Lecture 5: Unit of Selection Who/what benefits from adaptation? Nucleotide – Gene – Cell –...
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Transcript of Lecture 5: Unit of Selection Who/what benefits from adaptation? Nucleotide – Gene – Cell –...
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
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
Initial success, ultimate failure
A A A S A
A A A
S S S S S S
S S S
EXTINCT
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
S
empty
A
Overuse resources
Extinct unless migrate
Infected by S
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
Invest now
Future Reproduction
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
• Size often correlates with survival
• Growth vs. Repro etc…• Restraint at time X may benefit ind. at time Y
% s
urvi
ving
# offspring / size offspring # offspringN
sur
vivi
ng
Kin Selection
Altruism: incur cost (c) to bestow benefit (b)Contradicts Darwinism: Unlikely NS fix altruistic
alleleNepotistic alleles: tend to help siblingShould feed own offspring or sibling? Offspring p(n) = 0.5 (meiosis)Sibling p(n) = 0.5Allele says either because they are equal
Hamilton’s Rule of Inclusive Fitness
If : rb > c or r > c/b then helpr = 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 fitnessN.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