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Population Genetics II.

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Population Genetics II. Natural selection and the genetic makeup of populations: Natural selection + limited gene flow = adaptation to local conditions – PowerPoint PPT presentation

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Title: Population Genetics II.


1
Population Genetics II.
Natural selection and the genetic makeup of
populations
Natural selection limited gene flow
adaptation to local conditions
Example metal tolerance on a site contaminated
with mine tailings
2
Clines
The gradient in allele occurrence across a set
of populations due to natural selection
Example leaf hairiness in Spergula arvensis
Hairy plants increase In frequency with latitude
3
Another example of a cline
Flower scent in Polemonium viscosum high
altitude bumblebee pollination low altitude
flies also pollinate it
4
Clines illustrate directional selectionover a
range of environmentalconditions.
directional selection?
Better survival and reproduction for
organisms at one end of the phenotypic
distribution
5
Other types of selection
Stabilizing selection The mean phenotype is
favored by natural selection
Disruptive selection The extremes of the
phenotypes have higher fitness than mean
phenotype
Fitness?....
6
A graphical illustration
(Silvertown and Charlesworth 2001, p. 77)
7
Some real examples
Directional selection widowbirds
Galapagos Finch beaks
8
Some real examples
Stabilizing selection flowering time in Senecio
integrifolius
Another example birth weight in human babies
9
Some real examples
Disruptive selection Garter snake coloration
and behavior
10
Other types of selection
  • Balancing selection the heterozygote
  • advantage

Example sickle cell anemia.
11
Other types of selection
Negative frequency dependence the rare alleles
have the advantage
Example Self-incompatibility alleles in plants
Mechanism by which genetic diversity is
maintained
12
Other types of selection
Positive frequency-dependent selection
Ipomea purpurea flowers
Can lead to loss of genetic diversity
13
The mutation-selection balance and mutation load
Even while natural selection acts to
eliminate deleterious mutations, new
mutations are being formed
An equilibrium exists between the creation
of new mutations and their elimination
by natural selection
This is the mutation selection balance
14
The mutation-selection balance and mutation load
The mutation load is made up of the
generally low frequencies of deleterious alleles
found in the population
Alleles that are dominant are removed
more quickly, and maintained at
lower frequencies, than recessive
deleterious alleles
15
Population genetics and conservation
Population genetics is used throughout conservati
on biology, in a number of different ways
Determining level of isolation Burrowing owls
16
Population genetics and conservation
Genetics is used to infer evolutionary
relation- ships and define management units.
Example coho salmon stocks evolutionarily
significant units
17
Population genetics and conservation
Captive breeding programs
Minimize inbreeding and maintain as much genetic
diversity as possible
18
Population genetics and conservation
Measures of genetic diversity indicate
past events and possible management issues
19
Population genetics and conservation
Patterns of genetic diversity among populations
  • Inferring rate of spread of organisms
  • Inferring past geological/climatic processes

20
Ungraded homework assignment
List the three major types of selection and give
an example of each.
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