Sexual Selection and Altruism

Transcription

1 Sexual Selection and Altruism

2 Sex in Humans: XX and XY

3 Sex in Humans: XX and XY

4 Y = just a trigger for stuff on X

5 Sexual Selection "Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is, individual males have had, in successive generations, some slight advantage over other males, in their weapons, means of defense, or charms; and have transmitted these advantages to their male offspring."

6 Morphs Every organism has adaptations that function to facilitate reproduction Members of a population/species live in the same environment, so why do some animals have different adaptations than others? Morphs: age, sex, others SEX: male and female adaptations are different WHY?

7 Parental Investment Any investment by the parent in an individual offspring that increases the offspring s chance of surviving (and hence reproductive success) at the cost of the parent s ability to invest in other offspring (Trivers 1972)

8 In sexuallyreproducing species, the relative size of gametes define who is male and who is female. Sperm vs. Egg

9 Nurturant Females In most animals, and almost all mammals, females provide far more parental investment than just the egg Internal fertilization protects, but at a cost Cod vs. gorillas Humans (mammals): Prolonged internal gestation (pregnancy) Placentation Lactation

10 Competitive Males Males are fighting with each other to mate with as many females as possible More females = more offspring (sharp contrast to females)

11 Potential Offspring Numbers Ishmael the Bloodthirsty: polygynous emperor of Morocco had 888 kids 19th century Muscovite woman: 69 kids (27 births, many twins, triplets, and quads) Male variance is higher than female variance Ghengis Kahn

12 High male Variance: Elephant seals One$breeding$season,$ 115$males$were$ present,$but$the$5$ highest$ranking$ones$in$ the$hierarchy$(the$big,$ tough$ones)$performed$ 123$of$144$observed$ copulaaons$

13 High Male Variance: Humans Xavante$of$Brazil$ 184$married$males,$74$ were$polygynously$ married$ Men$with$4H5$wives$ (chiefs)$had$2h3$ames$as$ many$children$as$other$ men$ Male$variance:$12.1$ Female$variance:$3.9$

14 Sexual Selection and Parental Investment Theory For members of the sex that invests more in offspring, reproductive success is limited by the amount of resources an individual can secure for itself and its offspring For members of the sex that invests less in offspring, reproductive success is limited by the number of mates one can acquire

15 Sexual Selection and Parental Investment Theory What of it? Selection acted on males differently than it acted on females Specifically, differences in parenting strategies cause differences in adaptations Sex that invests more: adaptations to survive and get resources for offspring Sex that invests less: adaptations to help them get as many mates as possible It explains why, in many species, males look and behave differently than females

16 Sexual Selection and Parental Investment Theory Explains primary sex differences (uteruses vs. testes) Explains secondary sex differences Differences in weaponry (intrasexual selection) Differences in ornaments (intersexual selection) When the sexes have different adaptations, they are sexually dimorphic

17 Sexual Selection and Parental Investment Theory Intrasexual selection occurs when members of one sex fight with each other to gain sexual access to members of the other sex Results in weaponry Examples

18 Sexual Selection and Parental Investment Theory: Intrasexual Selection Hamadryas baboons

19 Sexual Selection and Parental Investment Theory: Intrasexual Selection Sea Skates

20 Sexual Selection and Parental Bears Investment Theory

21 Sexual Selection and Parental Investment Theory: Intrasexual Selection Male elephants

22 Sexual Selection and Parental Investment Theory: Intrasexual Selection In human females, physical aggression is unusual though not unheard of. When it does happen, it is often over men, resources provided by men, or insults to one s reputation.

23 Sexual Selection and Parental Investment Theory: Intrasexual Selection Female vs. female: gossip?

24 Sexual Selection and Parental Investment Theory: Intrasexual Selection Australopithecines

25 Sexual Selection and Parental Investment Theory: Intrasexual Selection Gibbons: lack of differentiation in size and weaponry

26 Sexual Selection and Parental Investment Theory: Intrasexual Selection Gorillas (rank/age-based differentiation)

27 Sexual Selection and Parental Investment Theory: Intrasexual Selection Human weaponry and size

28 Mortality in Humans Intrasexual selection often causes members of the competitive sex to die younger than the competed-for sex

29 Sexual Selection and Parental Investment Theory Intrasexual selection occurs when members of one sex fight with each other to gain sexual access to members of the other sex Intersexual selection.

30 Sexual Selection and Parental Investment Theory Intersexual selection results in ornaments in one sex absent or reduced in the other

31 Sexual Selection and Parental Investment Theory Intersexual selection: ornaments are not always pretty by human standards

32 Sexual Selection and Parental Investment Theory Intersexual selection: why ornaments? Arbitrary (you just know you want your offspring to inherit those traits) Good genes Low parasite load Handicap principle Combination

33 Sexual Selection and Parental Investment Theory Intersexual selection + Intrasexual selection = large, aggressive males plus ornaments mandrills

34 Sexual Selection and Parental Investment Theory Why not just say females are limited by resources and males are limited by access to females? The exceptions that prove the rule phalarope seahorse

35 Also related to sexual selection and parental investment Differences in human mating patterns due to sex differences in parental investment: Polygyny (1 male, multiple females) is common across human cultures Polyandry (1 female, multiple males is very rare in humans) Polygamy refers to either polygyny or polyandry, but most people often use it to refer to the more common system of polygyny Monogamy and serial monogamy also common because human fathers do often invest substantially in offspring (more than in most other animals); more on this topic later

36 Also Related to Sexual Selection and Parental Investment: Paternity Uncertainty Due to differences in sexual anatomy (e.g., humans and other mammals), paternity confidence is typically less than maternity confidence If fathers invest, as they do in humans, the father needs to be sure he is, in fact, the father more on this topic later Sexual jealousy and acute anger at cuckoldry more on this topic later

37 Male competition Sperm is produced in testes Testes are larger in less monogamous species Among primates Testes account for:.018% of body weight in gorilla.079% of body weight in human.269% of body weight in chimpanzee

38 Why sex at all? Handout: Zimmer ideas?...

39 The Problem of Altruism Kin Selection Reciprocal Altruism

40 Altruism Doing something that benefits another individual s reproductive success at a cost to one s own reproductive success If natural selection is all about competition to reproduce, how could there possibly be adaptations that cause one to be altruistic? Two solutions recall Dawkins

41 Kin Selection William Hamilton (bees) Not about helping the individual but about helping the gene Hamilton s Rule: C<Br You share genes with your relatives A gene that causes its bearer to be altruistic will only spread in a population if the cost it imposes on the altruist (C) is less than the benefit it provides to the recipient (B), multiplied ( devalued, because it is a fraction) by the coefficient of relatedness (r) Relatedness coefficients: all about halving r for every parent/offspring generation, and every sibling-sibling relationship (whiteboard)

42 Kin Selection Mom, dad, full siblings: 50% (1/2) Grandparents with grandchildren; 1st uncles and aunts with their nieces and nephews: 25% (1/4) First cousins: 12.5% (1/8) Identical twins: 100% (but really in the EEA?) Issues of step relatives much more on this topic later, but their r is 0 Issues of half-siblings:.25 (each kid got a different random sample of the genes of only one parent; and that parent only contributed half of each kid s genes to them) Issues of marrying relatives (e.g., Chagnon s work with the Yanomamo, who ideally marry one type of first cousin throws a monkey wrench into r calculations, but it can be done); Yanomamo villages with higher average r s, as averaged across all possible dyadic relationships, experience lower degrees of conflict Bees: weird; workers are more closely related to each other than to the queen; Hamilton s work

43 Kin Selection Subtheory Parent-Offspring Conflict results in conflict among ever very closely-related individuals You are more closely related to yourself than your siblings But mom is equally related to you and your siblings Mom wants to divide her resources equally among her kids Each kid wants more of mom s investment for self than for siblings Probably explains: E.g., weaning is conflict between E.g., sibling rivalry

44 Reciprocal Altruism Altruism among Unrelated individuals I.e., exchange, reciprocity, social exchange Trivers (of Sexual Selection and Parental Investment theory fame)

45 Reciprocal Altruism Four conditions must be met Cost to altruist is less than benefit to recipient There must be individual recognition There must be frequent role reversal Cheaters must be punished vampire bats

46 Altruism: for Reputation? ( Indirect Reciprocity ) Gandhi$ Scrooge$ Gordon$Moore,$Intel$rich$guy$

47 Reciprocal Altruism in Humans We are off the charts Economic experimental games are a recent research method used to study these issues (in class exercise ) The Ultimatum Game: A offers B a percentage of a pool; B accepts or rejects; If B accepts, they both get what A said they would; if B rejects, both get nothing (note that it is in the rational selfinterest a term favored by economists of B to accept any offer greater than zero) The Dictator Game: A offers B a percentage of a pool; B must take the offer, end of story (note that it is in the rational self-interest of A to offer nothing) For these games, the offer and accept rates, and the pool contributions, vary substantially within and between cultures Issues of reputation, anonymity, cultural values of fairness, and cultural values of obligatory regifting, the size of the reward (10 cents vs. $100) are important factors