Lecture 4

Question 1

What happens to genetically declining populations

Answer 1

• Genetic drift enhanced
  
  • Chance changes in gene frequency increase in size
    - Loss of genetic variation as alleles fix randomly
• Inbreeding more common
  
  • Reduce of heterozygosity
  • Inbreeding depression from exposure of rare, recessive alleles
  • Loss of heterozygote advantage for fitness

Question 2

Demographic effects of declining population

Answer 2

• Potential for allee effects enhanced
  
  • Reproductive rate depends positively on density
  • At low densities, low reproductive rates
  • Likely in colonial breeders
• Increase risk of demographic stochasticity
  
  • Random differences among individuals in survivorship and reproduction create variability in the population growth rate
  • Happens even if individuals have same average survival and reproduction
  • Most important in small populations

Question 3

Genetics vs Demographics

Answer 3

• Genetics: Influence of population decline on Ne and its consequences should predominate in leading to extinction
• Demography: Effects of demographic stochasticity should predominate and lead to extinction before effects of genetics can manifest

Question 4

Cheetah case study

Answer 4

• Genetically depauperate(lack genetic variation)
• Proportion of polymorphic loci in population: 2-4%
• Proportion of heterozygous loci per individual: 0.04-1.4%
• Lack of variation in MHC locus: captive cheetahs don’t reject skin grafts from others
• Captive cheetahs have low sperm count and high disease load. Suffer from inbreeding depression, can’t defend against diseases

Question 5

Early view

Answer 5

• Historical bottleneck lowered Ne
• Inbreeding during times of low N and Ne further reduce both N and Ne
• Disease and MHC symptoms suggest interactive effect of loss genetic diversity and threats to population persistence

Question 6

Complications

Answer 6

• Dead cheetahs from wild nambian populations: no sign of disease, high viral load, or infectious disease induced mortality
• 75% of mortality of cubs is due to predation. Low genetic variability unlikely to affect such predation effects
• Refined genetic diversity estimates
  
  • MHC allele number: low compared to other mammals
  • Nucleotide diversity: similar to other cats, high compared to dogs
  • Amino acid variation at MHC: similar to most other cats

Question 7

Conclusions

Answer 7

• Genetic diversity does not appear to affect cheetah disease susceptibility or demography in the wild
• Depending on how one estimates genetic diversity
  - Allele number, nucleotide diversity, amino acid sequence diversity
• Cheetahs show either reduced genetic diversity or diversity similar to other organisms

Question 8

Modern View

Answer 8

• Demography vs. genetics is not either-or scenario
• Inbreeding depression affect demographic parameters
• Demographic stochasticity exacerbate lower Ne

Question 9

Extinction Vortex

Answer 9

• Small population -> Inbreeding and random genetic drift -> loss of genetic variability -> reduction of individual fitness -> lower reproduction and higher mortality -> smaller population

Question 10

Population decline in Florida scrub jay

Answer 10

• Flordia scrub jay are threatened non-migratory species
• Range-wide population decline due to human mediate habitat destruction
• Individuals do not move as breeding adult
• Females tend to disperse farther so they have more genetic structure

Question 11

Male and female differences dispersal and genetic consequences

Answer 11

• Males disperse shorter distances than females
• At short distance, identity by descent stronger in males reflecting female biased dispersal
• Identity by descent: identical stretch of DNA inherited from common ancestor
• At given distance under 1000m, males more related to each other than females

Question 12

Z chromosome and autosome show different patterns of IBD

Answer 12

• Females are ZW, males are ZZ
• Z chromosomes spend 2/3 of time in males
• Females disperse further than males
• Expect greater identity by descent for Z, at small distances as a result of these forces

Question 13

Flordia Scrub Jay ecological genetics

Answer 13

• Less migration in males lead to higher identity by descent in male individuals
• Z chromosome and autosome show different patterns of identity by descent

Question 14

Immigrants tend to be less heterozygous

Answer 14

• Immigrants come from smaller populations where inbreeding has reduced heterozygosity

Question 15

Identity by descent increasing

Answer 15

• Identical stretch of DNA inherited from common ancestor
• Immigration decline: IBD goes up
• IBD higher from resident-resident pairs than those involving immigrant birds
  
  • Inbred birds from other populations are homozygous, but at different stretches of genome than resident birds

Question 16

Inbreeding coefficients going up

Answer 16

• Reduced immigration
• Immigrants generally of lower heterozygosity
• Increase inbreeding depression

Question 17

Inbreeding depression impact on population

Answer 17

• Reduced immigration in population increases inbreeding
• Inbreeding depression negatively impacts hatching success

Question 18

Scrub jay summary

Answer 18

• Population had been large, demographically stable
• Reduced immigration into population due to habitat destruction/fragmentation
• Inbreeding depression negatively affects fitness components
• Small inbred population bring new hybrid superiority/hybrid vigor back into main, large population

Question 19

Florida Panthers

Answer 19

• Declining numbers and developmental abnormalities suggest inbreeding depression
• Genetic rescue: Translocate 8 female pumas from Texas
  - Goals: Increase genetic diversity, improve population numbers, reverse indications of inbreeding depression

Question 20

Observations

Answer 20

• Number of admixed individuals increase
• N increase from 26 -> 102
• Ne increase from 16 -> 32

Question 21

Genetic rescue as time-buying strategy

Answer 21

• Genetic rescue temporarily increase population size -> temporarily reduce probability of extinction and increase probability of establishment of an endangered population
• May give more time to fix environmental and other problems that have cause endangerment
• For long-term genetic health of endangered species, recovery of populations to effective population size resembling that before endangerment is necessary, goal that is dependent upon eliminating or greatly reducing, the factors that cause endangerment