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.