Genetic diversity among populations (and factors that increase or decrease diversity)
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Genetic diversity among populations
(and factors that increase or decrease diversity)
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p = .5q = .5
p = .55q = .45
p = .7q = .3
p = .8q = .2
Populations that are spatially isolated will tend to diverge genetically
• genetic drift
• natural selection and local adaptation
p = .5q = .5
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Genetic diversity among populations• Increases due to isolation, followed by
– genetic drift– inbreeding– selection– local adaptation
• Decreases due to gene flow (migration) as migrants move between populations, they homogenize
allele frequencies among populations
• larger populations diverge slowly through drift – few migrants needed to counteract
• small populations diverge rapidly through drift – more migrants needed to counteract
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Changes in genetic diversity among populations
m = proportion of population that migrates
Nm = number of migrants randomly exchanged per generation Ne = 1,000, m = 0.01, Nm = 10Ne = 100, m = 0.01, Nm = 1
approx 1 migrant/generation will maintain same alleles among populations (= qualitative similarity)
but 10 migrants per generation may still permit significant differences in allelic frequencies ( = quantitative dissimilarity)
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Changes in genetic diversity among populations
m = proportion of population that migrates
Nm = number of migrants randomly exchanged per generation Ne = 1,000, m = 0.01, Nm = 10Ne = 100, m = 0.01, Nm = 1
“in the absence of natural selection, the amount of genetic divergence among demes is a function of the absolute number of migrants exchanged (Nm), not the proportion of exchange (m)” (Allendorf 1983)
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p = 0.4q = 0.6
p = 0.55q = 0.45
p = 0.75q = 0.25
p=0.5q=0.5
Isolation by distance
p = 0.5q = 0.5
p = 0.65q = 0.35
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Isolation by distance
http://www.youtube.com/watch?v=31qBrRawDK8
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Isolation by distance
Mussel ‘lures’ with glochidia
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Elliptio dilatata
Genetic distance between populations
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Clegg, S.M., S.M. Degnan, J. Kikkawa, et al. 2002. Genetic Clegg, S.M., S.M. Degnan, J. Kikkawa, et al. 2002. Genetic consequences of sequential founder events by an island-consequences of sequential founder events by an island-colonizing bird. PNAS 99:8127-8132colonizing bird. PNAS 99:8127-8132
silvereye (Zosterops lateralis)
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Issues with genetic diversity among populations outbreeding depression/hybridization• local adaptation
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Issues with genetic diversity among populations outbreeding depression/hybridization• local adaptationexample: ibex extirpated from Czechoslovakia (Capra ibex ibex)
- transplanted from Austria successfully (Capra ibex ibex)
- then added bezoars (C. i. aegagrus) and Nubian ibex (C. i. nubiana)
- fertile hybrids rutted in early fall instead of winter (as natives did)
- kids of hybrids born in February, coldest
month of year, entire population
went extinct
David Hall
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Issues with genetic diversity among populations outbreeding depression/hybridization• local adaptation
• co-adapted gene complexes
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Loss of fitness due to -
Inbreeding:accumulation of homozygous recessivesloss of superior heterozygotes
Outbreeding:reduced fitness of F1 generation - disruption of local adaptation - epistatic interactionsreduced fitness of F2 generation - breakup of co-adapted gene complexes
More empirical studies on inbreeding than outbreeding….
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inbreedingdepression
outbreedingdepression
Rep
rodu
ctiv
e su
cces
s
inbreeding random mating inter-breeding
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Why do we care?
Inbreeding: may be last recourse for endangered population
Outbreeding: dangerous consequence of saving small popns.
“Genetic pollution” controversy
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Florida panther:
declined due to habitat loss, poaching, road killsevidence of inbreeding:
low fertility, sperm abnormalities, cowlicks, kinked tails,cardiac defects, undescended testicles, high disease rate
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Florida panther:
Genetic studies indicated low variability:
P H DNA HFlorida 4.9 1.8 10.4Western US 9.9 4.3 29.7Other felids 8-21 3-8 45.9
Found to have hybridized with S. American sub-species; introgressed animals with higher P
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Florida panther:
Outbred with sub-species from Texas - added 8 females in 1995
F1 hybrid kittens do not have cowlinks or kinked tailsTexas genes are now 15-29% of total
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Cheetah (Acinonyx jubatus):
O’Brien et al. 1983. The cheetah is depauperate in genetic variation - using protein electrophoresis
# # % poly. av.Species popns. N loci loci HDrosophila 43 >100 24 43.1 0.14Mus 2 87 46 20.5 0.088Homo sapiens many >100 104 31.7 0.063Felis catus 1 56 55 22 0.076Cheetah 2 55 47 0.0 0.0
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Cheetah (Acinonyx jubatus):
Menotti-Raymond and O’Brien et al. 1993. - using protein electrophoresis, high-resolution PE, and mtDNA
# # % poly. av.Species popns. N loci loci HDrosophila 43 >100 24 43.1 0.14Mus 2 87 46 20.5 0.088Homo sapiens many >100 104 31.7 0.063Felis catus 1 56 55 22 0.076Cheetah 2 55 47 0.0 0.0
Drosophila 1 20 54 11.1 0.04Mus 1 72 4.1 0.02Homo sapiens 1 34 many 1.2 0.063Cheetah 1 6 155 3.2 0.013
Felis catus 1 17 46.0Cheetah 3 76 45.0
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O’Brien et al. 1983. The cheetah is depauperate in genetic variation - assumed to be result of small N, bottleneck, then inbreeding - highly vulnerable to disease outbreaks (50% mortality in one captive population)
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O’Brien et al. 1983. The cheetah is depauperate in genetic variation - assumed to be result of small N, bottleneck, then inbreeding - highly vulnerable to disease outbreaks (50% mortality in one captive population)
Merola, 1994. A reassessment of homozygosity …. - carnivores tend to show low levels of genetic variation
(several have lower levels of H and P than cheetah) - measures of fluctuating asymmetry indicate cheetah is not
suffering from low homozygosity or genetic stress - sperm deformities – do not affect fertility, may be normal in felids - low litter sizes – in captivity (high in wild) - susceptibility to disease – may be due to captive contact (in wild,
cheetahs avoid conspecifics)
Concluded that conservation is better directed at habitat