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Darwin’s Puzzle: Why are Males and Females Different?. Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex . 1st ed., Murray, London. Parental Investment and Sexual Selection. Trivers 1972. Assumption. - PowerPoint PPT Presentation
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Darwin’s Puzzle:Why are Males and Females
Different?
Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. 1st ed., Murray, London.
Parental Investment and Sexual Selection
Trivers 1972
Assumption
• Assumption: 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?
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)
Sperm vs. Egg
In sexually-reproducing species, the relative size of gametes define who is male and who is female.
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
Competitive Males• Males are fighting with each other to mate with as
many females as possible• More females = more offspring (sharp contrast to
females)
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 copulations
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 it’s offspring
• For members of the sex that invests less in offspring, reproductive success is limited by the number of mates one can acquire
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
• 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”
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…
Sexual Selection and Parental Investment Theory
• Intrasexual selection:weaponry/size differentiation
Sexual Selection and Parental Investment Theory
• Intrasexual selection: lack of differentiation
Sexual Selection and Parental Investment Theory
• Intrasexual selection: behavior
Sexual Selection and Parental Investment Theory
• Intrasexual selection• Causes members of the competitive sex
to die younger than the competed-for sex
• …willingly!!!
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….
Sexual Selection and Parental Investment Theory
• Intersexual selection: ornaments in one sex and not the other
Sexual Selection and Parental Investment Theory
• Intersexual selection: ornaments are not always pretty by human standards
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
Sexual Selection and Parental Investment Theory
Sexual Selection and Parental Investment Theory
Intersexual selection+
Intrasexual selection=
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
seahorse
phalarope
Next Topic
• Altruism
The Problem of Altruism
Kin Selection
Reciprocal Altruism
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…
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 to the altruist (C) is less than the benefit to the recipient (B), multiplied (devalued) by the coefficient of relatedness (r)
Kin Selection
• Mom, dad, full siblings: 50% (1/2)• Grandparents, 1st uncles and aunts,
and half siblings: 25% (1/4)• First cousins: 12.5% (1/8)• Identical twins: 100% (very un likely to
be a significant factor in the evolution of nonhuman primate behavior)
• Bees: weird; workers are more closely related to each other than to the queen; Hamilton’s work
Reciprocal Altruism
• Altruism among Unrelated individuals
• I.e., “exchange”
• Trivers (again)
Reciprocal Altruism
• Not very common in nonhuman primates
• Very common in humans
• Chimps exchange coalitional support
• See also capuchins in Perry book (later)