Genetics and extinction

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Genetics and extinction
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Introduction
Inbreeding and loss of genetic diversity are
inevitable in small populations of threatened
species. Reduce reproduction and survival in
the short term Diminish the capacity of
populations to evolve in response to
environmental change in the long
term Contributed to previous extinctions
Constitute part of the threat to endangered
species.
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Genetics and the fate of endangered species
Contribution of genetic factors - considered to
be minor. Lande (1988) demographic,
environmental fluctuations, and catastrophes,
would cause extinction before genetic
deterioration became a serious threat to wild
populations. Theoretical and empirical evidence
- supporting the contention that genetic changes
in small population are intimately involved with
their fate.
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Specifically Many surviving populations have
now been shown to be genetically compromised
(reduced genetic diversity and inbred). Inbreedin
g causes extinctions in deliberately inbred
captive populations. Inbreeding has contributed
to extinctions in some natural populations. Compu
ter projections based on real life histories,
indicate that inbreeding will cause elevated
extinction risks in realistic situations faced by
natural populations. Loss of genetic diversity
increases the susceptibility of populations to
extinction.
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Inbreeding - production of offspring by
individuals related by descent, e.g.
self-fertilization, brother-sister,
parent-offspring, etc. Inbreeding reduces
reproduction and survival (reproductive fitness)
inbreeding depression.
Evidence that inbreeding adversely affects most
wild populations. Crnokrak Rolf (1999)
reviewed 34 species, for inbreeding depression in
natural situations. In 90 cases inbred
individuals showed inbreeding depression.
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Significant inbreeding depression has been
reported in at least 15 taxa.
Example - desert topminnow fish, Vrijenhoed
(1994). Prior to a drought that eliminated their
habitat, the sexually reproducing topminnow
numerically dominated the asexual species.
Following the drought, populations of both
species were re-established. Asexual species
numerically dominated the sexual species. The
sexual species had lost much of its genetic
variation (and was inbred) as a consequence of
small number of individuals taking part in the
re-establishment (founding) events. After
deliberate replacement of 30 of the sexual fish
with 30 outbred individuals from elsewhere, to
restore genetic diversity, the sexual species
regained numerical dominance.
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Genetic diversity is the extent of heritable
variation in a population, or species, or across
a group of species.
Genetic diversity is required for populations to
evolve in response to environmental change.
Consequently, if there is no genetic diversity
in a populations or species, it is likely to go
extinct in responses to major environmental
change. Genetic diversity is lost in small
random mating populations at the same time they
become inbred, so the two processes are closely
related.
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Measures of inbreeding
There are several ways to measure the extent of
inbreeding in a population

• The inbreeding coefficient (F)
• Average inbreeding
• Inbreeding relative to random breeding

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The inbreeding coefficient (F) The inbreeding
coefficient of an individual refers to how
closely related its parents are. When parents
are unrelated, offspring F 0, while when
inbreeding is complete F 1. Level of inbreeding
for different kinds of relationships among
parents are Parent Offspring
F Unrelated 0 Brother-sister, mother-son, or
father-daughter 0.25 Half-brother-half-sister
(half-sibs) 0.125 First cousins 0.0625 In
breeding accumulates in closed populations (those
without immigration) and complete inbreeding can
eventually be reached with repeated inbred
matings
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Average inbreeding A second measure of
inbreeding is the average inbreeding coefficient
of all individuals in a population. In small
closed populations, average F will inevitably
rise as mates become increasingly related.
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Inbreeding relative to random breeding The
third way to measure inbreeding is to compare the
average relatedness of mates (parents) to what
one would expect if the population was breeding
randomly.
If the population is mating at random, the F
0. If mates are more closely related than
expected under random matings, then the
population is said to be inbreeding and Fgt0. If
individuals in the populations are actively
choosing less related mates, then the populations
is said to be avoiding inbreeding and Flt0. This
F value refers to the average F of the
population, while the inbreeding coefficient is
calculated for a particular individual. Levels
of inbreeding can be determined from pedigrees,
or inferred from heterozygosities for genetic
markers.
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Relationship between inbreeding and extinction
First line of evidence on the relationship
between inbreeding and extinction came from
deliberately inbred populations of laboratory and
domestic animals and plants. Between 80 and
95 of deliberately inbred populations die out
after eight generations of brother sister
mating. For example, 338 populations of Japanese
quail, inbred by continued brother sister
mating, were all extinct after four generations.
Such extinctions could be due to either
inbreeding, or demographic stochasticity
(fluctuations in birth and death rates and
sex-ratios) or a combination of these effects.
However, under circumstances where demographic
stochasticity is excluded, inbreeding clearly
increased the risk of extinction in captive
populations.
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Rate of inbreeding and extinction risk Natural
populations of outbreeding wild animals and
plants are usually subjected to slower rates of
inbreeding, dependent on their population sizes.
Slower inbreeding allows natural selection more
opportunity to remove genetically compromised
individuals (and thereby remove deleterious
alleles). However, even slow rates of
inbreeding increase the risk of extinction it
just takes longer for inbreeding to accumulate
and extinction to occur.
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For example, 15 of 60 fruit fly populations,
inbred due to sizes of 67 individuals per
generation, went extinct within 210 generations.
In a similar manner, 5 of 6 replicate housefly
populations of size 50 went extinct over 64
generations. A comparison of extinction risks
for the same amount of inbreeding - but due to
slower double first-cousin versus faster brother
sister inbreeding, revealed no significant
difference in extinction risk. Accordingly
differences in the effects of rate of inbreeding
on extinction risk for the same amount of
inbreeding are unlikely to be large.
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Do taxonomic groups differ in susceptibility to
inbreeding depression? Inbreeding depression
for wild populations of homeotherms,
poikilotherms and plants do not differ
significantly. Nor are there any significant
differences in inbreeding depression under
captive conditions among mammalian orders. A
comparison of extinction proneness due to
inbreeding in 25 captive populations failed to
find significant differences among mammals,
birds, invertebrates and plants
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Inbreeding depression in plants is typically
higher for gymnosperms than angiosperms. This
could be related to a higher level of polyploidy
(more than two doses of each chromosome, e.g., 4n
vs. 2n) in the latter than the former. Since
the rate of increase in homozygosity is slower in
polyploids than in diploids, polyploids are
expected to suffer less inbreeding depression.
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Inbreeding and extinction in the wild Since
inbreeding leads to elevated extinction risks in
captive populations, it is logical to extrapolate
this to wild populations to inbreeding and
consequent elevated extinctions risk Computer
projections have predicted that inbreeding will
increase extinction risks for wild
populations. Many small surviving wild
populations have now been shown to be genetically
compromised. Direct evidence of inbreeding
and loss of genetic variation contributing to
the extinction of populations in nature has
been presented
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Computer projections Computer projections
incorporating factual life history information
are often used to assess the combined impact of
all deterministic and stochastic factors on the
probability of extinction of populations.
Information on population size, births and
survival rates and their variation over age and
years together with measures of inbreeding
depression, changes in habitat quality, etc. form
the input.
Stochastic models are then run through repeated
cycles to project the fate of populations into
the future. Mills Smouse (1994) used computer
simulations to show that inbreeding generally
increases the risk of extinction, especially in
species with low reproductive rates. These
simulations encompassed only a 20-year time
frame, representing less than five generations
for the types of life cycles they simulated.
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Direct evidence of extinctions due to inbreeding
and loss of genetic diversity Experimental
populations of the evening primrose plant founded
with a low level of genetic diversity (and high
inbreeding) exhibited 78 extinction rates over
three generations in the wild, while populations
with lower inbreeding showed only a 21
extinction rate. Differences in inbreeding as
well as differences in genetic diversity were
presumed to be involved.
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Circumstantial evidence for extinctions due to
inbreeding The responses of populations to
environmental stochasticity (random unpredictable
variation in environmental factors), demographic
stochasiticity and the impact of catastrophes are
not independent of inbreeding and genetic
diversity. Inbreeding, on average, reduces
birth rates and increases death rates and may
distort sex-rations. It therefore interacts
with the basic parameters determining population
viability such as population growth rate and
variation in population size. 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 population founded
from unrelated pairs.
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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 as the
extinction vortex. The complicated interactions
between genetic, demographic and environmental
factors can make it extremely difficult to
identify the immediate cause(s) for any
particular extinction event.
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Smaller populations are expected to be more
extinction prone than larger ones for
demographic, ecological and genetic reasons.
Berger (1990) found a strong relationship
between population size and persistence in North
American bighorn sheep. All populations of lt50
became extinct within 50 years. Mammalian
extinctions in national parks in western North
America were related to park area, and presumably
population sizes. Extinctions were more frequent
for populations with smaller initial population
sizes, or large fluctuations in population size,
than in those with longer generation times.
Declines in population size or extinction in
the wild have been attributed, at least in part,
to inbreeding in many populations including
bighorn sheep, Florida panthers, Isle Royale gray
wolves, greater prairie chickens, heath hens,
middle spotted woodpeckers, adders, and many
island species.
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Extinction proneness of island populations Recor
ded extinctions since 1600 reveal that a majority
of extinctions have been of island forms, even
though island species represent a minority of
total species in all groups. For example, only
20 of all bird species live on islands, but 80
of bird species driven to extinction have been
island dwellers. Vertebrates, endemic island
species (species not found elsewhere) are more
prone to extinction than non-endemic species.
Human factors have been the major recorded
causes of extinction islands over the past 50 000
years. The human impacts have typically driven
down population sizes to the point where
stochastic factors come into play. The
mechanisms underlying susceptibility of
island populations to extinction are
controversial.
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Ecologists stress the susceptibility of small
island populations to demographic and
environmental stochasticity. However, this
susceptibility is also predicted on genetic
grounds. Island populations are expected to be
inbred due to both low numbers of founders on
remote islands (often a single inseminated female
animal or a single plant propagule), and
subsequent small populations sizes. There is
essentially little critical 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. Island populations typically have less
genetic diversity and are more inbred than
mainland populations. A meta-analysis of data
involving 202 island populations revealed that
82 had lower levels of genetic diversity than
their mainland counterparts. Island populations
are significantly inbred with endemic island
populations more so than non-endemic.
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Inbreeding in many island populations is at
levels where captive populations show an elevated
risk of extinction. The levels of inbreeding in
these island populations are not in accord with
predictions that demographic and environmental
stochasticity and catastrophes will drive
populations to extinction before genetic factors
become a problem. Endemic island populations
have generally existed on islands at restricted
population sizes for longer than non-endemics -
expected to be more inbred - are expected to be
more prone to extinction than non-endemic for
genetic reasons. There are no obvious
demographic or environmental reasons why
endemic and non-endemic island populations
should differ in extinction proneness - this
indicates that genetic factors are, at least
partly, responsible for the extinction
proneness of island populations.
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Humans are fragmenting habitat throughout the
world. This results in islands (remnants,
reserves, national parks, etc) in a sea of now
inhospitable landscape. Consequently, these
population fragments share many characteristics,
including susceptibility to extinction, with
their island counterparts. This is the case for
isolated populations of greater prairie chickens
in Illinois and adders in Sweden. Both
populations declined due to inbreeding depression
and only recovered following introduction of
additional genetic diversity.
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Relationship between loss of genetic diversity
and extinction Natural populations face
continuous assaults from environmental changes
including new diseases, pests, parasites,
competitors and predators, pollution, climatic
cycles such as the El Nino La Nina cycles, and
human-induced global climate change. Species
must 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 responses
to such environmental changes. Evolutionary
responses to environmental change have been
observed in many species. For example, over 200
species of moths have evolved black body colours
to aid in camouflage in response to industrial
pollution.
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Small populations typically have lower levels of
genetic diversity than large populations. This
is due to sampling of alleles in the parental
generation in production of offspring. During
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. Over
time genetic diversity will decline, with loss
being more rapid in smaller than in larger
populations. Genetic variation allows
populations to tolerate a wide range of
environmental extremes. Humans are generating
increasing rates of environmental change. For
example, increasing levels of greenhouse gas are
causing global climate change. If populations are
to cope with these factors they require genetic
diversity.
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There are compelling theoretical predictions that
loss of genetic diversity will reduce the ability
of populations to evolve in response to
environmental change. Experimental evidence
validates these predictions. Consequently, we
expect a similar relationship between loss of
genetic diversity and extinction rate due to
environmental change.
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