Social interactions between organisms present the opportunity for conflict and cooperation...

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




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




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?




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




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


performerof action

recipient

sharedparent

X

Path analysis:

(1) start with performer

(2) trace all paths of descent through pedigree to recipient

half-siblings


performerof action

recipient

sharedparent

X

Path analysis:


(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


performerof action

recipient

sharedparent

X

Path analysis:




(4) multiply the odds for each path

1/21/2

half-siblings

r = (½) (½) = ¼


performerof action

recipient

sharedparent

X

Path analysis:




(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


performerof action

recipient

mother

X

full siblings

X

fatherfather mother


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


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


Mothers-daughters and sisters were also more likely to cooperate in chasing trespasser squirrels off their territory


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)


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


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


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


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

Social interactions between organisms present the opportunity for conflict and cooperation...

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