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

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 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?

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

Does inbreeding increase extinction risk?Field evidence

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

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?

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

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

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

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 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

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

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

Evolution requires:

variation, heritability, selection

VP = VG + VE +2CovGE

VG = VA + VD + VI

h2 = VA/VP

S

Evolutionary potential of quantitative traits

Heritability – the relationship between the traits of offspring and parents

The slope is a measure of heritability (h2)

Fig 5.5

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 body size bill sizeBirds (n=19) 0.245 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% 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

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

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

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

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

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

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

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

>2000mm120+ raindays

<1500mm<100 raindays

D. birchii - rainforest restricted fly

H = 0.65A = 8.4 allele/loci

Hoffmann et al 2003 Science

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

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

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?

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

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