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Genetic Drift
What is it?
A sampling error problem
Imagine you are flipping a coin…
Genetic drift is important only when populations
are SMALL
ways for a population to end up small…?
Bottlenecks
Founder events
Imagine you have all the genes from a single
locus of a single population in a bottle…
Is genetic drift an example of evolution?
Examples of bottlenecking/founder events……
Humans
•Pingelap Atoll 20,000 pop dropped to 20 in 1775Typhoon
At least 1 of the 20 carried an allele that
when homozygous=loss of vision
Conservation examples…
Elephant seals
Northern’s reduced to 20(?)
in the 1890s
Now about 30,000
24 loci
Southern pop has abundant
variation
Cheetahs
52 loci and found no diversity
skin transplants not rejected (MHC)
Low sperm counts
High infant mortality
Disease susceptibility
Florida Panther=Puma=Cougar=Mountain lion
Roelke et al. (1993).
Nearly as low as Cheetahs
With Fresh Blood, Inbred Florida Panthers Rebound
By Robert Roy Britt
LiveScience Managing Editor
posted: 18 August 2005
10:26 am ET
A controversial breeding program has improved the genetic diversity of inbred Florida panthers and the endangered
animals are on the rebound, scientists announced today.
As few as 30 wild panthers roamed the Florida Everglades in the early 1990s. Abnormalities such as low sperm
counts and heart defects were becoming common, studies found, and the kittens had low survival rates.
In 1995, researchers outfitted some female Texas panthers with radio collars and introduced them into four sections
of the Florida Everglades. Some Florida panthers were also tagged.
Researchers monitored the cats and kittens and found that the hybrids had better survival rates, presumably
because they were more genetically diverse. By 2003, three of the Texas panthers were still alive, and they were
removed -- scientists figured enough fresh blood had been injected into the Florida population and they wanted to
keep outside genetic exposure to a minimum.
Today, there are at least 87 wild panthers in Florida.
"More than three times as many hybrid kittens appear to reach adulthood as do purebred ones," Pimm and his
colleagues write in a paper that will be published early next year in the British journal Animal Conservation.
And the hybrids are on the move. The purebred panthers rarely ventured beyond protected areas north of interstate
75 and west of State Highway 29. Hybrid offspring have moved south and east into new sections of the Everglades
National Park and Big Cypress National Preserve. They're pushing into places that are less hospitable, but they're
doing so with success, the scientists said.
The federal government has decided that the hybrids will be called Florida panthers for purposes of determining their
endangered species status.
Mongolian Wild Horse/Przewalski’s Horse
Foals captured in Mongolia in 1800s and shipped to
Europe
(After 1945 only two captive populations in zoos remained, in Munich
and Prague. The most valuable group, in the Ukraine, was shot by
German soldiers during WWII. ….By the end of the 1950s, only
12 individual Przewalski's horses were left in the world). Wikipedia…
Went extinct in wild in 1960s
9 used to start a breeding program
Reintroduced to Mongolia
Currently reproducing in wild!
Review
In very small populations (bottlenecks, founder
events) variation is lost (alleles are lost)
AND
rare alleles can become common
common alleles might become less common
Due to random changes in gene frequency
not a result of selection, mutation or
immigration.
Gene flow
movement of …..individuals or gametes….
between populations
tends to reduce differences between populations
that have accumulated because of natural
selection or genetic drift…..
Gene flow is a big issue in the 21st century!
How are we genetically modifying crops?
•Herbicide tolerance-roundup ready corn
•Insect resistance…
HT=herbicide
tolerance
Bt =insect
resistance
http://blogs.ei.columbi
a.edu/2011/06/23/clim
ate-ready-crops-thepros-and-cons/
Most of the world‘s major crops naturally hybridize
with wild relatives somewhere in the world……
(corn, mustards, sunflowers, squash, carrots and
strawberries)
Why is this a problem?
Mexico to start experimental planting of GMO corn
http://www.reuters.com/article/idUSN0424072320090604
Thu Jun 4, 2009 6:24pm EDT
MEXICO CITY, June 4 (Reuters) - Mexico, considered the birthplace of corn, is reviewing more than
two dozen requests to begin experimental planting of genetically modified crops, the agriculture
ministry said on Thursday.
He said U.S. biotech food producer Monsanto Co (MON.N) is one of the companies who have
applied for a permit to begin planting.
Supporters of GMO food, whose DNA is altered to be resistant to pests, say they boost
yields. More than 70 percent of U.S. corn is genetically modified.
But farmers in Mexico's rural south, where corn has been grown for thousands of years,
worry GMO corn will cross-pollinate with native species and alter their genetic content.
Corn was first planted in Mexico some 9,000 years ago and the country is now home to
more than 10,000 varieties. The grain was adopted by Spanish conquistadors in the early
1500s and eventually spread to the rest of the world.
Under the current rules, GMO corn seeds are not allowed into certain parts of the country
that are determined to be "centers of origin" for genetically unique corn strains found only
in Mexico.
Genetic Variation
What is it?
Why is variation important?
Where does new variation come from?
Mutation
Sexual reproduction-Recombination-Shuffling of alleles.
What was the Red Queen Hypothesis for the evolution of
sex?
How is variation preserved? (back of Chapter)
Diploidy…why does this preserve variation?
Selection itself may preserve variation-how?
Niels Dingemanse University of Groningen, Netherlands
Genetic variation underlying exploration behavior in
small birds!
Some highly exploratory others more cautious
When food scarce-who does well?
When food common –who does well?
Gene flow study with this
Species…(Gene Flow p410)
Evolutionary Psychology…
“Why do we all have different personalities?
Why hasn't natural selection homed in on optimum character traits
instead of allowing so much variability?
As the study of personality moves to a more scientific footing, we
are starting to understand the underlying neurobiology, and to see
that each personality trait is beneficial in certain circumstances
and costly in others.
We may tend to think that some personality types are more
desirable than others, but these new insights make it clear that there
is no "best" personality. It really does take all sorts to make a
world.”
Nettle, New Scientist Feb 2008
Another way selection
preserves variation
Aa “do best” or have
highest fitness
(AA get Malaria, aa get
sickle cell disease)
What is this called?
Frequency dependent selection
Rare individuals have higher fitness..
“it is good to be unusual”
EX. Scale eating fish in text p415