Download Three Possible Outcomes of Selection

Three Possible Outcomes of Selection
Disruptive selection
(3) types of selection: Directional
Directional
Frequency
# offspring
small
Body size
(or any range of phenotypes)
large
Environmental change = selection for optimal phenotype,
Results in a directional shift of gene frequencies
Odocoileus virginianus
white-tailed deer
http://animaldiversity.ummz.umich.edu
Bergmans Law = large bodies are beneficial in colder climates
(3) types of selection: Stabilizing
Directional
Stabilizing
Frequency
# offspring
small
Body size
large
Selection for the mean or median gene frequency
the mean increases, the outliers decrease = overall decrease in
genetic diversity
Percentage survival to age 4 weeks
Birth weight in humans: Stabilizing selection?
Birth weight in pounds
(Karn and Penrose, 1951)
(3) types of selection: Disruptive
Directional
Stabilizing
Disruptive (selectional
dimorphis m)
No selection
Frequency
# offspring
small
Body size
large
Individuals at both extremes of a range of phenotypes are
favored over those in the middle
Accentuates or preserves discontinuities in populations
Time-------------->
Fossils: Speciation and the phylogenetic “tree”
Extinction
Speciation event
Lineage
<---------Morphology ------->
Recognizing species in fossil records
New
Time
Extinction: loss of species, loss of lineage
Lineage
Speciation e vent/origination
Old
Morphology
• Difficult to detect
• Does morphology imply breeding barrier?
• How to recognize species in the fossil record?
Adaptive radiation
(after Futuyma 1998; based on Purves et al., 1997; Grant, 1986)
Premating Mechanism: seasonal isolation
late spring
breeding
Bufo fowleri
Fowler's Toad
Bufo americanus
American Toad
early spring
breeding
Character displacement: Finch variation
(Lack, 1947)