Download Evolution Pt II

Sources of Genetic Variation
Factors increasing variation
•Mutation
•Migration
Factors decreasing variation
•Natural selection
•Genetic drift in small populations (<1000)
Evidence of Selection in
Natural Plant Populations
Selection Among Populations
The Common Garden Experiments
of Clauson, Keck and Heiesy (1948)
Differences in phenotype across a gradient:
Yarrow (Achiella spp) as an example
What is the source of variation?
• Different species – genetic variation?
• Same species – phenotypic plasticity?
Common Garden Experiment
Step #1: Obtain Plants from Source Populations
Stanford – 100’
Mather – 4600’
Timberline – 10,000’
Common Garden Experiment
Step #2: Produce Clones
Source Plant
Clones
(e.g., piece of root)
Location #1
Location #2
Common Garden Experiment
Step #3: Plant clones in common gardens
Source Plant
Clones
(e.g., piece of root)
Common Gardens
Location #1
Location #1
Location #2
Location #2
Stanford Common Garden
Mather Common Garden
Timberline Common Garden
Interpretation of Results: Pure Plastic Response
Source Plant
Clones
Common Gardens
?
Location #1
Location #1
?
Location #2
Location #2
Interpretation of Results: Pure Genetic Response
Source Plant
Clones
Common Gardens
?
Location #1
Location #1
?
Location #2
Location #2
Plastic response
Genetic
response
Experimental Outcome:
Growth of Mather
Achiella Clones
A Second Example
Potentilla glandulosa
Copyright © 1997-2001 by Jane Strong and Tom Chester
http://tchester.org/sgm/conditions/blooms/idyellow.html
Lowland Plant
Lowland Ecotype
©Brother Alfred Brousseau, St. Mary's College
Montane Plant
©Brother Alfred Brousseau, St. Mary's College
Experimental Outcome:
Growth of Potentilla Clones
Interpretation Part I
• Not a pure plastic response
• Not a pure genetic response
What is the relationship
between these organisms?
Separate experiments
show that crosses
between different source
populations produce
viable offspring
Interpretation Part II
• These are not different
species
What then are they?
Ecotypes the middle ground
• Genetically distinct organisms
• Phenotypically distinct in terms of
• Morphology
• Physiology
• Phenology
• Occur in distinct habitats
• Differences can be traced to ecological
differences in home habitat
• Plants are potentially interfertile
(i.e., same biologicial species)
An Interpretation
Individuals or Ecotypes
Selection Within a Population
Purple loosestrife (Lythrum salicaria):
an aggressive invasive species
Purple Loosestrife and Tristyly
Three flower types (morphs)
• ♀  Pistal positions differ
• ♂ Anther positions differ
Pollination patterns
• No self pollination
• Each morph can pollinate
the other two morphs
Less frequent morphs have
higher fitness
Impact of Frequency-Dependent Selection on
Invading Populations of Purple Loosestrife
• Study system with 24 newly invaded
sites censused over a 5 year period
• Low evenness during year zero
• Evenness predicted to increase due
to frequency dependent selection
among morphs
No change line
(y=x)
Prediction is met, indicating a
change in population due to
natural selection
Selection At a Global Scale
Convergent Evolution
Example #1: Desert plants
Euphorbiaceae: Africa
Cactaceae: N. America
Example #2: Alpine plants
Campanulaceae: Africa
Asteraceae: S. America
Life Histories and Tradeoffs
Key Stages in the Life-History of a Plant
Seed Maturation
Flowering
seed phase
Growth
Dispersal
Dormancy
Pollination
Germination
The Ideal Plant
• Grow large rapidly
• Live forever
• Reproduce early and often
Life Histories and Tradeoffs
Impact of
Limiting Resources
General Scheme of Resource Allocation
Reproduction
Growth
• Pollen
• Nectar
• Ovules
• Seeds
• Leaves
• Stems
• Roots
• Rhizomes
3
2
Maintenance
• Structural support
• Storage
• Defenses
• Basal metabolism
1
General order in which resources are used
Available resource
General Scheme of Resource
Allocation through time
Time
Resource Allocation for a
Typical Annual Plant
Time
Resource Allocation over a typical
year for a Stress Tolerating Plant
Time
Tradeoffs and Limiting Resources
vs