GE CLST 70B Lecture 7: GE70B Lecture 7

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Lecture 7
Darwin's Finches
The Grant's have been measuring finches on the Galapagos for decades (multiple generations)
Drought hit in 1977, only large seeds available
Average beak size got bigger
Variation in beak size
More individuals produced than could survive
Variation related to survival
Variation is heritable
Industrial Melanism
Light peppered moth was more common
During Industrial Revolution, pollution killed light lichens that grew on trees + soot blackened the
Selected for darker peppered moths bc now they blended in better
Bernard Kettlewell tested + confirmed this hypothesis experimentally
Reverse happened after the UK passed a clean air act
Similar change found in Michigan moths after the US passes a clean air act
4 premises for selection met
o Individuals vary: moths vary in color pattern
o More individuals produced than can survive: predation
o Variation influences survival and reproduction: darker or lighter color is less preyed on than
the opposite phenotype
o Variation in features is heritable: yes
Limits to Selection
Constraints imposed by phylogenetic history
o Evolution builds on what's there
o Some traits are just "adequate"
Lack of appropriate genetic variation
Selection can't act on traits that appear post-reproductively
o Deleterious genes have already been passed on
Development interactions among organs or structures that necessitate trade-offs
o i.e. larger babies tend to survive better than smaller ones, but it it's too big the mother
might die in childbirth
Selection depends on strength of selection and the heritability of the trait
Modes of Selection
For quantitative (continuously varying) trait
Directional: one extreme phenotype is most fit
Stabilizing/Balancing: intermediate phenotype is most fit
o Human birthweight
o Sickle-cell hemoglobin
Diversifying: two or more extreme phenotypes are more fit than the intermediates
o Darwin's finches
Adaptive landscape: created when selection acts on multiple traits
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