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Harvard University
Human Evolutionary Biology
Human Evolutionary Biology 1329
Richard Wrangham

Evolution by Natural Selection Darwin’s observations and postulates: -finches, beak side correlates to what size of seed they eat - More offspring are produced than can survive on the amount of resources available (Malthus). Competition ensues. - There is variability in nature. Each individual is unique (in sexually producing species) - Breeders can artificially select for desired characteristics by only allowing those with the desired trait to breed -postulates: Evolution occurs by natural selection under the three conditions:- Individuals vary within a species - Variation is heritable - Individuals vary in their reproductive success (RS)* -when true: THEN: Traits favorable to survival and reproduction will be passed down to offspring more frequently than unfavorable variants, so the frequency of that trait will increase in the species. - Over long geological time periods, successful variants may accumulate to cause such great changes that new species arise. - Because offspring tend to inherit their parents’ traits, those traits that better promote reproduction* tend to be preserved and come to predominate. Alfred Russell Wallace - Envisioned natural selection as a feedback mechanism keeping species and varieties adapted to their environment. Important terms - Evolution: descent with modification; each generation the population has a different genetic composition, such that a species may eventually change into new species - Biological Species Concept: Groups of actually or potentially interbreeding populations which produce fertile offspring and are reproductively isolated from other such groups. (Ernst Mayr) - Natural selection: non-random differential propagation of alleles. -Adaptations: traits favored by natural selection -Adaptive: promoting increased success at survival and reproduction - Natural Selection acts on the phenotype/ changes occur at the genotype - Survival of the fittest* . . ..Fittest= individuals who survive and reproduce. Does NOT mean “Only the Strong Survive” Selection Pressures that select for survival and reproduction can be similar or different. Ability to forage influences Survival & Reproduction - Natural selection is ONE of the main processes by which evolution occurs. It is the most interesting one because it produces adaptations. • Natural selection does NOT always produce change (only if the environment is changing): it favors traits that promote reproduction in the current environment. • Deleterious genes can persist in the genome - The Danger of Teleology:The concept that final causes/goals exist in nature Natural selection, Sexual selection, Runaway selection -sexual selection: traits that contribute to mating opportunities -male-male combat -mate choice (usually female choice) -mutually antagonistic sexual co-evolution -runaway selection: sexy sons and picky daughters -Fisher's explanation is that selection of some traits is a result of sexual preference; that members of the opposite sex find a trait desirable. This preference makes the trait advantageous, which in a circular fashion makes having a preference for the trait advantageous.-->greater preference and more pronounced traits, until the costs of producing the trait balance the reproductive benefit of possessing it. -horns, tails Individual and gene selection -natural selection-the nonrandom differential propagation of alleles. Alleles that cause the individual bearing them to produce more copies of the same allele are favored by natural selection Individual vs Group selection: according to darwin’s postulates, there must be variation between individuals for selection to work. for group selection, there must be variation between groups. migration can result in as much variation within as between groups. within-group selection working in the opposite direction from between group selection. so individual selection likely to prevail. -genes do not have motives. -nature is not always harmonious: group selection often erroneously used to explain observed patterns of behavior. most apparent group benefits are incidental effects of adaptations benefiting individuals. -natural selection: how good you do relative to others selection on social behavior: social behavior allows individuals to negotiate living in groups. selection can favor altruism. Kin Selection, Inclusive Fitness & Hamilton’s rule kin selection: kin share many of the same alleles. kin tend to associate with one another more than with non-kin. hamilton’s rule: C cost by a sufficient amount. Scientific Method & types of Reasoning -hypothesis: a statement about how two or more variables relate to one another. with prediction. -methods: the procedures used to collect data and test hypotheses. -data: information that has been systematically collected in order to test a hypothesis or describe a phenomenon. -inductive reasoning: the scientist notices patterns in the data which lead him to formulate a hypothesis to test. -deductive reasoning: prior to collecting the data. the scientist derives a hypothesis, based on theory, which he tests with data Primate Characteristics -brain (sensory and neural) -olfaction center in brain reduced. visual regions of brain evolved/binocular stereoscopic vision. expansion of the neocortex life history -slow. long gestation, long infancy, extended juvenility, delayed reproduction but long reproductive life span feeding; -many foraging strategies and tactics, extensive food processing, derived gut morphology. locomotion; behavioral flexibility: all contribute to the behavioral flexibility that characterizes the primate order. the order primates: phylogeny. -anthropoids. -strepsirhines -eyes not fully-frontated. wet noses. limited facial mobility. olfaction important. tapetum. -ring tailed lemur. -tarseirs: primate odd balls. nocturnal, carnivorous, head rotates 180, dry nose, pair bonded, no tapetum. strep brain and body. solitaryish. both sexes disperse. -//catarrhines, -old world monkeys: downward facing nostrils. arboreal and terrestrial. ischial callosities. cercopithecines (macaques, baboons, mangabeys, guenons, patas monkeys, cheek pouches) - vervets, hamadryas baboon (harem) and colobines.: leaf eating monkeys (sacculated stomach) UM/MF -hanuman langur, guereza, -apes: no tail, big brain to body ration. very slow life history. lesser apes: duet territorial defense. pair bonded. both sexes disperse. (gibbons, siamang) great apes: chimps, bonobos, gorillas, orangutans, humans . females or females and males disperse. platyrrhines -new world monkeys. side facing nostrils. prehensile tail. arboreal. -tamarins and marmosets. red howler. spider monkeys. squirrel monkeys. white faced capuchin. titi monkey. clade vs grade: clade: phylogenetic relationships. grade: non-phylogenetic relationship (folivorous, social org, diurnality) Logical Fallacies -phase of ignorance: humans are unique in having social org. phase of human exceptioonalism: humans unique in having culture. then comparative primatology. -dangers: superficial similarities (eg monogamy, different in gibbons and people) exaggeration of patterns as cross-cultural universals(monogamy: polygyny and polyandry also occur) imposing western patterns on the species as a whole (sexual mores) -naturalistic fallacy: equating IS with OUGHT. equating what is NATURAL with what is RIGHT. concern for unpleasant behaviors likeinfanticide and rape. -nature-nurture fallacy. false dichotomies. all traits result from the interaction of genes and environment. -the EEA: environment of evolutionary adaptedness. the set of adaptive problems faced by members of that species over evolutionary time. important fro understanding the adaptive properties of a species. largest sample is not representative of our remote ancestors. missing habitats. (is forager right for EEA?? complex HG=stratified foragers. only independent foraging peoples in Africa: hadza and san -social org of SAN. no full time foraging since 1980s. hunter gatherer residential groups are bands. bands average 25-40 individuals both among san and worldwide. occupies a camp for several weeks at a time. abandoned when women run out of food supplies. band composition: primary and distant kin of a family unit =40-50% of the co-resident adult members. but bands are not pure kin groups. greater tendency for closely related men to co-reside than women but not strongly patrilocal. many brothers live in other camps. high mortality for infants. old people exist. u shaped mortality. lower than wild chimps at all ages. -childcare by foragers: mostly by mothers. juvs can stay in camp while mothers forage. fathers, grandparents, kin help. -aggression is rare and disliked. mostly among men. often over women. no strict leadership. bullies or assertive individuals are laughed at or excluded. as a group, men dominate women. individually, personalities determin influence. -cooperation. uniquely developed in humans. joint action. sexual division of labor. teaching, important in humans, unknown in primates. -san: husbands and wives marry with informal ceremonies: husband pays for wife by months of bride service. helping her parents. little within band aggression. frequency and intensity of polygyny (rare to common, multiwife). attitudes to premarital and extramarital sex (tolerant to controlling) Complex foragers -fisher foragers: asmat. permanent structures. warfare endemic. art. -high pop density, reliable food storage and resources, increased sedentism, territorial compression, ownership and control of resource patches -suggest social org possibilities beyond nomadic HG horticulturalists/slash and burn farmers -not in eea. multiyear villages. polygyny. patrilocal. instense warfare. big man not chief. low pop density. little food storage. men clear land and warriors. movie. large ind villages. flexible alliances. -yanomamo. pastoralism herd animals -patrilineal descent systems. local warfare. male male alliances. women domestic, men herds. low female choice. lsoc org like hamadryas baboons. male kinship. males unite in patrilineal clans. defend access to females. why male? women small eco role. man need herd to buy woman and maintain food. Primate and Human Social Organization Grouping patterns -foragers. -daily sub grouping: women, babies and older children forage together. men alone or in small groups hunting. old women and toddlers in camp. -residential or local group (camp or band) median group size 29.5. groups can spend time together. -ethno-linguistic group. median size 895. might never assemble as a group. shared dialect, art, customs. -HUMAN: fission-fusion +group territorialty. cooperative breeding. nuclear families +sex div of labor. -more similar to apes than monkeys. atomistic fission fusion like chimps. tendency for male kin to breed in same group. -unique human: more variable social org than NHP. central place eating and sleeping. SDL with sexes working for each other. much more coop. Foraging patterns -SAN: mostly carried out by women and men separately. extensively cooperative. women forage as a group. men forage alone or as a group. both sexes snack out of camp, but bring most food back to camp. food is shared in camp. juveniles (pre adolescent) and the very old get little of their own food. men forage further than women. san women: typical forage abundant roots or seeds. men search for high risk high gain items. animal prey and honey. highly cooperative. sharing a kill: cultural rules influence division. biggest share often within family. other kin get big shares. non kin get some. -hadza: sex differences in diet. men eat more meat. women more tubers. Interaction between Resource Distribution/Monopolizability & Social Organization/Interactions Logical Inferences about the LCA Aggression in Primates -behavior directed towards causing harm to another individual. -agonistic: a system of behaviors which are adaptive in conflict situations between members of the same species. Types -male male competition. larger canines and body size in multF, one male. -displaced agg. A>B>C. A pisses off B, B attacks C. -hierarchy-reinforcing agg. intimidation. random. -maternal agg.-mama bear protect baby bear. -eviction agg. -familial agg. -infanticide. if baby is not theirs to increase future paternity. sexually selected male reproductive strategy. -siblicide. let weaker die so you get resources if limited. -abuse. -policing. physically impartial intervention into conflicts. -inter-group agg. resource holding potential: ability of an animal to win an all-out fight if one were to take place Costs and Benefits -benefit for winner: immediate: contested resource. reputation/ rank/ winner effect -costs for loser: reproductive disruption (females) -delayed puberty, decreased levels of estrogen and progesterone, increased incidence of anovulatory cycles, impaired implantation, greater risk of miscarriage, prolonged interbirth intervals, accelerated reproductive senescence. -loser effect: chronic activation of HPA axis. STRESS. less fitness. social withdrawal. -costs for everyone: energy mobilization for fight, predation while fighting, (reaction time graph), opportunity costs, risk of injury/death Formalized Signaling: makes other guy back down and avoid fight. signal hawk intentions. -agonistic behavior: stare, open mouth threat, bigger open mouth threat, escalation: lunge, ground slap, charge, slap, bite. submission can neutralize agg. Warfare and Aggression in Chimpanzees and Humans Hawk position vs Dove position and assumptions -maynard smith and price. first application of game theory to understanding animal behavior. -2 doves: display, random takes prize (v), neither injured. average payoff=v/2 because each wins half the time. -2 hawks: escalate, fight, one wins. other injured and bears costs (c). average payoff is v/2-c/2 or (
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