1. 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 benefitsPerformer suffers ReceiverCooperationAltruism benefits ReceiverSelfishSpiteful suffers Social interaction

2. 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 benefitsPerformer suffers ReceiverCooperationAltruism benefits ReceiverSelfishSpiteful suffers Social interaction

3. 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 benefitsPerformer suffers ReceiverCooperationAltruism benefits ReceiverSelfishSpiteful suffers Social interaction

4. 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 benefitsPerformer suffers ReceiverCooperationAltruism benefits ReceiverSelfishSpiteful suffers Social interaction

5. 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 benefitsPerformer suffers ReceiverCooperationAltruism benefits ReceiverSelfishSpiteful suffers Social interaction

6. 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

7. 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

8. 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

9. performer of action recipient shared parent X Path analysis: (1) start with performer (2) trace all paths of descent through pedigree to recipient half-siblings

10. Coefficient of relationship performer of action recipient shared parent X Path analysis: (1) start with performer (2) trace all paths of descent through pedigree to recipient (3) determine odds that an allele was passed along each arrow 1/2 - odds of any allele being passed from parent to offspring are normally ½, which means 50% of the time half-siblings

11. Coefficient of relationship performer of action recipient shared parent X Path analysis: (1) start with performer (2) trace all paths of descent through pedigree to recipient (3) determine odds that an allele was passed along each arrow (4) multiply the odds for each path 1/2 half-siblings r = (½) (½) = ¼

12. Coefficient of relationship performer of action recipient shared parent X Path analysis: (1) start with performer (2) trace all paths of descent through 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 1/2 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

13. Coefficient of relationship performer of action recipient mother X full siblings X father mother

14. Coefficient of relationship performer of action recipient mother X 1/2 (½) (½) = ¼ full siblings X 1/2 father 1/2 performer of action recipient (½) (½) = ¼ add the probabilities for each path: r = ¼ + ¼ = ½ fathermother

15. 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

16. 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

17. 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

18. 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

19. Kin Selection in Belding’s Ground Squirrel (1)Females were much more likely to call than males

20. (2) Females were much more likely to call when they had a close relative nearby Kin Selection in Belding’s Ground Squirrel

21. 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

22. 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

23. 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

24. 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 Asister B mother (diploid) X father (haploid) 1/2 Path #1.

25. Sex determination in social insects Sisters are highly related to each other in haplodiploidy sister Asister B mother (diploid) X father (haploid) 1/2 Path #1. sister Asister 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)

26. 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 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 mother (diploid) X father (haploid) 1/2

27. Sex determination in social insects sisterbrother 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 mother (diploid) X father (haploid) 1/2

28. 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

29. 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

30. 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

31. 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