Social interactions between organisms present the opportunity for conflict and cooperation
Interaction between individuals can have 4 possible outcomes on the fitness of the 2 individuals involved:
Performer benefits Performer suffers
Receiver Cooperation Altruismbenefits
Receiver Selfish Spiteful suffers
Social interaction
Cooperation = mutualism, fitness gain for both participants Example: mutualism between corals and zooxanthellae (algae) - hard corals cannot obtain enough nutrients by feeding - symbiotic dinoflagellates provide carbohydrates from photosynthesis in exchange for nitrogen from the coral
Performer benefits Performer suffers
Receiver Cooperation Altruismbenefits
Receiver Selfish Spiteful suffers
Social interaction
Selfish actions benefit the performer but hurt the receiver
examples: predation and parasitism - one organism exploits another to increase its own fitness - sexually antagonistic actions that benefit one sex at the expense of the other
Performer benefits Performer suffers
Receiver Cooperation Altruismbenefits
Receiver Selfish Spiteful suffers
Social interaction
Spiteful actions hurt both parties - no known examples from nature
Text claims “an allele that results in fitness losses for both actor and recipient would quickly be eliminated by natural selection”
- Do you believe this is necessarily true? Could an allele persist, despite hurting the performer?
Performer benefits Performer suffers
Receiver Cooperation Altruismbenefits
Receiver Selfish Spiteful suffers
Social interaction
Altruism is the case where the performer suffers in order to benefit the recipient of the action
Example: a dolphin rams into a shark that is trying to eat me; I give you a cookie
Performer benefits Performer suffers
Receiver Cooperation Altruismbenefits
Receiver Selfish Spiteful suffers
Social interaction
Altruism is the case where the performer suffers in order to benefit the recipient of the action
Problematic for Darwinian evolution: how can an allele persist if it produces fitness benefits for others, but hurts the one who actually carries the allele?
Darwin wrote this “..at first appeared to me… actually fatal to my whole theory,” because altruism is so common in nature - birds help their parents raise young instead of reproducing
- alarm calling in colonial mammals (beavers, ground squirrels)
- adoption of orphaned animals by mature females
Social interaction
Traits that lower personal fitness can survive natural selection if they increase the fitness of close relatives
You only pass 50% of your alleles to each offspring
On average, you share 50% of your alleles with each sibling
Therefore, when your parents produce another child, it’s the same as if you reproduced yourself: a new individual is created that shares 50% of your alleles
Similarly, if a sibling produces 2 offspring it’s equivalent to you producing one offspring yourself
Evolution of altruism
Genetic model was proposed in 1964, showing how an allele that promoted altruism could spread
Depended on the coefficient of relationship, r
- probability that 2 copies of an allele, in 2 different individuals, are identical by descent from a common ancestor
How does this relate to F, the coefficient of inbreeding?
A coefficient of relationship, r, is calculated using a pedigree that shows how the performer and recipient of the action are related
Coefficient of relationship
Coefficient of relationship
performerof action
recipient
sharedparent
X
Path analysis:
(1) start with performer
(2) trace all paths of descent through pedigree to recipient
half-siblings
Coefficient of relationship
performerof action
recipient
sharedparent
X
Path analysis:
(1) start with performer
(2) trace all paths of descentthrough pedigree to recipient
(3) determine odds that an allele was passed along each arrow
1/21/2
- odds of any allele being passed from parent to offspring are normally ½, which means 50% of the time
half-siblings
Coefficient of relationship
performerof action
recipient
sharedparent
X
Path analysis:
(1) start with performer
(2) trace all paths of descentthrough pedigree to recipient
(3) determine odds that an allele was passed along each arrow
(4) multiply the odds for each path
1/21/2
half-siblings
r = (½) (½) = ¼
Coefficient of relationship
performerof action
recipient
sharedparent
X
Path analysis:
(1) start with performer
(2) trace all paths of descentthrough pedigree to recipient
(3) determine odds that an allele was passed along each arrow
(4) multiply the odds for each path
(5) add the odds for all paths together
1/21/2
half-siblings
r = (½) (½) = ¼
- since we have only one path, odds that an allele was passed from the common parent to both half siblings is just r = 1/4
Coefficient of relationship
performerof action
recipient
mother
X
full siblings
X
fatherfather mother
Coefficient of relationship
performerof action
recipient
mother
X
1/21/2
(½) (½) = ¼
full siblings
X
1/2
father
1/2
performerof action
recipient
(½) (½) = ¼
add the probabilities for each path: r = ¼ + ¼ = ½
father mother
Hamilton’s Rule: an allele for altruistic behavior will spread if:
(B)(r) – C > 0 - where B is the benefit to the receiver- and C is the cost to the performer
if (benefit)(degree of relatedness) – (cost) > 0, the allele spreads - so, you will help a relative if the cost is low and they gain a lot
largest value for r is usually 0.5, for full siblings benefit must be at least twice the cost for full siblings to find it worthwhile to help each other
Coefficient of relationship
An individual’s fitness can be divided into 2 components:
(1) Direct fitness – results from personal reproduction
(2) Indirect fitness – results from extra reproduction by relatives, made possible by an individual’s actions
Inclusive fitness
When natural selection favors the spread of alleles that increase indirect fitness, kin selection results
Kin selection = natural selection based on indirect fitness gains
- most instances of apparent altruism result from kin selection
Example: alarm calling - in many mammals, individuals that see a predator approach produce an alarm call to warn others of their species
- often, this exposes the caller to added danger
Kin Selection
Highly social mammal; breeds in colonies
Females tend to breed near their birthplace
Neighbors are often closely related
Produce alarm calls when predators approach
13% of alarm-callers are chased by predators; only 5% of non-callers are chased
How did this evolve?…
Alarm calling: Belding’s Ground Squirrel
Kin Selection in Belding’s Ground Squirrel
(1) Females were much more likely to call than males
(2) Females were much more likely to call when they had a close relative nearby
Kin Selection in Belding’s Ground Squirrel
Mothers-daughters and sisters were also more likely to cooperate in chasing trespasser squirrels off their territory
Kin Selection in Belding’s Ground Squirrel
Thus, the degree of cooperation depends on the degree of relatedness between females
Evolution of Eusociality
Social insects are the pinnacle of altruism
- most worker bees and worker ants never reproduce - help their queen for life
Eusociality describes social systems with 3 key characteristics:
(1) overlap in generations between parents and offspring
(2) cooperative brood care
(3) a non-reproductive worker caste
Found in social insects, snapping shrimp, and naked mole rats
Sex determination in social insects
The unusual genetic system of social insects makes eusociality a likely consequence of kin selection
In haplodiploidy, sex is determined by chromosome number
- males develop from unfertilized eggs, are haploid
- females develop from fertilized eggs, are diploid
Sons do not have fathers, only mothers
Sisters all get the same set of chromosomes from their father, but have 50% chance of getting same allele from their mother
Sex determination in social insects Sisters are highly related to each other in haplodiploidy
Odds that one of sister A’s alleles came from mom = ½
Odds that mom gave an allele to sister B = ½
odds of identical-by-descent allele for Path #1 is (½) (½) = ¼
sister A sister B
mother(diploid)
X
father(haploid)
1/21/2
Path #1 .
Sex determination in social insects Sisters are highly related to each other in haplodiploidy
sister A sister B
mother(diploid)
X
father(haploid)
1/21/2
Path #1 .
sister A sister B
mother(diploid)
X
father(haploid)
1/2 1
Path #2 .
Odds that one of sister A’s alleles came from dad = ½
Odds that dad gave an allele to sister B = 1 (his haploid genome)
Sex determination in social insects
Because of this system, females are more related to their sisters (r = ¾) than to their own offspring (r = ½)
sister sister
mother(diploid)
X
father(haploid)
1/2
Combined odds of sisters sharing identical alleles:
(Path #1 odds) + (Path #2 odds) = (1/4) + (1/2) = 3/4
1
sister sister
mother(diploid)
X
father(haploid)
1/21/2
Sex determination in social insects
sister brother
mother(diploid)
X
father(haploid)
1/2
Sisters are only distantly related to their brothers:
(1/2)(1/2) = 1/4 (only one path links sisters and brothers)
1
sister sister
mother(diploid)
X
father(haploid)
1/21/2
1/21/2
Conflict between queen and workers
Conflict of interest between queen and non-reproductive workers Queen is equally related to sons and daughters (r = 1/2) - she will favor a 1:1 sex ratio (equal # of daughters and sons)
Workers have r = 3/4 with sisters, but only r = 1/4 with brothers - their fitness will be maximized when the queen produces a 3:1 sex ratio (more daughters than sons)
Who wins the conflict?
- in one species of ant, the queen laid eggs in a 1:1 ratio, but at hatching the sex ratio was biased towards many more females
- workers selectively destroyed male larvae - assert their own reproductive agenda over the queen’s
Mechanisms of Kin Recognition
In order for kin selection to operate, organisms must be able to reliably recognize their relatives
Direct recognition: chemical or vocal cues (distinct smells, cries)
Indirect recognition: you’re near me, so you’re probably a relative
- thus, many adult birds will feed any young in their nest
Mechanisms of Kin Recognition
One mechanism for direct kin recognition uses MHC proteins
Major Histocompatibility Complex (MHC) glycoproteins are important to immune response
- MHC receptors bind to bits of virus inside an infected cell
- move to surface, flag down killer T cells to nuke the cell
Are responsible for tissue rejection after organ transplant
- there are so many alleles, only close relatives are likely to share alleles
- these genes are probably good markers for kinship
Mechanisms of Kin Recognition
Experiment: Do mice that nest communally use MHC recognition to distinguish relatives from non-relatives?
- MHC proteins are released in mouse urine (signal is there)
- mice can tell full from half-siblings by their MHC genotype - mothers tend to place their young in nests that contain other
young sharing their MHC alleles Thus, mice use MHC proteins to identify allelic similarities
among offspring they may end up nursing