genetic diversity among populations (and factors that increase or decrease diversity)

Genetic diversity among populations

(and factors that increase or decrease diversity)

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

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

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)

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)

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

Isolation by distance

http://www.youtube.com/watch?v=31qBrRawDK8

Isolation by distance

Mussel ‘lures’ with glochidia

Elliptio dilatata

Genetic distance between populations

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)

Issues with genetic diversity among populations outbreeding depression/hybridization• local adaptation

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

Issues with genetic diversity among populations outbreeding depression/hybridization• local adaptation

• co-adapted gene complexes

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….

inbreedingdepression

outbreedingdepression

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inbreeding random mating inter-breeding

Why do we care?

Inbreeding: may be last recourse for endangered population

Outbreeding: dangerous consequence of saving small popns.

“Genetic pollution” controversy

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

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

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

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

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

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)

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