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Genetic Diversity
Factors that make populations
vulnerable to extinction
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Environmental fluctuations
Catastrophes
Demographic uncertainties
Genetic problems
Habitat fragmentation
Heath Hen – Extinction Vortex
Minimum Viable Population Size
• Another definition - often defined as 95%
probability of 100 year survival, but can
also plan for longer survival (500 or 1000
years)
• MVP is usually determined by modeling
Forces which may cause extinction
1) deterministic - something essential is
removed (habitat loss) or something lethal
is added (pollutant, disease, introduced
species) - presumably we can act to
minimize these risks
Forces which may cause extinction
2) stochastic (random) - environmental, catastrophic,
demographic and genetic - this is what we need to
worry about and what is hardest to prevent
• environmental randomness effects resources and
conditions and we can't do much about it
• catastrophic randomness - floods, fires, hurricanes,
volcanoes - can't really prevent but can spread
individuals around to minimize the impact
• demographic - just natural random variation in
birth and death rates can lead to extinction
• genetic - lack of genetic variability can lead to
problems of inbreeding and poor response to
diseases and environmental change
Bighorn Sheep and MVP
Bighorn Sheep and MVP
Grizzly Bear and 50/500 Rule
MVP – 50/500 Rule?
Reductions in Polymorphism
Reductions in population size can lead to
losses of genetic polymorphism
Two special cases of reductions in population
size are:
1. A few individuals move to a new area and
start a new population that is isolated from
other populations – founder effect
2. We can also experience a population
bottleneck where a formerly large
population is drastically reduced in size
Founder Effect – Galapagos Tortoise
Founder effect – Amish and
Polydactyly
Population Bottleneck – Northern
Elephant Seal
Reductions to Polymorphism
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Genetic drift - random changes in gene
frequency in a population – nondirectional
evolutionary change which occurs due to
sampling errors in mating and mortality can result in a loss of polymorphism
Effective Population Size
• Effective population size - the number of
reproductive individuals within a population
taking into account unequal representation
of the sexes in reproduction or unequal
representation of generations over time
Sewall Wright
at University
of Chicago
In 1928
Sewall
Wright in
1980’s
Effective Population Size in the
Social Tuco-Tuco
Effective Population Size in the
Social Tuco-Tuco
Using a pedigree chain to
calculate inbreeding
Inbreeding in European Royalty
Gene Flow
• The movement of alleles from one
population to another
Rates of Gene Flow
– Ne (effective population size) = 120
Gene Flow in Conifers
Gene Flow in Kingsnakes
Figure caption for Kingsnakes
• a, b, The venomous eastern coral snake (Micrurus
fulvius; a) and its non-venomous mimic, the scarlet
kingsnake (Lampropeltis triangulum elapsoides; b). c,
Geographical distributions of model (yellow) and
mimic (yellow, green). For simplicity, sampling
locations for genetic analyses are shown for allopatry
only (open circles, western allopatry; filled circles,
eastern allopatry). d, e, Gene flow among L. t.
elapsoides (number of migrants per generation, Nm,
shown as means 95% confidence interval) from
sympatry into each of two allopatric recipient
populations (eastern (d) and western allopatry (e)),
as measured in mitochondrial (mtDNA) and nuclear
(nucDNA) genomes (the three estimates in each
population and genome reflect three runs of
MIGRATE-n). Asterisks indicate estimates
significantly greater than 0.
Value of genetic diversity
• Why are genes valuable? Or
• What kind of value do genes have?
Value of genetic diversity
• Genes have instrumental value – they are
valuable for what they do
Grassy Stunt Virus in Rice
Oryza nivara
Native habitat of O. nivara in
Uttara Kannada, India and Distribution