BIO220H1 Chapter Notes - Chapter 4: Stabilizing Selection, Disruptive Selection, Directional Selection

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20 Apr 2012
BIO220 Lab Chapter 4 Implications of Climate Change
Climate affects physiology/development of a species, alters patterns in natural
selection , inducing adaptive evolutionary shifts
Selective agents act on target of selection
Patterns of selection frequently shift/fluctuate, targets of selection can change
Measuring selection
Measurements of the trait that is target of selection
Estimates of reproductive success of individuals with each value of the trait
Standardize each trait
Absolute fitnessL total number of offspring an individual produces
Relative fitness: total number of offspring and individual produces divided by
average number of offspring produced in the population
Fitness functions
Relationship between the variation in the trait and fitness (fitness function)
Directional selection: for greater values
Stabilizing selection: intermediate values favorable
Disruptive selection: extreme trait values have higher fitness
No selection: distribution of trait values is not under selection
Notes on “Rapid evolution of flowering time by an annual plant in response to a climate
Abbreviated growing seasons caused by drought evolution of earlier onset of
Intermediate flowering time of ancestor x descendant hybrids: additive genetic
basis for divergence
Flowering time was heritable
Descendants flowered earlier than ancestors
Higher survival of postdrought than predrought
Higher survival of Dry site than west site genotypes: adaptation to local conditions
Evolution is not always a slow, gradual process but can occur on contemporary
time scales in natural populations
Evolution of shorter FT during drought
Long season should select for delayed flowering: plants that spend more time in
exponential growth phase will accumulate disproportionately more resources and
thus produce many more offspring once flowering begins.
Seasons are short: plants that delay flowering are unable to mature seeds before
conditions deteriorate lethally.
Dry site higher fitness, better adapted to faster-drying soils
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