Inbreeding -- Production of offspring by

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Inbreeding -- Production of offspring
by individuals related by descent. Genetic
Diversity -- Extent of heritable variation in a
population, or a species, or across a group of
species and include heterozygosity allelic
diversity haplotype diversity nucleotide
diversity
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Rates of Inbreeding Extinction Risks Slower
inbreeding allows natural selection
more opportunity to remove genetically
compromised individuals (an therefore deleterious
alleles). For fitness components such as
survival fecundity, slower rates of inbreeding
generally lead to less inbreeding depression than
fast inbreeding for the same total amount of
inbreeding. Even slow rates of inbreeding though
increase risk of extinction.
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Response of populations to environmental
and demographic stochasticity and the impact of
catastrophes are not independent of
inbreeding and genetic diversity. Inbreeding, on
average, reduces birth rates, increases death
rates, and may distort sex ratios. Inbreeding
therefore interacts with the basic parameters
determining population viability, such as
population growth rate.
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Adverse effects of inbreeding on
population growth rates probably occur in most
naturally outbreeding species. Experimental
populations of mosquito fish founded From
brother-sister pairs showed 56 lower Growth in
numbers than populations founded from Unrelated
individuals (Leberg 1990). Strong reductions in
pop. Growth were also found In flour beetle
populations inbred to small numbers. They even
detected adverse effects at F 0.1 (McCauley
Wade 1981).
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If populations become small for any reason,
they become more inbred and less
demographically stable, further reducing
population size and increasing inbreeding. This
feedback between reduced population size, loss of
genetic diversity, and inbreeding is referred to
the Extinction Vortex.
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Smaller populations are expected to be more
extinction prone than larger ones for
demographic, ecological, and genetic
reasons. Since the 1600s, the majority of
recorded extinctions have been of island forms
even though island species represent a minority
of total species of all groups. Human factors
have been the major recorded causes of
extinctions on islands over the past 50,000 years.
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The mechanisms underlying susceptibility
of island populations to extinction are
controversial. Ecologists stress the
susceptibility of small island populations to
demographic and environmental stochasticity. How
ever, this susceptibility is also predicted
on genetic grounds. Island populations are
expected to be inbred due to low number of
founders and subsequent small population size.
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There is little evidence to separate the
effects of non-genetic factors from the effects
of inbreeding and loss of genetic
diversity. Inbreeding can certainly diminish the
resistance of a population by reducing its
reproductive rate and survival such that it is
more susceptible to non-genetic factors.
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Island populations typically have less
genetic diversity and are more inbred than
mainland populations. Analysis of 202 island
populations revealed that 82 had lower levels of
genetic diversity than their mainland
counterparts. Inbreeding in many island
populations is at levels where captive
populations show an elevated risk of extinction.
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Species must be able to evolve to cope with
these new conditions or face extinction!! To
evolve, species require genetic
diversity. Naturally outbreeding species with
large populations normally possess large stores
of genetic diversity that confer differences
among individuals in their response to such
environmental changes.
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Due to sampling of alleles in the parental
generation to produce offspring, small
populations typically have lower levels of
genetic diversity than large populations. Due to
this random sampling process, some
alleles increase in frequency, others decrease
and some alleles may be lost entirely. The
smaller the population the more change there will
be between the parental and offspring gene pools.
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Over time, genetic diversity will decline,
with Loss being more rapid in smaller than
larger Populations.