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Levels of Selection
• Gametic Selection: meiotic drive or segregation distortion is selection among gametes.
• Cell Selection: selection among cell lineages in tissue formation.
• Individual Selection: Darwinian selection.• *Group Selection*:Selection among groups.
– Kin Selection– Interdemic or Inter-population Selection
• Species Selection: selection among taxonomic lineages.
Necessary Properties for Evolution by Natural Selection
• Phenotypic Variation: There must be differences among the units in phenotype.
• Fitness Variation: The phenotypic differences must result in differences among the units in birth and death rates.
• Genetic Variation: The differences in phenotype that cause the differences in fitness must be heritable:
Levels of Selection
• Selection Among Individuals within Groups is Standard Darwinian Selection
• Selection Among Groups of Individuals is Controversial.
“I confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile
females in insect-communities: for these neuters often differ widely in instinct and in structure from
both the males and fertile females, and yet, from being sterile, they cannot propagate their kind.”
Darwin 1859
Problem: Sterile female workers have a fitness = 0 and cannot propagate their own kind.
“This difficulty, … as I believe, disappears, when it is remembered that selection may be applied to the
family, as well as to the individual, and may thus gain the desired end.”
Darwin 1859
Solution to Problem: Selection applied at a higher level of biological organization: Selection among
families or among colonies.
“ (Problem=) Breeders of cattle wish the flesh and fat to be well marbled together: an animal thus
characterised has been slaughtered, but (Solution=) the breeder has gone with confidence to the same
stock and has succeeded. .”Darwin 1859
Typical Darwinian style of argument: Uses example from artificial selection of domesticated
plants and animals.
“Here is a better and real illustration: according to
M. Verlot, some varieties of the double annual Stock from having been long and carefully selected to the right degree, always produce a large proportion of
(Sterile Offspring = ) seedlings bearing double and quite sterile flowers; but they likewise yield some single [flowers] and fertile plants (= Reproductive
Offspring). These latter, by which alone the variety can be propagated.”
Darwin 1859
These latter, by which alone the variety can be propagated, may be compared with the fertile male and female ants, and the double sterile plants with the neuters of the same community. As with the
varieties of the stock, so with social insects, selection has been applied to the family, and not to the individual, for the sake of gaining a serviceable
end.
Darwin 1859
Benefactors Recipients
Bear fitness cost of altruismand bestow fitness benefit
on group members
No fitness cost because they are not altruistic, but a fitness benefit,
reap fruits of theBenefactors in the group
Wbenefactor < Wrecipient
Group 1: Pbenefactors = 0.80 Group 2: Pbenefactors = 0.20
Two kinds of Phenotypic Variation: (1) individuals differ from one another; (2) groups differ from one another
in frequency of benefactors and recipients .
Individual Selection Within Groups Opposes Benefactors
Before:
After:
Individual Selection Within Group 1:
pbenefactors = 0.78 – 0.80 = - 0.02 Individual Selection Within Group 2
pbenefactors = 0.17 – 0.20 = - 0.03
Average pindividual = (- 0.02)(0.6) + (- 0.03)(0.4) = - 0.027 < 0
Benefactors0
1
Directional Individual Selection within groups: Benefactors reduce their own relative fitness and increase fitness of others.
Low Fitness
High Fitness
Fit
nes
s of
In
divi
dual
Recipients
Behavior Phenotype
Group 1: Many Benefactors Group 2: Few Benefactors
Group Fitness Variation: group growth rate increases with the frequency of benefactors.
Before:
After:
AfterReplication:
pgroup = {(0.8)*(1.2) + (0.2)*(0.8)}/2 – 0.5 = +0.06 > 0
Group Selection Favors Benefactors:Groups with more benefactors grow faster thanGroups with few benefactors.
More Benefactors0
1
Directional Group Selection among groups: Groups with a high frequency of benefactors Grow faster and to larger size.
Low Fitness
High FitnessF
itn
ess
of G
rou
p
Few Benefactors
Group Behavior Phenotype
Directional Group Selection F
itn
ess
of G
rou
p
Group Behavior Phenotype
0
1
Directional Individual Selection
Fit
nes
s of
In
divi
dual
0
1
Individual Behavior Phenotype
TWO Opposing Levels of Selection
Selection Among Groups: pgroup = {[(0.8)*(1.2) + (0.2)*(0.8)]/2} – 0.5 = +0.060 > 0
Selection Within Groups: pindividual = (- 0.02)(0.6) + (- 0.03)(0.4) = - 0.027 < 0
ptotal = pindividual + pgroup = - 0.027 + 0.060 = +0.033When Group Selection is stronger than Individual
Selectionthe Benefactor Type INCREASES.
ptotal = /Total}{
After
/Total}{
Before
-
Total Selection: Sum of Within and Among Group Selection
Opposing Evolutionary Forces• Natural Selection versus Deleterious Mutation
Reach a balance where every copy of a deleterious allele removed from a population by selection is replaced by mutation.
• Natural Selection versus Sexual SelectionTraits that increase male mate numbers at the expense of
male viability spread through a population.• Group Selection versus Individual Selection
Traits that increase the fitness of the group can spread at the expense of the individual when group selection > individual selection.
Traits that increase the fitness of the individual can spread at the expense of the group when individual selection > group selection = Tragedy of the Commons
Selection at any higher level than that of an individual is essentially "impotent“ and is "not an appreciable factor in
evolution" (1966:8; cf., Williams 1992).
“Many, perhaps most, evolutionary biologists believe that it [group selection] is only rarely an important force of evolution.”
(Futuyma 1998, p. 352)
Group vs. Individual Selection Controversy
It is sometimes said, “Group selection was vanquished in the 1960s by William Hamilton and
his ‘gene’s eye view’ of selection, which is the basis of Richard Dawkins’ notion of the selfish gene.”
“The fundamental unit of selection, and therefore of self-interest, is not the species,
nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity.”
(Dawkins, 1976)
Group vs. Individual Selection
Property Individual GroupPhenotypic Variation Large Small
FitnessVariation Large Small
Association between
phenotype and fitness
Common Rare
The Biology of Group Phenotypes
• Artificial Individual and Group Selection on Migration in the flour beetle,
Tribolium confusum (Craig 1982)
Individuals differ in tendency to emigrate and the differences are genetic
Phenotypic variation
Vmigration > 0
And it is heritable
VGenetic > 0
“Stay at home” beetle
“Traveling” beetle
Variation in Group Density leads to Variation in Group Migration
Low density = every beetle is morelikely to “stay home”
High density = every beetle is morelikely to be “traveling”
Variation in Group Density leads to Variation in Group Migration
Low density = every beetle is morelikely to “stay home”
High density = every beetle is morelikely to be “traveling”
Group Density Variation
VDensity > 0
And Density is heritable
VGenetic > 0
High Density
Low
High
High Density Causes Increased Migration
Low Migration
High Migration
Mig
rati
on P
hen
otyp
e of
Gro
up
Low Density
“Social” Environment
0
1
Directional Individual Selection imposed by Dr. Craig: Experimentor creates variation in fitness: VW > 0 and the association between migration and fitness
Low Fitness
High Fitness
Fit
nes
s of
In
divi
dual
“Stay at home” beetle“Traveling” beetle
High Migration = High Density
0
1
Directional Group Selection imposed by Dr. Craig: Group (High)Experimentor creates variation in group fitness: VGroup W > 0
and association between Migration and Group Fitness
Low Fitness
High Fitness
Fit
nes
s of
Gro
up
Low Migration =Low Density
0
1
Low Fitness
High Fitness
Fit
nes
s of
Gro
up
High Migration = High Density
Low Migration =Low Density
Directional Group Selection imposed by Dr. Craig: Group (Low)Experimentor creates variation in group fitness: VGroup W > 0
and association between Migration and Group Fitness
“Stay at home” beetle High Fitness
Individual Selection Group SelectionNone NoneControl
NoneInd Low
Group Low Low MigrationHigh FitnessNone
Ind & Group Low“Stay at home” beetle
High Fitness
Low MigrationHigh Fitness
Ind Low Vs Group High “Stay at home” beetle
High Fitness
High MigrationHigh Fitness
High MigrationHigh Fitness
Group High None
Experimental Test for measuring Migration
160 beetles Empty
Experimental Test for measuring Migration
160 beetles Empty
Pipe Cleaner and string
Experimental Test for measuring Migration
Less than 160 beetles Migrators
Pipe Cleaner and string
Results of 14 Generations of Artificial Selection
Treatment
Migrators
Out of 160
% of
Control
Only Individual Selection (Low)
91.95 - 4.9
Only Group
Selection (Low)
67.85 -29.9
Only Group
Selection (High)
118.2 +22.2
Ind versus Group
(Low vs High)
112.4 +16.2
Ind & Group (Low)
80.79 -16.5
No Selection
Fewer thanControl
Much Fewer Than Control
Much More Than Control
Much More Than Control
Surprizing!
Experimental Test for measuring Offspring Density
30 Adult beetles
Wait 40 Days
Experimental Test for measuring Offspring Density
30 Adult beetles
40 Days
Hundreds of Offspring
Results of 14 Generations of Artificial Selection
Treatment
Offspring
Density
% of
Control
Only Individual Selection (Low)
846 +1.6
Only Group
Selection (Low)643 -22.8
Only Group
Selection (High)886 +6.4
I vs G
(Low vs High)891 +7.0
I and G (Low) 710 -14.8
No Selection
Equal toControl
Much Fewer Than Control
More Than Control
More Than Control
Surprizing!
1% ~10 offspring