Adaptation and levels of selection What is an adaptation? What is natural selection? On what does...
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Transcript of Adaptation and levels of selection What is an adaptation? What is natural selection? On what does...
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Adaptation and levels of selection
• What is an adaptation?• What is natural selection?• On what does selection act?
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What is an adaptation?
• A feature designed to improve survival and/or reproduction of an organism (or other entity)
• Most behavior is likely to be adaptive because it influences how an animal acquires mates, finds food and avoids predators
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How do adaptations evolve (or what does evolution by natural
selection require)?
• Variation• Differential reproduction or survival• Transmission to the next generation
(heredity)
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Evolution by natural selection
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Evolution by natural selection
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Evolution by natural selection
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Evolution by natural selection
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Selection and Galapagos finches
14 species evolved in about 500,000 years
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Selection on beak depth in G. fortis
4% change in two years, due to change in seed sizes
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Selection on G. fortis
Trait Selection h2*
Bill length 0.45 s.d.u. 0.66Bill depth 0.58 s.d.u. 0.69Tarsus length 0.27 s.d.u. 0.82
*h2 estimated from midparent-offspring regression
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Measuring selection• Directional selection
– Extreme trait has highest fitness, i.e. lifetime reproductive success (LRS)
– S = cov (trait,fitness)– Cov(x,w) = (xi-X)(wi-W)/n
• Stabilizing selection – Intermediate trait has highest LRS – S = cov (trait2,fitness)
• Disruptive selection– High and low traits have highest LRS– S = cov (trait2,fitness)
Trait value
Life
time
repr
oduc
tive
succ
ess
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Stabilizing selection on song repertoires
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Disruptive selection for body size in male bluegill sunfish
Three male morphs: sneaker, female mimic, territorial.
males
female
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On what does selection act?
• Organism• Population (group)• Kin group• Cytoplasmic elements• Cells• Genes
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What is an organism?
• An organism contains one or more closely related (usually genetically identical) cells descended from a single progenitor cell
• Has a repeating life-cycle• Either consists of, contains, or works for the
welfare of germ-line cells
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Why do adaptations typically benefit an individual organism?
• Because individuals inherit characteristics due to genetic transmission. Traits are typically heritable by individuals not by groups.
• Genes replicate and change in frequency after selection on individuals. Individuals replicate faster than groups.
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Group
• Theoretically possible if groups differ in survival or reproduction
• But, unlikely to be caused by differential extinction because individuals die faster than groups.
• Possible if groups exhibit differential productivity. Experimentally demonstrated in flour beetles due to changes in cannibalism
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The fallacy of group selection
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Honeybees as superorganisms
Social insects are often composed of closely related individuals
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Kin• Selection can favor altruistic behavior when
animals interact with kin because they share copies of genes that are identical by descent.
• The condition for altruism to spread is given by Hamilton’s rule: rB > C – B = increase in recipient’s LRS– C = decrease in donor’s LRS– r = genetic relatedness
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Relatedness
• Probability that the alleles at a locus are identical by descent in two individuals
• For diploids, r can be calculated by raising 1/2 to a power equal to the number of links in a pedigree separating two relatives and then summing independent paths involving each common ancestor
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Estimating relatedness
a b
1 2
Ancestor b: (1/2)(1/2) = 1/4r12 = 1/4 + 1/4 = 1/2
Full sibs (diploid)
1/2
Ancestor a: (1/2)(1/2) = 1/4
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Estimating relatedness
a b
2
Ancestor b: (1/2)4 = 1/16 1/16 + 1/16 = 1/8
1
Cousins (diploid)
r12 =
Ancestor a: (1/2)4= 1/16
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Kin selection in lions
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Females nurse their sister’s cubs
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Males help brothers in coalitions
Inclusive fitness:
IFi = wi + rijwj
wi = offspring producedby individual i