Demography and Extinction

Persistence of populations

  • importance of genetics in population persistence controversial, critics said

    • inbreeding unlikely to affect fitness in the wild

    • inbreeding may affect individual fitness but not population viability

    • demographic effects more important

    • hard to separate genetic and environmental factors when assessing population viability

    • inbreeding depression initially causes subtle reductions in birth and death rates that interact with other factors

    • obvious indications of inbreeding depression aren’t detectable until severe

  • extinction a demographic process influenced by genetic effects under some circumstances: need to identify when genetic concerns will affect population persistence

Inbreeding depression and extinction

  • when is inbreeding likely to affect population viability

    • have to have inbreeding

    • inbreeding depression must occur

    • traits affected by inbreeding depression must reduce population viability

      • when will inbreeding depression affect pop viabilty and how important will that effect be?

    • usually true in small pops

What is a viable population? Demographic criteria

  • IUCN most generally accepted demographic criteria

    • incorporate short term urgency and long-term concerns

    • considers generation time

    • shorter time periods recommended b/c errors in models are propagated in longer time periods

  • Genetic criteria

  • Soule et al. 1986

    • captive breeding programs

    • 90% of H over 200 years

  • Long-term persistence: adaptation to change

    • target pop sizes should be 1,000-5,000 individuals to balance loss of variation with new variation due to mutation

Population viability analysis (PVA)

  • models to assess whether pops are likely to persist

  • used to identify spp of concern, set prioritiesm develop recovery plans

  • includes chance events (stochasticity) in demography, env, natural catastrophes, and genetics

  • includes synergistic interactions btwn demographic events and genetic effects

  • vortex simulation model created by Bob Lacy

    • free

    • powerfull

    • easy to use

    • widespread

    • need info on many factors

  • identify threats and management actions to reduce threats and increase prob of persistence

  • sensitivity testing

    • try a range of values for uncertain parameters to determine how they might affect the results

    • which aspects contribute most to vulnerability? target these for management

    • If uncertain parameters are important, focus research

Inbreeding depression isnt the only way that loss of genetic variation affects population viability

  • separate mating types

    • 50% of plants have genetic incompatibility mechanisms

    • self-incompatibility locus (S)

    • pollen grains can only fertilize plants that do not have the same S allele

    • homozygotes cannot be produced at this locus

    • smaller pops have fewer S alleles than larger pops

    • reducing the number of S alleles reduces the freq of compatible matings

Haplodiploid species

  • sex determined by genotype at one or more loci, commonly one locus with 10-20 alleles

    • ants, bees, wasps, thrips, whitefly and others

  • at sex determining loci:

    • heterozygotes = female

    • haploids and homozygous diploid = male

  • loss of allelic variation caused by bottlenecks or drift increases homozygosity at sex determining loci and increases the number of males

  • diploid males sterile

  • fewer females

    • reduced foraging productivity

    • reduced pop size

    • skewed sex ratio and reduced Ne

Loss of evolutionary potential

  • reduces ability to respond to future changes (which will occur eventually)

  • Depends on heterozygosity and number of alleles

  • E.g. Mhc and disease resistance

mtDNA

  • mutations in mtDNA may affect fitness

  • sperm are powered by mitochondria, reductions in power output reduces male fertility

  • pop viability reduced by mtDNA haplotypes that lower fitness

  • Ne for mtDNA about 1/4Ne for nuclear genes

  • more sensitive to drift and bottlenecks

  • increase in haplotypes that reduce male fertility

    • reduce number of progeny

    • increase variability in male reproductive success and therefore reduce Ne

Mutational meltdown

  • small pops accumulate deleterious mutations b/c natural selection is overpowered by drift

  • pop has decreased fitness and becomes smaller

  • rate that deleterious mutations accumulate accelerates

  • feedback process

  • could substantially decrease long-term viability over the course of 100s/1000s of gens