does reduced genetic diversity or inbreeding increase extinction risk? inbreeding does not always...
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Does reduced genetic diversity or inbreeding increase extinction risk?
Inbreeding does not always cause declines in pop’n size
Barrow island rock wallaby pop’nSmall, highly inbred, low genetic diversity
persisted > 1600 yrs
Mauritius kestrel6 generations with N< 50Very low genetic diversityPopulation still recovered
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Does inbreeding increase extinction risk?
Circumstantial evidence 1
Small populations are more prone to extinctions
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Does inbreeding increase extinction risk?
Circumstantial evidence. 2
Number spp. extinct since 1600 % on islands
Mammals 85 60Birds 113 81Molluscs 191 79Flowering plants 384 36
Island populations that are usually more inbred and less genetically diverse than mainland populations are more prone to extinctions
Q. Why isn’t this conclusive? What else is different about island populations?
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Does inbreeding increase extinction risk?Circumstantial evidence. 3Island endemics are more inbred and more prone to extinction than non-endemics
Higher extinction rate of endemic island species is predicted by genetic, but not demographic or ecological considerations
Endemics
Non-endemics Freq
uen
cy
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Does inbreeding increase extinction risk?Field evidence
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Do small populations have lower genetic diversity? YES
Does lower heterozygosity correlate with reduced survival or reproduction
YESDoes inbreeding reduce survival or
reproduction? YES
Does reduced genetic diversity or inbreeding increase extinction risk?
SOMETIMES
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TODAYDoes the loss of genetic diversity limit the ability of species to adapt to change
Change and evolutionQuantitative traits: the basicsDataThe unresolved issue
FridayPopulation size and evolutionary potential
How big is big enough?
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Environmental change
New diseases eg canine distemper virus
Pests and parasites eg Toxoplasma gondii
Competitors and predators eg foxes
Pollution
Human induced climate change
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Evolutionary responses to change
Peppered moth 1848 first melanic recorded1900 melanic form 99% in midlands2000 melanic form down to 10%
Eg1 rapid evolutionary changes in response to industrial pollution
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Evolutionary responses to change
Eg2 change in host preference in response to human induced habitat change
CheckerspotEuphydras editha
Original host: Collinsia parvifloraHabitat change – cattle ranching reduces host abundance, introduces weed Plantago lanceolata
P. l
C. p
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Q. Does inbreeding and the loss of genetic variation observed in small populations reduce their ability to adapt?
Answering this Q requires a DETOUR
into the genetics of quantitative traits
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What do we know about genetic variation for quantitative traits?
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Quantitative characters - Continuous distributionInfluenced by many loci
Affected by the environment
Phenotype = Genes it inherits + Environment P = G + E
Phenotypic variance = Sum contributions fromgenetic diversityenvironment + interactions betweengenes and environment
VP = VG + VE + 2.CovGE
Covariance between genetic and env effects
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VP = VG + VE + 2.Cov
GE
genetic:Additive genetic variation
alleles acting independently
Dominance variance
alleles affected by other alleles
Interaction variance
alleles affected by alleles at other loci
VA
VD
VI
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Additive Genetic Variation
Single locus model - additive effects d=0
Allele 1 freq = p, Allele 2 freq = qFreq heterozygote = 2pq
VA = 2pqa2
Variance is highest when heterozygosity is maximumVariance depends on a
half difference in mean of 2 homozygotes
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Evolution requires:
variation, heritability, selection
VP = VG + VE +2CovGE
VG = VA + VD + VI
h2 = VA/VP
S
Evolutionary potential of quantitative traits
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Heritability – the relationship between the traits of offspring and parents
The slope is a measure of heritability (h2)
Fig 5.5
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Estimating heritabilities
= regression of mean offspring on mean parent
= 2x regression offspring on one parent
= 2x correlation between full sibs
= 4x correlation between half sibs
Heritability estimates may be biased by
shared environments, maternal effects
and are specific to a particular pop’n in a particular environment
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Magnitude of Heritabilities
Most quantitative traits in outbreeding spp have heritable variation h2 > 0
Heritabilities are consistently lower for characters related to reproductive fitness than more periperal traits
Heritability of fitness trait body size bill sizeBirds (n=19) 0.245 0.572 0.674
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Evolutionary change - R
R=Sh2
h2=slope
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Predicting response to selection imposed by climate oscillations in Darwin’s finches
1976 – 1978 drought --> 85% mortalitysurvivors had wider beaks than the original
population S= 0.25 mmheritability = 0.745Predicted response = R = S.h2 =0.25.0.745 =
0.19mmobserved change = 0.25 mm
1984-86 selection favoured small bill width
S = -0.10Predicted response = R = S.h2 = -0.10x0.745 = -
0.07mmObserved response = - 0.16 mm
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Quantitative traits - key points
VA determines ability of a pop’n to evolve
VA is dependent on the heterozygosity of loci that affect that trait
Population size influences inbreeding and the loss of heterozygosity so…..
Small populations may have a reduced ability to adapt to environmental change
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Isle Royale wolf population was founded by 1 pairThis bottleneckreduces heterozygosity ( Ht/H0= 1-1/2N = 1-1/4 = ¾)VA = 2pqa2 and h2 = VA/VP
so reduces evol potential by 1/4 via h2increases inbreedingwhich reduces juv survival (if F =0.25 by 50%)reduces competition to replace parents (2/302/15)and reduces selection pressure
so reduces evol potential via S and Ne remains low (25 vs 5000)reduces heterozygosity loss = ∑[1-(1/2Ne)]t-1
so evol potential is further reduced via h2
Loss of evolutionary potential (R) in small populations
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What assumptions are involved in using these eqn’s to make this sort of argument? 1. Genetic drift is the major evolutionary force. Alleles are effectively neutral ie not selected upon
2. Mating is randomNo inbreeding avoidance
3. Loss of heterozygosity in quantitative trait loci conforms to theory based on neutral alleles
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Does inbreeding and the loss of genetic variation reduce the ability to adapt?
1. Experiments2. Field data on small and large
populations3. Selection experiments on targeted
traits
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Frankham et al. 1999
Frequ
ency
EXPERIMENTAL EVIDENCE
Wild- control
Bottleneck 1 pair 1 gen
Inbred - homozygous
Increase to same pop’n size
Increase NaCl conc’n from 0% until extinction
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Evolutionary potential in small populations
50 gen predictions based on R=S h2 ∑[1-(1/2Ne)]t-1
R50/R1 - cumulative response after 50 gendivided by response in first gen
Data from
Mice, flies, beetles, maize
Evolutionary potential is proportional to Ne
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FIELD DATA
Vulnerability to dieback root rot fungus
Jarrah Mortality<30% to >90%Variation in resistance is heritable
Wollemi pine - 40 adults No genetic diversity - 100s of markers No variation in resistance – 100% die No evolutionary potential
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Adaptation to climatic stress in Drosophila
RAIN
>2000mm120+ raindays
<1500mm<100 raindays
D. birchii - rainforest restricted fly
H = 0.65A = 8.4 allele/loci
Hoffmann et al 2003 Science
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Adaptation to climatic stress in Drosophila
RAIN
wet
Less wet
Dessication resistance (hours to 50% mortality) increases with latititude
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Adaptation to climatic stress in Drosophila
Molecular variation H=0.65 A=8.4
Quantitative variation
Dessication resistance h2=0
Wing size h2= 0.386-0.706
expected
Response to selection
Evolutionary potential is best estimated in targeted ecological traits using spp in threatened habitats
50 generations
30 selection events
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Do small populations have higher levels of inbreeding, reduced heterozygosity and lower levels of genetic variation?
YES
Does inbreeding/loss of heterozygosity reduce a population’s ability to adapt?
YESWhat is the unresolved issue?
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How closely correlated are molecular and quantitative measures of genetic variation?
Reed and Frankham meta-analysis - 71 datasetsmean corr r = 0.22H and life history traits r = -0.11 nsH and morph traits r = 0.30
Molecular measures of variation provide a very imprecise measure of evolutionary potential
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After reviewing this lecture you should now be able to:
Calculate Ne, H, F
Understand how/why Ne influences heterozygosity, inbreeding and evolutionary potential
Explain why it may be important to conserve genetic variation
Argue why genetic data should/should not inform conservation actions