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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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Adaptation and levels of selection
• What is an adaptation?• What is natural selection?• On what does selection act?
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
How do adaptations evolve (or what does evolution by natural
selection require)?
• Variation• Differential reproduction or survival• Transmission to the next generation
(heredity)
Evolution by natural selection
Evolution by natural selection
Evolution by natural selection
Evolution by natural selection
Selection and Galapagos finches
14 species evolved in about 500,000 years
Selection on beak depth in G. fortis
4% change in two years, due to change in seed sizes
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
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
Stabilizing selection on song repertoires
Disruptive selection for body size in male bluegill sunfish
Three male morphs: sneaker, female mimic, territorial.
males
female
On what does selection act?
• Organism• Population (group)• Kin group• Cytoplasmic elements• Cells• Genes
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
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.
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
The fallacy of group selection
Honeybees as superorganisms
Social insects are often composed of closely related individuals
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
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
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
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
Kin selection in lions
Females nurse their sister’s cubs
Males help brothers in coalitions
Inclusive fitness:
IFi = wi + rijwj
wi = offspring producedby individual i
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