and are eliminated by selection
2. Neutral mutations appear and are fixed or lost by chance
3. Advantageous alleles are swept to fixation by selection

Neutral theory: 2 is more important

Selectionist theory: 3 is more important

6.3     NONRANDOM MATING

1. does not by itself cause evolution
2. causes genotype frequencies to change, altering proportions of phenotypes, and therefore affects natural selection
3. inbreeding, mating among genetic relatives, increases the frequency of homozygotes, decreases the frequency of heterozygotes.  Reduced heterozygosity is most dramatic with selfing.  [Fig. 6.19]--- frequency of A₁A₂ = 2pq/2ⁿ, where n is the generation
4. genotype frequencies change, but allele frequencies do not [Table 6.1]

Empirical Research on Inbreeding: The Malaria Parasite

1. Life cycle of Plasmodium [Fig. 6.20]
2. Allele frequencies for three genes in Plasmodium [Table 6.2]
3. Homozygote frequencies are much higher than expected; heterozygote, much lower for three loci in Plasmodium [Table 6.3]

• panmixia = random mating
• F: inbreeding coefficient, the probability that the two alleles within an individual are identical by descent (autozygous)
• F provides a measure of the extent to which inbreeding has increased the frequency of homozygotes and decreased the frequency of heterozygotes relative to Hardy-Weinberg.
• " F = 0 for no inbreeding; F = 0.5 for selfing (the probability that the offspring of a self fertilizing plant will inherit two copies of its parents genes is 50%)

Computing F

• Estimating inbreeding from pedigrees [Fig. 6.21]
• must know pedigree (humans, captive populations, long-term studies)
• siblings share 50% of their genes by descent with each other and with parents; (identical twins share 100% of their genes)

Inbreeding Depression Is A Consequence of Being Diploid

• inbreeding depression = inbred offspring are inferior to outbred offspring in survival & reproduction. By increasing the frequency of homozygotes, inbreeding can expose deleterious recessive alleles to selection [Figs. 6.22, 6.23, 6.24]
• Inbreeding depression widespread in plants and animals. Evolution has selected for various mechanisms of inbreeding avoidance

6.4 CONSERVATION GENETICS of the Illinois Greater Prairie Chicken

• The Illinois greater prairie chicken is in decline because of 
  1. destruction of prairie habitat
  2. reduced population size
  3. fragmented populations of few individuals on small islands of prairie embedded in seas of farmland
  4. fragmentation of populations with reduced migration/gene flow among populations, leading to genetic drift
• as population size declined, drift led to inbreeding depression
  1. individuals homozygous for deleterious recessive alleles increase
  2. deleterious mutations  become fixed due to drift
  3. populations enter positive feedback loop (mutational meltdown, extinction vortex)
     1. as inbreeding depression increases, population size drops causing
     2. effect of drift to increase – more deleterious mutations become fixed
     3. as more deleterious mutations become fixed, inbreeding depression increases
     4. Back to a.
• A long-term study documents inbreeding depression: Steady decline in hatching success of Jasper County populations by 1970 (fig. 6.25)
• Other evidence of genetic drift
  1. less genetic variability in Jasper population than in larger populations elsewhere
  2. comparison with DNA from museum specimens from 1930's and 1960's showed significant decline in genetic variability over time 
• Developed a conservation strategy: Increase gene flow by introducing individuals from other areas to the Jasper population
• hatching rate increased from 40% to 90%

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