Transcript PowerPoint Presentation - What is an adaptation?

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
Bill length
Bill depth
Tarsus length
Selection
0.45 s.d.u.
0.58 s.d.u.
0.27 s.d.u.
h2*
0.66
0.69
0.82
*h2 estimated from midparent-offspring
regression
• Directional selection
– Extreme trait has highest fitness, i.e.
lifetime reproductive success (LRS)
– S = cov (trait,fitness)
– Cov(x,w) = S(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)
Lifetime reproductive success
Measuring selection
Trait value
Stabilizing selection on song repertoires
Disruptive selection for body size in
male bluegill sunfish
males
female
Three male morphs: sneaker, female mimic, territorial.
On what does selection act?
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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
Full sibs (diploid)
a
b
1
2
1/2
Ancestor a: (1/2)(1/2) = 1/4
Ancestor b: (1/2)(1/2) = 1/4
r12 = 1/4 + 1/4 = 1/2
Estimating relatedness
Cousins (diploid)
a
b
1
Ancestor a: (1/2)4= 1/16
Ancestor b: (1/2)4 = 1/16
r12 = 1/16 + 1/16 = 1/8
2
Kin selection in lions
Females nurse their sister’s cubs
Males help brothers in coalitions
Inclusive fitness:
IFi = wi + Srijwj
wi = offspring produced
by individual i