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
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
Another example of a cline
Flower scent in Polemonium viscosum:
high altitude: bumblebee pollination
low altitude: flies also pollinate it
Clines illustrate directional selection
over a range of environmental
conditions….
…directional selection?
Better survival and reproduction for organisms
at one end of the phenotypic distribution
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?....
A graphical illustration…
(Silvertown and Charlesworth 2001, p. 77)
Some real examples:
Directional selection: widowbirds
Galapagos Finch beaks
Some real examples…
Stabilizing selection:
flowering time in Senecio integrifolius
Another example: birth weight in human babies
Some real examples…
Disruptive selection:
Garter snake coloration and behavior
Other types of selection
•Balancing selection: the heterozygote
advantage
Example: sickle cell anemia.
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
Other types of selection
Positive frequency-dependent selection
Ipomea purpurea flowers
Can lead to loss of genetic diversity
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
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
Population genetics and
conservation
Population genetics is used throughout
conservation biology, in a number
of different ways
Determining level of isolation:
Burrowing owls
Population genetics and
conservation
Genetics is used to infer evolutionary relationships and define management units.
Example: coho salmon stocks
evolutionarily significant units
Population genetics and conservation
Captive breeding programs
Minimize inbreeding and maintain as much
genetic diversity as possible
Population genetics and
conservation
Measures of genetic diversity indicate past
events and possible management issues
Population genetics and conservation
Patterns of genetic diversity among populations:
•Inferring rate of spread of organisms
•Inferring past geological/climatic processes
Ungraded homework assignment
List the three major types of selection and
give an example of each.