Transcript Lecture 5

Lecture 5:
Unit of Selection
• Who/what benefits from adaptation?
Nucleotide – Gene – Cell – Organism – Group – Species
• What is the unit of selection?
• Can a benefit at one level be detrimental to other
levels? Conflict?
Examples
• Segregation distorter genes
– Benefit one gene at expense of others
• Cell lines vs. rest of body
– Must be able to reproduce from >1 cell line
– Theoretical at the moment
Reproductive Restraint
(Group Benefit)
NS: allele  in freq if bearer  fitness vs.
those w/o the allele
NS = maximization of reproduction
Why repro restraint? altruism?
Conflict b/w ind. benefit & other levels?
Interdemic/ Interpopulational
Selection
VC Wynne-Edwards:
• Non-breeding in
seabirds – how explain?
• NS acts on the level of
group
• Social behaviour
regulates pop’n density
Reproductive restraint
Evidence:
Northern Flicker :
avg: 6-8 eggs
can lay up to 71
Altruism
Detriment to ind. fitness for benefit of others
Florida Scrub Jay
-young, sexually
mature helpers
• Infer: restraint evolves through
pop’n selection not ind. selection
• Why? Unrestrained pop’n growth depletes
resources & leads to extinction
• Implication: selection operates to the
detriment of the ind. but benefit of the group
• How likely is this really?
Interdemic Selection Model
Selfish gene (S) =  repro rate
A A
A S A
S S S
S S S
AA A
S
EXTINCT
S S
Initial success, ultimate failure
Problem
Group selection cannot counteract individual
selection b/c…
Rate of allele freq. Δ :
ind. selection >>>> group selection
Individual generation time is shorter
More individuals
Patch Model
(Maynard-Smith)
Alleles: A = altruistic S = selfish
empty
S
A
Overuse resources
Infected by S
Extinct unless migrate
Goes to fixation
Group Selection
• Weak force
• Only if migration is very low & group
extinction rates very high
• Group selection may exist, it just cannot
counteract individual selection
Traits that benefit other than
individual
1. Actually does benefit individual
2. Life history analysis
3. Kin selection
Life History Trade-offs
• Current vs. Future reproduction
– “cost of repro” hypothesis
Future Reproduction
Invest now
Future Survival
Evidence:
future fecundity
• Collared flycatcher:
Gustaffson & Part (1990)
• Manipulated clutch size,
• birds with  clutches had
 clutches for next 3 years
Number vs. Size of Offspring
% surviving
• Size often correlates with survival
# offspring / size offspring
# offspring
• Growth vs. Repro etc…
• Restraint at time X may benefit ind. at time Y
Kin Selection
Altruism: incur cost (c) to bestow benefit (b)
Contradicts Darwinism: Unlikely NS fix altruistic
allele
Nepotistic alleles: tend to help sibling
Should feed own offspring or sibling?
Offspring p(n) = 0.5 (meiosis)
Sibling p(n) = 0.5
Allele says either because they are equal
Hamilton’s Rule of Inclusive
Fitness
If : rb > c or
r > c/b then help
r = degree of relatedness
b = benefit (#surviving offspring)
c = cost (#lost offspring)
e.g. Costs c to help sister (r = 0.5), each gene suffers
c but gains b x 0.5
If b/2 >c help b/c  inclusive fitness
N.B. r = probability that two individuals have
homologous alleles identical by descent
Inclusive Fitness
Def’n: fitness of gene or genotype in ind. &
relatives
e.g.: Cousin : b x 0.125 > c
As r  b must  or c must  to  fitness
“I would give my life for 3 brothers or 9
cousins”
Beldings Ground Squirrel
Alarm calling:
Cost:  predation risk
Benefit: ???
Likelihood of calling
depends on knowledge of
relatedness (philopatry?)….
thus  inclusive fitness
Spadefoot Toad Cannibalism
• Two morphs: omnivore vs. cannibal
• Cannibals tended not to eat their siblings
(Pfennig 1999)
• Calculated that rb > c
European Wood Mouse Sperm
• Promiscuous females
• Sperm trains: faster than single sperm
• Release before reaching egg…many
unsuccessful, but improves brothers’ chances