Download Group Selection

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
Wbenefactor < Wrecipient
Benefactors
Bear fitness cost of altruism
and bestow fitness benefit
on group members
Recipients
No fitness cost because
they are not altruistic,
but a fitness benefit,
reap fruits of the
Benefactors in the group
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:
Dpbenefactors = 0.78 – 0.80 = - 0.02
Individual Selection Within Group 2
Dpbenefactors = 0.17 – 0.20 = - 0.03
Average Dpindividual = (- 0.02)(0.6) + (- 0.03)(0.4) = - 0.027 < 0
Fitness of Individual
Directional Individual Selection within groups:
Benefactors reduce their own relative fitness and
increase fitness of others.
High Fitness
1
Low Fitness
0
Benefactors
Recipients
Behavior Phenotype
Group 1: Many Benefactors
Group 2: Few Benefactors
Before:
After:
Group Fitness Variation: group growth rate
increases with the frequency of benefactors.
Group Selection Favors Benefactors:
Groups with more benefactors grow faster than
Groups with few benefactors.
After
Replication:
Dpgroup = {(0.8)*(1.2) + (0.2)*(0.8)}/2 – 0.5 = +0.06 > 0
Fitness of Group
Directional Group Selection among groups:
Groups with a high frequency of benefactors
Grow faster and to larger size.
High Fitness
1
Low Fitness
0
More Benefactors
Few Benefactors
Group Behavior Phenotype
Directional Individual Selection
Fitness of Individual
Fitness of Group
Directional Group Selection
1
0
Group Behavior Phenotype
1
0
Individual Behavior
Phenotype
TWO Opposing Levels of Selection
Total Selection: Sum of Within and Among Group Selection
Selection Among Groups:
Dpgroup = {[(0.8)*(1.2) + (0.2)*(0.8)]/2} – 0.5 = +0.060 > 0
Selection Within Groups:
Dpindividual = (- 0.02)(0.6) + (- 0.03)(0.4) = - 0.027 < 0
Dptotal =
{
/Total} - {
After
/Total}
Before
Dptotal = Dpindividual + Dpgroup = - 0.027 + 0.060 = +0.033
When Group Selection is stronger than Individual
Selectionthe Benefactor Type INCREASES.
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 Selection
Traits 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
Group vs. Individual Selection
Controversy
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)
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
Group
Phenotypic
Variation
Large
Small
Large
Small
Common
Rare
FitnessVariation
Association
between
phenotype and
fitness
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
“Traveling” beetle
Phenotypic variation
Vmigration > 0
And it is heritable
VGenetic > 0
“Stay at home” beetle
Variation in Group Density leads to
Variation in Group Migration
High density = every beetle is more
likely to be “traveling”
Low density = every beetle is more
likely to “stay home”
Variation in Group Density leads to
Variation in Group Migration
High density = every beetle is more
likely to be “traveling”
Group Density Variation
VDensity > 0
And Density is heritable
VGenetic > 0
Low density = every beetle is more
likely to “stay home”
Migration Phenotype of Group
High Density Causes Increased Migration
High High Migration
Low Migration
Low
High Density
Low Density
“Social”
Environment
Fitness of Individual
Directional Individual Selection imposed by Dr. Craig:
Experimentor creates variation in fitness: VW > 0
and the association between migration and fitness
High Fitness
1
Low Fitness
0
“Traveling” beetle
“Stay at home” beetle
Fitness of Group
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
High Fitness
1
Low Fitness
0
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
Fitness of Group
High Fitness
1
Low Fitness
0
High Migration =
High Density
Low Migration =
Low Density
Control
Individual Selection
None
Ind Low
Group Selection
None
None
“Stay at home” beetle
High Fitness
Group Low
None
Low Migration
High Fitness
Group High
None
High Migration
High Fitness
Ind & Group Low“Stay at home” beetle
Low Migration
High Fitness
High Fitness
Ind Low
Vs Group High
“Stay at home” beetle
High Fitness
High Migration
High Fitness
Experimental Test for measuring Migration
160 beetles
Empty
Experimental Test for measuring Migration
Pipe Cleaner and string
160 beetles
Empty
Experimental Test for measuring Migration
Pipe Cleaner and string
Less than 160 beetles
Migrators
Results of 14 Generations of Artificial Selection
Treatment
Only Individual
Selection (Low)
Only Group
Selection (Low)
Only Group
Selection (High)
Ind versus Group
Migrators D% of
Out of 160 Control
91.95
- 4.9
(Low)
Fewer than
Control
67.85
-29.9
Much Fewer
Than Control
118.2
+22.2
Much More
Than Control
112.4
+16.2
Much More
Than Control
80.79
-16.5
(Low vs High)
Ind & Group
No
Selection
Surprizing!
Experimental Test for measuring Offspring
Density
Wait 40 Days
30 Adult beetles
Experimental Test for measuring Offspring
Density
40 Days
30 Adult beetles
Hundreds of Offspring
Results of 14 Generations of Artificial Selection
1% ~
10 offspring
Treatment
Only Individual
Selection (Low)
Only Group
Selection (Low)
Only Group
Selection (High)
I vs G
(Low vs High)
I and G (Low)
Offspring
Density
846
D% of
Control
+1.6
No
Selection
Equal to
Control
-22.8
Much Fewer
Than Control
+6.4
More
Than Control
891
+7.0
More
Than Control
710
-14.8
Surprizing!
643
886