There are limits to physiological adaptations.

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There are limits to physiological
adaptations. If environmental changes are
greater than any individual can cope with then
the species becomes extinct. Evolutionary
change through natural selection is an
alternative means (vs. physiological
adaptation) for adjusting to environmental
change. This is Adaptive Evolution!
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Natural Selection -- differential reproduction
survival of different genotypes. When adaptive
evolutionary changes occur over long periods of
time, they may allow a population to cope with
conditions more extreme than any individual could
originally tolerate. Adaptive evolution is
observed when large genetically variable
populations are subjected to altered biotic or
physical environments.
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Conservation Importance of Adaptive
Evolution Preservation of ability of species to
evolve in response to new environments. Loss of
adaptive evolutionary potential in
small populations. Most endangered species exist
on the periphery of their historic range so they
must adapt to what was previously marginal
habitat.
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Genetic adaptation to captivity and its
deleterious effects on reintroductions. Adaptatio
n of translocated populations to their new
environment.
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Conservation biology is concerned with
preserving species as dynamic entities that can
evolve to cope with environmental
change. Retaining the ability to evolve requires
the preservation of genetic diversity. Consequent
ly, we must understand the factors that influence
the evolution of natural populations.
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An evolving population is a complex system
influenced by mutation, migration, selection,
and chance operating within the context of
the breeding system. To understand this
complexity, we use modelling with no factors, one
factor, two factors etc. In its simplest form,
evolution involves a change in gene frequency and
its importance can be summarized as
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Mutation is the source of all genetic diversity
but is a weak evolutionary force over
the short-term. Selection is the only force
causing adaptive evolutionary change. Migration
reduces differences between populations generated
by mutations, selection, and chance.
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Chance effects in small populations lead to
loss of genetic diversity and reduced
adaptive evolutionary potential. Fragmentation
and reduced migration lead to random
differentiation among subpopulations derived from
the same original source population.
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Selection arises because different genotypes
have different rates of reproduction and
survival (reproductive fitness) and such
selection changes allele frequencies. Selection
operates at all stages of life-cycle. In animals
this involves mating ability and fertility of
males and females, fertilizing ability of
sperm, number of offspring per female, survival
or offspring to reproductive age and longevity.
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The most intensive selection that can apply
against a recessive allele is when all
homozygotes die (lethal). For example, all
individuals homozygous for Chondrodystrophic
dwarfism (dwdw) in endangered California condors
die around time of hatching. Modeling impact of
selection against Chondrodystrophy in California
condors.
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The frequency of the dw allele in the next
Generation (q1) is q1 q/(1 q) The
change in frequency (?q) -q2/(1 q) Thus, the
lethal allele always declines in
frequency. Importance it becomes progressively
harder to reduce the frequency of the
deleterious recessive allele as its frequency
declines!
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Prior to industrial revolution, its peppered
wings provided camouflage as it rested on
lichen-covered tree trunks. Sulfur pollution
killed most lichen and soot darkened tree
trunks. Previously rare dark variants
(melanics) were now better camouflaged. Melanic
form was first reported in 1848 but by 1900 they
represented 99 of all moths in this part of
England.
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Simple model for this type of selection Beginnin
g frequencies of melanic (M) and typical
(m) Alleles with frequencies of p and q,
respectively. Assumptions Large random mating
population no migration no mutation selection
occurs on adults but before reproduction mm
individuals have a relative fitness of 1 -
s where s is the selection coefficient.
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1848, frequency of M p 0.005 and typicals
had Only 70 survival of melanics (s 0.3)
then p1 p/(1 - sq2) 0.005/(1 - (0.3 X
0.9952) 0.0071 Change in frequency (?p)
p1 - p 0.0071 - 0.005 0.0021
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Models of 4 different degrees of dominance
are given in Figure 6.5 In each case, the
selection coefficient (s) represents the
reduction in relative fitness of the genotype
compared to that in the most fit genotype
(Fitness 1.0). Values of s range from 0 to 1.
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Additive Case -- heterozygote has a
fitness intermediate between the two
homozygotes. Completely Dominant Case --
heterozygote has a fitness equal to the A1A1
homozygote. Partial Dominance Case --
heterozygote has a fitness nearer one of the
homozygotes than the other with its position on
the scale depending on the value of
h. Overdominant Case -- heterozygote has
higher fitness than either homozygote.
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The length of time it takes for an allele
frequency to change by a given amount of
selection depends upon the intensity of selection
and on the mode of inheritance. For a recessive
lethal allele we can determine the number of
generations to change an allele freq. from q0 to
qt as t 1/qt - 1/q0.
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