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Questions?. Genetic variation & inbreeding. Genetic Variation. “Genetic variation is the stuff of diversification, of adaptation, of speciation, of evolution: without it, a population or species lacks the raw material to respond to changing conditions.” L. Scott Mills. Motivating Examples.

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Genetic variation inbreeding

Genetic variation & inbreeding

Genetic variation

Genetic Variation

“Genetic variation is the stuff of diversification, of adaptation, of speciation, of evolution: without it, a population or species lacks the raw material to respond to changing conditions.”

L. Scott Mills

Motivating examples

Motivating Examples

Florida panthers

Florida Panthers

  • Isolated for >100 years

  • Major bottleneck (30-50 individuals)

  • Breakdown in typical dispersal strategies

Florida panthers1

Florida Panthers

  • Inbreeding depression

Greater prairie chicken

Greater Prairie Chicken

  • Populations in Illinois were reduced by loss of habitat farmland.


1962 – 2000 birds  1994 population was <50.

Population declined despite efforts to control predators and increase quality and quantity of habitat.

DNA from this population showed a decrease in genetic variation compared with museum specimens from the 1930s and contemporary large populations.

50% of eggs failed to hatch, suggesting fixation of harmful alleles.

Mechanisms of Evolution – genetic drift

Greater prairie chickens genetic rescue

Greater prairie chickens – Genetic rescue

  • 1992 – 1996 birds from MN, KS, NE translocated to IL population

  • Translocations of prairie chickens from neighboring states increased low egg fertility and hatching success

  • HOORAY! But the final moral of the story was…

But lots of contradictory examples

But lots of contradictory examples…

  • Elk, deer, pronghorn, bison, turkeys

    • Faced huge bottlenecks

    • Restricted to a few remnant populations

    • Rebounded to staggering numbers!

  • What was different for these species?


  • Desert pupfish

    • Small isolated populations are the norm for pupfish

So what matters

So what matters?

  • Genetic Variation

  • Effective population size

    • And the number of generations spent at small Ne

  • Connectivity

Hardy weinberg equilibrium

Hardy-Weinberg equilibrium


Hardy weinberg equilibrium1

Hardy-Weinberg equilibrium

  • Observed heterozygosity

  • Expected heterozygosity

A quick exercise

A quick exercise

  • Work in pairs

  • Start with 2 individuals in a population both heterozygotes

  • Flip a coin to determine alleles passed on to the next generation (2 offspring per generation)

  • Proceed for 6 generations or until the alleles are fixed

  • What are the allele frequencies?

What affects genetic variation

What affects genetic variation?

  • Mutation

  • Gene flow

  • Natural selection

  • Genetic drift

Genetic drift

Genetic Drift

  • Random change in allele frequencies in small population

  • Causes populations to diverge

Measuring genetic drift

Measuring Genetic Drift

  • Effective population size

    • Equivalent number of individuals if all members of the population were randomly mating

    • Ne < N

    • Often ~0.2-0.3 for wildlife

    • Sometimes much smaller

Effective population size

Effective population size

  • Contributing factors

    • Breeding sex ratio

    • Fluctuations in population size

    • Variance in family size

N e and sex ratio

Ne and Sex ratio

Variance in population size

Variance in Population Size

  • We’ve seen arithmetic means and geometric means, now harmonic mean



  • Founder Effects

    • Reintroductions

    • Translocations

  • Genetic drift at work in the background



  • Inbreeding coefficient

    • F

      • I – individual

      • S – subpopulation

      • T – total population



  • FIS - inbreeding of an individual relative to its subpopulation

    • Typically near 0

  • FST - inbreeding of subpopulation relative to the total

    • Loss of diversity due to drift





Subpopulation heterozygosity

Total expected heterozygosity





Ne= 100

Inbreeding depression

Inbreeding depression

  • Does it occur?

Inbreeding depression1

Inbreeding depression

  • Does it occur?

    • Yes

  • Does it reduce vital rates?

Inbreeding depression2

Inbreeding depression

  • Does it occur?

    • Yes

  • Does it reduce vital rates?

    • Sometimes

  • Do reduced vital rates = reduced lambda?

Positive effects of inbreeding

Positive effects of inbreeding?

  • Purging

Outbreeding depression

Outbreeding depression

  • Potential loss of local adaptation

Genetic rescue

Genetic Rescue

  • Increased fitness following the arrival of new genes

  • Breaks deleterious effects of inbreeding



  • Not movement, not this time

  • The transfer of genes from one population to another

  • AKA Gene flow

Next connectivity

Next: Connectivity

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