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