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Small Populations and Losing Genetic Diversity do populations suffer a significantdeteriorationas a consequence of a
sudden decrease in numbers (a genetic
bottleneck) ?
eg. reduction in fitness of
cheetahs following a bottleneck
about 10,000 yrs BP
current populations have
deformed sperm; low genetic
variation; susceptibility to
disease; high mortality of cubs
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What genetic variables must be
considered by conservation ecologists ?
population size (bottlenecks)
population origin (founder effects)
genetic drift (loss of variability)
gene flow (gain of variability)
inbreeding depression (loss of fitness)
out-breeding depression (loss of fitness)
hybridization (loss of distinctness/gain in variability)
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genetic bottleneck
population has an abrupt contraction
in numbers
accompanied by change in genefrequencies
decline in genetic variation due to lossof numbers
Bottleneck
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population numbers may recover, but genetic
variation may be low
originalpopulation
bottleneck
recoveringpopulation
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bottleneck
recovery
isolation & low diversity
populations must remain small
& inbreed
Bottleneck options
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founder effect
establishment of new population by one or a few
individuals
low levels of genetic variation depending on the
genetic variation in the founder population
foundingpopulation recovered population
numbers
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may result in a disproportionate numberof
alleles rarein the parent population but nowcommonin the new population
founder effect
2 in 14
= 1 in 7
4 in 5
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inbreeding depression
when closely related individuals mate (inbreed),there is a high probability that genes will be
homozygous
some recessive alleles are deleterious
effects only apparent when they individuals are
homozygous for the recessive allele
may result in lower fitness ie. reduced
successful reproduction
more likely to occur in small populations such as
populations recovering from a bottleneck
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gene flow
gene flow is movement of genes amongpopulations, usually through dispersal and
migrationof individuals
high gene flow causes populations to becomegenetically similar
restricted gene flowallows populations to become
different
restricted gene flow can also lead to loss of genetic
diversity (genetic drift and inbreeding)
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genetic drift
random change in gene frequencies throughchance
more apparent in smallpopulations
occurs because there is an element of chancein
which individuals successfully breed
can lead to loss of genetic variation
larger populations sizes can buffer against loss
of genetic variation
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out-breeding
breeding between individuals from populationswhich have been isolated
many isolated populations have distinctivelydifferent gene pools
out-breeding introduces new alleles into the
population (gene flow ?)out-breeding may be desirable as it can increasegenetic diversity eg. Black-footed Rock-wallaby
out-breeding depression
may result in loss of fitness
may introduce genes not adapted to theparticular environment
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in nature, hybridization is prevented by a
barrier -geographic
seasonal
behavioural
animal species rarely hybridize
plants often hybridize
offspring in animals are usually infertile;
plant hybrids are often fertile
hybridization
production of offspring by parents fromdifferent species
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plant species hybridize more readily
eg. Buxton Gum (rare) x Swamp Gum
(common)
offspring frequently fertile
F1 offspring can breed back to the parents,
with other F1s,or with F2s
called introgression
results in populations
with a genetic range from
pure to very hybridized
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hybridization is uncommon in animals, but
can occur
sometimes offspring are fertile
introgression can occur
introgression is the breeding of F1, F2, F3 etc.
offspring with the parental species, and eachother, and results in a "hybrid swarm"
eg.Pacific Black Duck x Mallard Duck P1
P2F1
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hybridization in rare species presents some
"problems" for conservation
1. rare population may show "outbreeding
depression" - reduced "fitness" due to gamete
wastage and reduced seed set in the parent species
outbreeding depression is unlikely to be a problemin long-lived species; may be a problem in rare,
long-lived species
2. if hybrids are fertile, hybridization may lead toassimilation of the rare species through
introgression
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3.are hybrid populations legally protectedonce
they are not "genetically pure" ?
4.hybrid swarmsrepresent large pools of genetic
diversity - isnt this desirable ????
5.hybridization may lead to the evolution of new
species- one of the aims of biodiversity
conservation - what if it occurs with the loss onone or both parent species ??? How important is
this ?
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