bio 221 lec 13
What is the primary level at which natural selection acts?
The individual organism—traits are favored if they increase an individual's survival and reproductive success.
🧠 This is the basis of the “selfish gene” idea: genes succeed by helping individuals pass them on.
Can selection act below the level of the individual?
Yes—selection can act at the gene level, favoring “selfish” genes that replicate themselves, even at the cost of the organism.
🧬 Example: transposable elements
Can selection act above the level of the individual?
Yes—group selection can occur if traits that benefit the group’s survival outweigh costs to individuals.
🌍 Rare, but possible when groups with more cooperators outcompete others.
What are selfish genetic elements?
Genes that promote their own replication, even if they harm the fitness of the organism carrying them.
🧠 They succeed at the gene level, not necessarily the individual level.
Example of a selfish genetic element?
The paternal sex ratio (psr) chromosome in the wasp Nasonia vitripennis:
Transmitted through sperm
Destroys all paternal chromosomes in the zygote—except itself
How does the psr element ensure its own transmission?
By converting fertilized (would-be female) zygotes into haploid males, which can then pass on the psr element again.
⚠️ This benefits the gene, but disrupts normal sex ratios and can harm the population
What does group selection actually do?
It increases the fitness of the group, but it does not evolve “for the good” of the group or species.
🧠 Selection still favors traits that succeed evolutionarily, not morally.
What does group selection actually do?
It increases the fitness of the group, but it does not evolve “for the good” of the group or species.
🧠 Selection still favors traits that succeed evolutionarily, not morally.
Red flour beetles and cannibalism
They cannibalize eggs and pupae, even though it may reduce group survival.
❗This shows that individual selection can favor harmful behaviors if they increase individual success.
What does this reveal about group selection?
It’s often overridden by individual selection—traits that harm the group can still evolve if they benefit the individual.
🧠 Selection works on fitness, not cooperation unless cooperation pays off.
What experiment was done on red flour beetles to test group vs individual selection?
Beetles were reared under three conditions:
Selected for high reproduction
Selected for low reproduction
Control group
What were the results of the experiment?
Population sizes declined in all groups
Highest population size: high fecundity group
Lowest population size: low fecundity group
Highest cannibalism: control group → good for individuals, bad for group
What does this reveal about selection?
In the control group, individual selection for cannibalism harmed group size
In the high fecundity group, group selection reduced cannibalism, reinforcing group-level benefits
🧠 Group selection can align with or counteract individual selection depending on conditions.
What is altruism in an evolutionary context?
Altruism is behavior that appears to reduce the individual’s fitness while increasing the fitness of others.
🧠 “Self-sacrifice” in nature that still needs an evolutionary explanation.
What was Wynne-Edwards’ theory of altruism?
Proposed that group selection favors altruistic individuals:
Groups with more altruists would be more stable
These groups would persist longer and reproduce (bud off) more often
What was Williams' (and others') critique of Wynne-Edwards?
Group selection is vulnerable to selfish mutants or immigrants:
A single selfish individual can outcompete altruists within a group
This would prevent altruistic traits from persisting long-term
What’s the key disagreement between Wynne-Edwards and Williams?
Wynne-Edwards emphasized group-level benefits
Williams argued that individual-level selection is almost always stronger and will override group benefits
🧠 Unless altruism benefits the individual indirectly, it’s unlikely to evolve by group selection alone.
What did Thomas Malthus observe about population growth?
Human population could grow faster than resources, leading to famine and competition for survival—what he called the "struggle for existence."
How did Malthus influence Darwin?
Darwin used the idea of struggle for existence as the basis for natural selection—only individuals best suited to compete would survive and reproduce.
What problem did cooperation pose for Darwin’s theory?
Extreme cooperation, like altruism in social insects (e.g., sterile worker ants), seemed to contradict natural selection, which predicts selfish behavior that maximizes individual fitness.
Why is altruism a problem under classic natural selection?
Because altruism means reducing one’s own fitness to increase someone else’s, which should not be favored by natural selection—unless there’s an indirect benefit.
🧠 This puzzle led to theories like kin selection and inclusive fitness.
How can an allele for altruism spread in a population?
If it increases the fitness of other individuals who also carry that allele, even if it reduces the donor’s own fitness.
When is altruism more likely to evolve?
When the recipient is closely related to the donor—because relatives are more likely to share the same allele.
What condition must be met for altruism to be favored by selection?
The fitness benefit (B) to the recipient × relatedness (r) must be greater than the cost (C) to the donor:
📏 Hamilton’s Rule → rB > C
🧠 Helps explain how “selfish genes” can lead to unselfish behavior.
What is inclusive fitness?
The total fitness effect of an allele, including:
The individual’s own reproductive success
Plus the success of others (especially relatives) who carry the same allele
How is inclusive fitness applied to individuals?
An individual’s inclusive fitness = its own fitness + the fitness it helps produce in relatives
🧠 Genes can “win” by helping copies of themselves in others survive and reproduce.
What is Hamilton’s Rule?
An altruistic trait will evolve if:
📏 r × B > C
r = relatedness to recipient
B = fitness benefit to recipient
C = fitness cost to donor
✅ If this condition is met, the altruistic allele can spread by natural selection.
What is the coefficient of relatedness (r)?
It’s the fraction of genes shared by descent between a donor and recipient.
🧬 It reflects the probability that a gene in one individual is also present in a relative due to shared ancestry.
What are some common values of r in diploid organisms?
Identical twins: r = 1
Parent-offspring: r = 0.5
Full siblings: r = 0.5
Half-siblings: r = 0.25
Cousins: r = 0.125
Why does relatedness matter in evolution of altruism?
Because altruistic traits are more likely to evolve when helping relatives, since it increases the inclusive fitness of the shared genes.
🧠 Helping kin = helping copies of your own genes survive.
In Hamilton’s Rule, what does the benefit (B) represent?
The number of extra offspring (or equivalent fitness gain) the recipient gains due to the altruistic act.
How does relatedness affect the required benefit-to-cost ratio?
The more distantly related the recipient is, the greater the benefit must be to outweigh the cost.
🧠 Because r × B > C must still be true.
Is helping worth it for a full sibling vs. a cousin?
Helping a sibling (r = 0.5) gain 4 offspring: 4 × 0.5 = 2 > 1 → favors altruism
Helping a cousin (r = 0.125) gain 4 offspring: 4 × 0.125 = 0.5 < 1 → not favored
✅ Closer kin require smaller benefits for altruism to be selected.
How do bee-eaters show altruism in nature?
Young Hamiltonian bee-eaters often help older relatives raise offspring instead of breeding themselves during their first few years.
🧠 This behavior delays their own reproduction but increases the success of close kin.
What determines whether a bee-eater will help raise offspring?
The decision is based on relatedness:
More likely to help full siblings than cousins
Helping close kin provides greater inclusive fitness benefits, making it worth the cost
✅ Altruism is more likely when the benefit × relatedness outweighs the cost.
How does kin structure affect altruism in Belding’s ground squirrels?
Females stay in their natal colony → high relatedness
Males disperse → lower relatedness in new groups
🧠 So females are more likely to live with close kin.
Who performs most alarm calling, and why?
Females do most of the alarm calling, despite the high personal risk, because their warning protects close relatives, increasing their inclusive fitness.
✅ Kin-based altruism: help others survive when they likely carry your genes.
What did Gorrell et al. (2010) study in red squirrels?
Over 19 years, they observed five adoptions of orphaned red squirrel kits by unrelated adult females in the Yukon and measured whether these adoptions fit Hamilton’s Rule.
How did they apply Hamilton’s Rule (rB > C)?
r = relatedness of adopter to orphan
B = survival benefit to orphan
C = cost to adopter’s own offspring (estimated from known litter survival data)
✅ In all adoption cases, rB > C
❌ In non-adoption cases, rB < C
What conclusion did the study support?
Adoption decisions matched Hamilton’s Rule:
Squirrels were more likely to adopt when orphans were more closely related
No adoption occurred when the cost outweighed the genetic benefit
🧠 Even rare altruistic acts like adoption can follow kin selection logic.
What is eusociality?
The most extreme form of altruism, where individuals live in social groups and:
Only a few individuals reproduce
Most colony members are sterile and help raise the offspring of others and maintain the colony
What species show eusociality?
Insects: termites, all ants, some bees and wasps
Mammals: naked mole rats are a rare eusocial mammal
Why is eusociality considered extreme altruism?
Because individuals give up their own reproduction entirely to support the colony—maximizing inclusive fitness by helping close kin (often siblings) reproduce.
🧠 A lifetime of helping, not mating.
Why has eusociality evolved multiple times in Hymenoptera (ants, bees, wasps)?
Likely due to their haplodiploid sex determination system, which increases relatedness between sisters and makes helping more genetically rewarding than reproducing.
What is haplodiploidy?
Females are diploid: get genes from both parents
Males are haploid: develop from unfertilized eggs, and pass on 100% of their genome to daughters
🧬 This means sisters share ~75% of their genes:
50% from mom × 0.5 chance shared = 0.25
100% from dad × 1 = 0.5
👉 Total relatedness: r = 0.75
Why does haplodiploidy favor eusociality?
Because sterile female workers are more related to their sisters (r = 0.75) than they would be to their own offspring (r = 0.5), so helping the queen produce sisters can increase their inclusive fitness more than reproducing themselves.
🧠 Helping mom make sisters = passing on more of your genes.
How related are female workers to their brothers and sisters in haplodiploid Hymenoptera?
To sisters: r = 0.75
To brothers: r = 0.25
🧠 Workers share more genes with sisters, so helping raise sisters boosts their inclusive fitness more.
What sex ratio does the queen prefer?
The queen is equally related to sons and daughters (r = 0.5), so she favors a 1:1 male-to-female ratio of reproductive offspring.
What sex ratio do workers prefer?
Workers want a female-biased sex ratio to maximize their genetic payoff, since they are 3× more related to sisters than to brothers.
➡️ This creates queen–worker conflict over how many males vs. females are produced in the colony.
🧠 Same genes, different interests!
How does multiple mating by the queen affect worker relatedness?
If the queen mates with multiple males, the average relatedness between workers drops (from r = 0.75 to closer to r = 0.5).
🧠 Workers now share fewer genes on average, especially if they have different fathers.
How does this affect queen–worker conflict over sex ratios?
With lower relatedness among sisters, workers are less biased toward producing females—so the conflict with the queen is reduced.
➡️ Sex ratios shift closer to the queen's preferred 1:1 ratio.
✅ More mates for the queen = less incentive for workers to favor sisters.
What was observed in wood ant colonies with singly-mated queens?
Workers biased the sex ratio toward females by:
Allowing female eggs to survive
Withholding care from male larvae
➡️ Result: fewer reproductive males than the queen originally laid
What happened in colonies with multiply-mated queens?
Workers did not alter the sex ratio—they allowed equal care for male and female larvae.
➡️ Result: No bias in reproduction toward females.
What does this wood ant example demonstrate?
In singly-mated colonies, high sister relatedness (r = 0.75) leads workers to favor more sisters
In multiply-mated colonies, lower relatedness (r ≈ 0.5) reduces worker incentive to bias sex ratios
🧠 The observed change in sex ratio from egg-laying to adult stage reflects worker control and genetic interest.
What is reciprocal altruism?
A form of cooperation where individuals help others with the expectation of future help in return.
🧠 It can evolve if the benefit of help received > cost of help given and interactions are repeated.
When does reciprocal altruism work best?
When individuals interact repeatedly
When they can remember past interactions
When cheaters can be identified and punished
✅ Trust + memory = cooperation
How do vampire bats show reciprocal altruism?
Bats regurgitate blood meals to feed hungry roost-mates
They do this mostly with bats they’ve shared with or received from before
Bats that don’t reciprocate are eventually excluded from the group
🧠 Give today, get tomorrow—unless you cheat.
Do vampire bats share food based only on relatedness?
No—food sharing occurs even between unrelated individuals.
🧠 Bats share irrespective of kinship, though many roost-mates are kin, so kin selection may have contributed to the behavior’s evolution.
What did Carter & Wilkinson (2013) test?
They starved 20 bats of known pedigree and observed which bats shared food with them, to measure the influence of relatedness and reciprocal history.
What was the strongest predictor of food sharing?
The strongest predictor was having previously received food from that individual—not relatedness.
✅ Suggests reciprocal altruism is the main driver, with kin selection playing a supporting role.
What are some individual-level benefits of group living or cooperation (beyond kin selection)?
Protection from predators or harsh environments (e.g., huddling penguins)
Improved resource access through group hunting (e.g., sailfish hunting in packs)
Future opportunities (e.g., taking over territory or mating roles)
What’s an example of cooperation tied to future benefits?
In long-tailed manakins, two males perform coordinated dances:
Only the alpha mates
The junior male inherits the display ground when the alpha dies
🧠 Short-term cost for long-term gain.
Why must we be cautious when interpreting cooperation?
Because kin selection may still be at play.
✅ Even when cooperation looks self-serving, relatives may benefit, so inclusive fitness could still explain the behavior.
Why does group living involve both cooperation and conflict?
Individuals may cooperate overall, but still compete over key resources like:
Food
Mates
Access to breeding
🧠 Cooperation doesn’t eliminate competition—just changes its context.
What determines whether conflict escalates into fighting?
It depends on a cost-benefit analysis:
Fighting may bring rewards (e.g., dominance, mates)
But it also comes with potential injury or death, especially for weaker individuals
🧠 Individuals may back down if the cost of losing is too high.
How can we model the evolution of conflict strategies?
Using evolutionary game theory, such as the Hawk-Dove model, which compares:
Aggressive (“Hawk”) strategies
Peaceful (“Dove”) strategies
✅ Helps predict which behaviors are evolutionarily stable based on costs and benefits of conflict.
What is an Evolutionarily Stable Strategy (ESS)?
An ESS is a strategy that, if adopted by most of the population, cannot be invaded by an alternative (mutant) strategy because it yields the highest average fitness.
🧠 It’s not about winning every fight—it’s about long-term success.
What are pure vs. mixed strategies in game theory?
Pure strategy: individual always uses the same behavior (e.g., always fight or always flee)
Mixed strategy: individual varies behavior based on the situation or frequency of other strategies in the population
✅ Real animals often use mixed strategies.
What is the Hawk-Dove model?
A simple model of conflict:
Hawk: always fights aggressively, risking injury
Dove: avoids conflict, backs down when challenged
The payoff depends on benefit of winning (V) and cost of fighting (C)
🧠 When C > V, a mix of hawks and doves can be stable.
What are the possible outcomes in a Hawk-Dove encounter?
Encounter Outcome
Dove vs Dove No fight → they share the resource → each gets V/2
Hawk vs Dove Hawk wins all → Hawk gets V, Dove gets 0
Hawk vs Hawk Fight → each has 50% chance to win → average payoff = (V − C)/2
When is a mixed strategy stable (ESS)?
When the cost of fighting (C) is greater than the value of the resource (V), a mixture of Hawks and Doves in the population becomes stable.
✅ Too many Hawks = too costly → Doves do better
Too many Doves = Hawks exploit them → balance evolves
What are the payoffs in the Hawk-Dove game?
Player Opponent Payoff
Hawk Hawk (V − C)/2
Hawk Dove V
Dove Hawk 0
Dove Dove V/2
V = fitness benefit (value of the resource)
C = fitness cost of fighting
What happens when V < C?
Fighting becomes too costly, so:
Pure Hawk strategy is not stable (too much injury)
Pure Dove strategy is not stable (gets exploited by Hawks)
✅ Result: a mixed ESS evolves with both Hawks and Doves in the population
What’s the equilibrium frequency of Hawks in a population?
To find it: set average payoff of Hawk = average payoff of Dove:
Hawk: p(Hawk) × (V − C)/2 + p(Dove) × V
Dove: p(Hawk) × 0 + p(Dove) × V/2
Solving yields:
📊 Equilibrium frequency of Hawks = V/C
🧠 More valuable the resource or lower the cost, more Hawks in the population.
Why can’t a population of all Doves be evolutionarily stable?
Because if a Hawk mutant appears, it wins every encounter against Doves and gains full benefit (V), while Doves only share (V/2) among themselves.
🧠 Hawk invades easily → Dove is not an ESS.
Why can’t a population of all Hawks be stable when V < C?
Because Hawk vs. Hawk results in costly fights, with average payoff of (V − C)/2—which is lower than V/2, the Dove-Dove outcome.
🧠 Too many Hawks = everyone suffers → not stable.
Why can’t a population of all Hawks be stable when V < C?
Because Hawk vs. Hawk results in costly fights, with average payoff of (V − C)/2—which is lower than V/2, the Dove-Dove outcome.
🧠 Too many Hawks = everyone suffers → not stable.
What’s the stable outcome of the Hawk-Dove game when V < C?
A mixed ESS where both Hawks and Doves exist.
Rare Hawks do well because they beat Doves
But if Hawks become too common, their fitness drops due to costly fights
📊 Stable proportion of Hawks = V / C
✅ Natural selection favors a balance between aggression and avoidance.
Can Hawk ever be an ESS on its own?
Yes—if V > C, then (V − C)/2 > 0, so Hawks do well even when fighting each other.
✅ In that case, an all-Hawk population can be stable.
What happens if V < C?
Fighting is too costly, so pure Hawk isn’t stable
But pure Dove also isn’t stable—Hawks can still invade
➡️ So neither pure strategy works
What is the ESS when V < C?
A mixed strategy where individuals play Hawk with probability V/C
📊 On average, ~80% Hawk and 20% Dove if V = 0.8C
🧠 As found by Maynard Smith (1982)—this balances costs and benefits of aggression in the population.
What is a limitation of the basic Hawk-Dove model?
It assumes individuals can’t assess whether their opponent will fight or flee—they just choose a strategy blindly.
🧠 In reality, animals often evaluate their opponent before acting.
What is an assessment strategy?
A strategy where individuals assess the opponent’s strength or condition before deciding to fight or retreat.
✅ Often leads to less costly and more efficient outcomes than fixed strategies.
Why is assessment often an ESS?
Because it allows individuals to avoid unnecessary conflict:
Weak individuals back down
Strong individuals escalate when odds are good
📈 Leads to greater average fitness than always fighting or choosing randomly.
🧠 Think before you fight = win more, lose less.
What are the three male morphs of side-blotched lizards and their behaviors?
🟧 Orange males: Aggressive, large territories, many females (dominant)
🔵 Blue males: Less aggressive, small territory, guards one female
🟨 Yellow males: Sneaky, mimic females, steal matings from orange males
🧠 Each morph has a distinct reproductive strategy.
How does each morph interact in a rock-paper-scissors dynamic?
Orange > Blue: Overpowers them and steals mates
Blue > Yellow: Blue guards mates well
Yellow > Orange: Sneaks in while orange is distracted
🧠 No single morph wins long-term—success depends on who’s common or rare.
What evolutionary mechanism maintains this diversity?
Negative frequency-dependent selection:
Each morph does best when it’s rare
Fitness drops when it becomes common, so frequencies cycle over time
✅ This keeps all three strategies in the population.