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

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