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Lecture 17

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John Stinchcombe

BIO220: Lecture 17 Conservation Genetics Biodiversity: The number and kinds of living organisms in a given area  Interspecific variation or species diversity  Intraspecific variation or genetic diversity Within Genetic Diversity, can divide intraspecific genetic diversity into 3 parts: 1. Within individuals (heterozygosity)  having different alleles at same locus 2. Between individuals in the same population 3. Between populations of the same species Conservation Genetics: Focused on applying genetic methods to improve conservation, restoration and endangered species management What happens genetically to declining populations?  Genetic drift is enhanced in small populations and chance changes in gene frequency due to sampling/random events are going to be more important as the population gets smaller and smaller. o As the population gets smaller and smaller, there will be a loss of genetic variation as individual alleles will drift to fixation (instead of there being polymorphism at that locus)  Inbreeding becomes more common o Mating among individuals whoa re closely related are going to be more common as there are fewer individuals in the population and there’s some reproduction (and there aren’t that many individuals in the pop), the only individuals that someone has to mate with either a sibling, or cousin.  Inbreeding leads to reduction of heterozygosity  Inbreeding depression from exposure of rare, recessive alleles  Loss of heterozygote advantage for fitness  All these things combined have a net effect on changing genotype frequencies Demographic effects of declining populations  Allee Effects is enhanced o When population growth rates is a function of the density of the population. As a consequence, as the population shrinks, if it falls below a threshold size, there won’t be much reproduction or population growth. Because the reproductive rate is dependent on the population size. o Ex. Colonial breeders  gather in one location and breed; their behaviour are dependent on being in group aggregation o  when a small population grows faster when the organisms are at high population density, then it would be if the population was at low density  Increased risk of demographic stochasticity: o If we have completely random differences among individuals, we have an increased chance of extinction. This happens even if individuals have the same average in survival of reproduction. So these random chances (if you have a small population and by random chance, by random chance all the females are hit by lightning, there’s going to be no reproduction that year; This is more likely to have a negative effect then if there were only 20 individuals and you lose all the females compared to 1000 individuals, much less likely that a random event will kill all the females) So in the small populations, these random/stochastic events are going to have greater consequences. What matters more, genetics or demographics?  Genetics: o Influence of a population decline on effective pop size (Ne) and its consequences should predominate in leading to extinction  Demography: o The consequences of stochasticity and the random chance events are likely to lead populations lead to extinction before the genetic effects can even manifest in the population Case Study #1: Cheetahs  2-4% of polymorphic loci in population  Captive cheetahs  males have low sperm counts and are susceptible to high diseases  MHC locus  this locus determines how your immune system recognizes foreign objects in body. There’s not much variation at this locus in cheetahs  The Early view: o Cheetahs have been through a historical bottleneck and lowered the population efficiency (Ne) size o As a consequence of a low population efficiency size, there was inbreeding during times of low population density which further reduces population efficiency size and census size o Therefore, disease and MHC symptoms suggest an interactive effect of loss genetic diversity and threats to population persistence  The Complications: o You can do an autopsy on a dead cheetah (was it diseased, compromised immune system, was it dying of infectious disease)  you don’t see ANY of this o ~75%mortality of cubs is due to predation when mother is hunting  Low genetic variability unlikely to affect such predation effects o While the MHC allele number is low (like all cats); their nucleotide diversity is high (compared to dogs)  Conclusions: o In the wild, these factors don’t seem to mater o Cheetahs show either reduced genetic diversity or diversity similar to other organisms o So although genetic variability in the cheetahs are really low, should we even focus on it when compared to other animals, it’s doing okay?  The Modern View: o Demographic vs. genetics is not an either-or-scenario o Inbreeding depression is going to affect reproductive characters, fitness, offspring and will affect demographic parameters o Demographic stochasticity will exacerbate lower Ne (effective population size)  Extinction vortex: small population  inbreeding and random genet
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