l22 small populations

8/12/2019 L22 Small Populations

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Small Populations and Losing Genetic Diversity do populations suffer a significantdeteriorationas a consequence of a

sudden decrease in numbers (a genetic

bottleneck) ?

eg. reduction in fitness of

cheetahs following a bottleneck

about 10,000 yrs BP

current populations have

deformed sperm; low genetic

variation; susceptibility to

disease; high mortality of cubs


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What genetic variables must be

considered by conservation ecologists ?

population size (bottlenecks)

population origin (founder effects)

genetic drift (loss of variability)

gene flow (gain of variability)

inbreeding depression (loss of fitness)

out-breeding depression (loss of fitness)

hybridization (loss of distinctness/gain in variability)


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genetic bottleneck

population has an abrupt contraction

in numbers

accompanied by change in genefrequencies

decline in genetic variation due to lossof numbers

Bottleneck


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population numbers may recover, but genetic

variation may be low

originalpopulation

bottleneck

recoveringpopulation


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bottleneck

recovery

isolation & low diversity

populations must remain small

& inbreed

Bottleneck options


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founder effect

establishment of new population by one or a few

individuals

low levels of genetic variation depending on the

genetic variation in the founder population

foundingpopulation recovered population

numbers


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may result in a disproportionate numberof

alleles rarein the parent population but nowcommonin the new population

founder effect

2 in 14

= 1 in 7

4 in 5


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inbreeding depression

when closely related individuals mate (inbreed),there is a high probability that genes will be

homozygous

some recessive alleles are deleterious

effects only apparent when they individuals are

homozygous for the recessive allele

may result in lower fitness ie. reduced

successful reproduction

more likely to occur in small populations such as

populations recovering from a bottleneck


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gene flow

gene flow is movement of genes amongpopulations, usually through dispersal and

migrationof individuals

high gene flow causes populations to becomegenetically similar

restricted gene flowallows populations to become

different

restricted gene flow can also lead to loss of genetic

diversity (genetic drift and inbreeding)


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genetic drift

random change in gene frequencies throughchance

more apparent in smallpopulations

occurs because there is an element of chancein

which individuals successfully breed

can lead to loss of genetic variation

larger populations sizes can buffer against loss

of genetic variation


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out-breeding

breeding between individuals from populationswhich have been isolated

many isolated populations have distinctivelydifferent gene pools

out-breeding introduces new alleles into the

population (gene flow ?)out-breeding may be desirable as it can increasegenetic diversity eg. Black-footed Rock-wallaby

out-breeding depression

may result in loss of fitness

may introduce genes not adapted to theparticular environment


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in nature, hybridization is prevented by a

barrier -geographic

seasonal

behavioural

animal species rarely hybridize

plants often hybridize

offspring in animals are usually infertile;

plant hybrids are often fertile

hybridization

production of offspring by parents fromdifferent species


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plant species hybridize more readily

eg. Buxton Gum (rare) x Swamp Gum

(common)

offspring frequently fertile

F1 offspring can breed back to the parents,

with other F1s,or with F2s

called introgression

results in populations

with a genetic range from

pure to very hybridized


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hybridization is uncommon in animals, but

can occur

sometimes offspring are fertile

introgression can occur

introgression is the breeding of F1, F2, F3 etc.

offspring with the parental species, and eachother, and results in a "hybrid swarm"

eg.Pacific Black Duck x Mallard Duck P1

P2F1


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hybridization in rare species presents some

"problems" for conservation

1. rare population may show "outbreeding

depression" - reduced "fitness" due to gamete

wastage and reduced seed set in the parent species

outbreeding depression is unlikely to be a problemin long-lived species; may be a problem in rare,

long-lived species

2. if hybrids are fertile, hybridization may lead toassimilation of the rare species through

introgression


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3.are hybrid populations legally protectedonce

they are not "genetically pure" ?

4.hybrid swarmsrepresent large pools of genetic

diversity - isnt this desirable ????

5.hybridization may lead to the evolution of new

species- one of the aims of biodiversity

conservation - what if it occurs with the loss onone or both parent species ??? How important is

this ?

l22 small populations

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