Population Genetics II.
Natural selection and the genetic makeupof populations:
Natural selection + limited gene flow = adaptation to local conditions
Example: metal tolerance on a sitecontaminated with mine tailings
Clines
The gradient in allele occurrence acrossa set of populations due to naturalselection
Example: leaf hairiness in Spergula arvensis
Hairy plants increaseIn frequency withlatitude
Another example of a cline
Flower scent in Polemonium viscosum:high altitude: bumblebee pollinationlow altitude: flies also pollinate it
Clines illustrate directional selectionover a range of environmental
conditions….
…directional selection?
Better survival and reproduction for organismsat one end of the phenotypic distribution
Other types of selection
Stabilizing selection:The mean phenotype is favored bynatural selection
Disruptive selection:The extremes of the phenotypes havehigher 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 heterozygoteadvantage
Example: sickle cell anemia.
Other types of selection
Negative frequency dependence: the rarealleles have the advantage
Example: Self-incompatibility alleles in plants
Mechanism by which genetic diversity is maintained
Ipomea purpurea flowers
Other types of selectionPositive frequency-dependent selection
Can lead to loss of genetic diversity
The mutation-selection balance and mutation load
Even while natural selection acts to eliminatedeleterious mutations, new mutationsare being formed
An equilibrium exists between the creation ofnew mutations and their elimination bynatural selection
This is the mutation selection balance
The mutation-selection balance and mutation load
The mutation load is made up of the generallylow frequencies of deleterious alleles found in the population
Alleles that are dominant are removed morequickly, and maintained at lowerfrequencies, than recessive deleteriousalleles
Determining level of isolation:Burrowing owls
Population genetics and conservation
Population genetics is used throughoutconservation biology, in a numberof different ways
Population genetics and conservation
Genetics is used to infer evolutionary relation-ships and define management units.
Example: coho salmon stocks evolutionarily significant units
Population genetics and conservation
Captive breeding programs
Minimize inbreeding and maintain as muchgenetic diversity as possible
Population genetics and conservation
Measures of genetic diversity indicate pastevents 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.