Transcript 1091-L4(ConsGen3a)

Does reduced genetic diversity or
inbreeding increase extinction risk?
Barrow island rock wallaby pop’n
Small, highly inbred, low genetic
diversity
persisted > 1600 yrs
Mauritius kestrel
6 generations with N< 50
Very low genetic diversity
Population still recovered
Inbreeding does not always cause declines in pop’n size
Does inbreeding increase extinction risk?
Circumstantial evidence 1
Small populations are more prone to extinctions
Does inbreeding increase extinction risk?
Circumstantial evidence. 2
Number spp.
extinct since 1600 % on islands
Mammals
85
60
Birds
113
81
Molluscs
191
79
Flowering 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?
Does inbreeding increase extinction risk?
Endemics
Frequency
Circumstantial evidence. 3
Island endemics are more inbred and more prone to
extinction than non-endemics
Non-endemics
Higher extinction rate of endemic island species is
predicted by genetic, but not demographic or
ecological considerations
Does inbreeding increase extinction risk?
Field evidence
Do small populations have lower genetic
diversity?
YES
Does lower heterozygosity correlate
with reduced survival or reproduction
YES
Does inbreeding reduce survival or
reproduction?
YES
Does reduced genetic diversity or
inbreeding increase extinction risk?
SOMETIMES
TODAY
Does the loss of genetic diversity limit the
ability of species to adapt to change
Change and evolution
Quantitative traits: the basics
Data
The unresolved issue
Friday
Population size and evolutionary potential
How big is big enough?
Environmental change
New diseases eg canine distemper virus
Pests and parasites eg Toxoplasma gondii
Competitors and predators eg foxes
Pollution
Human induced climate change
Evolutionary responses to change
Eg1 rapid evolutionary changes in response
to industrial pollution
Peppered moth
1848 first melanic recorded
1900 melanic form 99% in midlands
2000 melanic form down to 10%
Evolutionary responses to change
Eg2 change in host preference in response
to human induced habitat change
Checkerspot
Euphydras editha
P. l
C. p
Original host: Collinsia parviflora
Habitat change –
cattle ranching reduces host abundance,
introduces weed Plantago lanceolata
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
What do we know about
genetic variation for
quantitative traits?
Quantitative characters - Continuous distribution
Influenced by many loci
Affected by the environment
Phenotype = Genes it inherits + Environment
P
=
G
+
E
Phenotypic variance = Sum contributions from
genetic diversity
environment +
interactions between
genes and environment
VP = VG + VE + 2.CovGE
Covariance between genetic and env effects
VP = VG + VE + 2.Cov GE
VA
VD
genetic:
Additive genetic variation
alleles acting independently
Dominance variance
alleles affected by other alleles
VI
Interaction variance
alleles affected by alleles at other loci
Additive Genetic Variation
Single locus model - additive effects d=0
Allele 1 freq = p, Allele 2 freq = q
Freq heterozygote = 2pq
VA = 2pqa2
Variance is highest when heterozygosity is maximum
Variance depends on a
half difference in mean of 2 homozygotes
Evolutionary potential of quantitative traits
Evolution requires:
variation, heritability, selection
VP = VG + VE +2CovGE
S
h2 = VA/VP
VG = V A + VD + VI
Heritability – the relationship between the
traits of offspring and parents
Fig 5.5
The slope is a measure of heritability (h2)
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
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
Birds (n=19)
0.245
body size bill size
0.572
0.674
Evolutionary change - R
R=Sh2
h2=slope
Predicting response to selection imposed by
climate oscillations in Darwin’s finches
1976 – 1978 drought --> 85% mortality
survivors had wider beaks than the original population
S= 0.25 mm
heritability = 0.745
Predicted response = R = S.h2 =0.25.0.745 = 0.19mm
observed change = 0.25 mm
1984-86 selection favoured small bill width
S = -0.10
Predicted response = R = S.h2 = -0.10x0.745 = -0.07mm
Observed response = - 0.16 mm
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
Loss of evolutionary potential (R) in small populations
Isle Royale wolf population was founded by 1 pair
This bottleneck
reduces heterozygosity ( Ht/H0= 1-1/2N = 1-1/4 = ¾)
VA = 2pqa2 and h2 = VA/VP
so reduces evol potential by 1/4 via h2
increases inbreeding
which reduces juv survival (if F =0.25 by 50%)
reduces competition to replace parents (2/302/15)
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
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 random
No inbreeding avoidance
3. Loss of heterozygosity in quantitative trait loci
conforms to theory based on neutral alleles
Does inbreeding and the loss of genetic
variation reduce the ability to adapt?
1. Experiments
2. Field data on small and large populations
3. Selection experiments on targeted
traits
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
Frankham et al. 1999
Evolutionary potential in small populations
50 gen predictions based on
R=S h2 ∑[1-(1/2Ne)]t-1
Data from
Mice, flies,
beetles, maize
R50/R1 -
cumulative response after 50 gen
divided by response in first gen
Evolutionary potential is proportional to Ne
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
Adaptation to climatic stress in Drosophila
RAIN
>2000mm
120+ raindays
<1500mm
<100 raindays
Hoffmann et al 2003 Science
H = 0.65
A = 8.4 allele/loci
D. birchii - rainforest restricted fly
Adaptation to climatic stress in Drosophila
RAIN
wet
Less wet
Dessication resistance (hours to 50%
mortality) increases with latititude
Adaptation to climatic stress in Drosophila
expected
Response to selection
50 generations
30 selection events
Molecular variation
H=0.65
A=8.4
Quantitative variation
Dessication resistanceh2=0
Wing size
h2= 0.386-0.706
Evolutionary potential is best estimated in targeted
ecological traits using spp in threatened habitats
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?
YES
What is the unresolved issue?
How closely correlated are molecular and
quantitative measures of genetic variation?
Reed and Frankham
meta-analysis - 71 datasets
mean corr r = 0.22
H and life history traits r = -0.11 ns
H and morph traits r = 0.30
Molecular measures of variation provide a
very imprecise measure of evolutionary
potential
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