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The big point of being alive is… is… To reproduce (for most organisms) Is this clear? 1 Let’ Let’s ask some questions • What do you mean by “reproduction” reproduction”? • But surely the purpose of life is to stay alive, right? • What about all those other things (eating, sleeping, traveling, building, etc)? • Maybe for other animals, but what about humans? • OK, so how does reproduction work? • Are there costs associated with sex and reproduction? • Has evolution changed the way humans have sex? Are there enough questions on this slide? What do you mean by “reproduction” reproduction”? • Life making copies of itself • Elemental building blocks are identical everywhere • Instructions for arrangement are in the genetic code • Replication and vertical transmission of DNA is key • “Selfish” Selfish” gene hypothesis: "our" genes build and maintain us in order to make more genes” genes” Genetic information is preserved over time 2 But surely the purpose of life is to stay alive, right? • On the surface, it may appear true • Most living things spend a lot of time to stay alive • Devote time to self-preservation • Why spend so much time devoted to self-preservation? • Living organisms are not immortal - can’ can’t live forever • Living things wear out (a.k.a. die) • To continue life, it must make fresh copies • Self preservation is necessary to stay alive long enough to make fresh copies • Even in death, some organisms preserve their future (the male praying mantis or black widow spider) Stay alive to make copies of your species What about all those other things organisms do? • engage in activities to enhance ability to stay alive with the goal of ensuring reproduction • “kamikaze reproduction” reproduction” of Pacific salmon: devotes years to staying alive, then commits suicide in efforts to reproduce • males compete with each other to gain access to females (physical or psychological competition) • do our lifestyle choices as humans reflect an ongoing effort to increase self-preservation and reproduction? Increase your chances of reproduction! 3 Maybe for other organisms, but humans??? • we paint pictures, go jogging. How does this help? • relatively little effect, maybe subtle • conscious overlay of culture upon subconscious drives • turn from biological basis to sociological fitness • more complex organisms control their environment and try to manipulate it to their advantage • humans are the ultimate environment manipulators • watch television: relaxation, increasing lifespan, bonding • painting pictures: relaxation, provide money, show fitness to potential mates • jogging: improve health, look for potential mates Humans are also biological creatures OK, so how does reproduction work? • species that don’ don’t reproduce, don’ don’t survive • “The hen is the egg’ egg’s way of making more eggs” eggs” (Samuel Butler) • The vehicle carrying genetic information must successfully get the egg fertilized, hatched (born), and far along enough in development to ensure it will produce another egg • The egg also wants more eggs like itself - the egg wants to pass it’ it’s genetic material to the next generation • Preservation of the species is paramount: individuals may die but the species lives on (herbivores in herds, fish and insect eggs produced in thousands, plant gametes in millions) 4 OK, so how does reproduction work? (part 2) • of course, individuals care about reproduction • that’ that’s the basis for self-preservation, remember? • the species doesn’ doesn’t carry the genes, individuals do • if organisms didn’ didn’t care about passing their genes onto the next generation, their species would disappear • without instinct to pass on it’ it’s genes, an individual doesn’ doesn’t pass them onto the next generation. The lack of instinct to reproduce dies with it. • individuals with the instinct to pass along genes are those that continue the species, thereby breeding in the instinct into all future generations Asexual reproduction - going it alone • reproduction is NOT synonymous with sex • asexual replication can occur by fission, budding, or cloning of the organism • fissioning: fissioning: splitting into two parts (amoeba) • budding: splitting off parts of the organism (hydra) • cloning: parthenogenesis without sperm (honeybees) • advantages: genes rarely mutate (good genes preserved), all time devoted to reproductive energy and creating offspring rather than finding a mate. • disadvantages: without disadvantages, there would be no reason for sexual reproduction to evolve. Short term benefit, long term losses 5 Sexual reproduction: sharing your legacy • most terrestrial reproduction is via sexual activity • each sex contributes half the genes into progeny • sperm and egg combine through intercourse • faster evolution - combine genes randomly • each offspring is a unique genetic assortment • variation / mutations possibly increase fitness • can also “edit out” out” mutations, since half of maternal and paternal genes aren’ aren’t passed along • mutations not passed along disappear from population • there is an inherent “cost of sex” sex” in this reproductive lifestyle that is absent from asexual reproduction Evolutionary advantage, but with a price The Cost of Sex (in mammalian reproductive terms) • male expends relatively little versus female • males: tens to thousands of calories per contact • time varies from 2-3 seconds (blue whale) to a few hours (tree sloth) not counting foreplay • female has far greater physical & temporal investment • females: carry fetus in womb, nourishing development, withstand birth, protect and rear offspring until capable of self-support • female investment can take weeks to years • differences in investment of time and energy lead to difference in way males versus females regard sex Cost prompts developing strategies 6 Reproductive strategies: gender wars • natural design: create reproductive strategies to favor the strongest, smartest, best adapted organisms • the most viable organisms to reproduce • fitness is improved with passing generations • males and females must desire reproduction to pass along genes, therefore strong urge for intercourse • without desire, reproductive strategies fail • sexual desire built into every organism • each organism want to find the best, most fit mate to combine genetic material with to improve progeny • reproductive strategy: mate with the best partner Reproduction evolves through competition Sexual desire: ensuring the next generation • an instinctive reaction in animals • based upon the perception of a suitable mate • suitable mate must meet or approximate set of criteria • individuals evaluate potential mates relative to available pool of possibilities to select superior candidates for mating • superior candidates should theoretically combine to produce progeny with best chance of survival • if criteria are not met, sexual desire does not occur • success depends upon reciprocal desire from mate • costs of sex differ between male and female, so criteria are vastly different between genders • intimately tied with numbers: number of offspring = increased chance for survival, superior gene mix, more generations Desire is shared, agendas may differ 7 The male agenda in sexual reproduction • most males are promiscuous - mate often as possible • genetically practical course of action • more female matings = more offspring with his genes • cost of sex lower for males than females • male criteria for sexual desire relatively simple: 1) healthy female can carry, give birth, support young 2) young enough for greater potential viability 3) impregnable condition • these criteria are readily apparent to the male - pheromones released by female cockroach - swelling and blush of genitalia in female chimpanzee As many superior matings as possible The male agenda in sexual reproduction (part 2) • the sex act is of paramount importance • the sex act is how the male impregnates the female • she must be receptive • he must try to find as many receptive mates as possible • he may try to prevent her from mating with other males • evolving strategies to prevent insemination by competitors - the basis of sperm competition • the more successful the male is at finding multiple mates, less energy will be spent guarding impregnated females But females have a different agenda… 8 The female agenda in sexual reproduction • bear the brunt of sex cost in terms of time and energy • can’ can’t create multiple offspring quickly with many mates • cannot abandon offspring or genes will die with infant • selective in choice of mate (or exercise choice in fertilization of egg) • criteria aimed at getting the best possible male 1) physically acceptable 2) contribute to offspring’ offspring’s welfare (e.g. leadership, status in group, fighting skill) 3) Possess attributes demonstrating fitness (e.g. peacock’ peacock’s conspicuous and unwieldy tail) Much more choosy in selecting a mate The female agenda in sexual reproduction (part 2) • sex act often brief, unimportant except for male orgasm • if better male comes along, multiple matings often occur • exercising choice over which sperm are allowed to reach the egg leads to sperm choice • divergent strategies mean evolution has driven development of mating and reproduction • females and males compete for selection of best mates, leading to evolution of strategies to ensure ability to procure and procreate with the best and the brightest Females exercise their own choices 9 Reproductive strategies • strategies include singing, display of physical character, dancing, pheromone release, flashing lights, or combat • males usually display to attract females, since females make the choice in mating • she is the arbiter of mating suitability, he applies for permission and awaits her decision • male application for permission is major part of strategy • he gets her attention to convince her that he’ he’s the best choice as a partner (fireflies, peacocks, leks) leks) • mammals often resort to challenges between males (rams, moose, elephant seals all fight) Males compete for opportunity to mate Reproductive strategies: part 2 (in mammals) • win her approval by success in combat (lions take over harem as dominant male, kills all prior male’ male’s progeny, re-impregnates all females) • female decides if male measures up • she can walk away or swat him if she’ she’s not interested • wolf pack uses “alpha” alpha” individuals - single male and female reproduce and pack rears their offspring • many herbivores use combat (deer, elk, sheep, horses, cattle all have male supremacy battles). Winner takes as many females as he can while keeping other males away as much as possible Females wait, watch, and choose 10 Reproductive strategies: role reversal? • in rare cases the female pursues the male • seen in species where the male is nurturing parent (e.g. phalarope or seahorse) • male is “resource” resource” for female in gene propagation • female fights off other females until eggs are laid, then leaves to find another mate • however, the female still chooses the male Generally, female behavior is not for convenience of males. Males must adapt their reproductive strategies to suit female behavior to ensure their success Human reproduction • anatomy of humans is radically different from other mammals • different anatomy affects human sexual activity • typical land mammal anatomical structure enhances reproduction: typically females walk on four legs, rear legs perpendicular to spine, vagina under tail and flush to body surface • position of limbs makes support of weight possible during intercourse without much effort, limbs don’ don’t impede access for intercourse, can walk (or run) away if not interested Human anatomy puts different spin on sex 11 Human reproduction: part 2 • vagina well protected by tail, easily accessible during mating season • primates primarily walk on all four extremities, males mount from rear, female easily supports male weight with strong rear legs, vagina near tailbone and protected by callous rump and limbs unless presented to male - similar to other mammals • humans body seems designed to impede intercourse • upright stance is major difference for humans - less of a problem for males (protruding out), but upright stance pushed legs out to sides and vaginal opening further forward Female anatomy evolved compromises Human reproduction: part 3 • vagina retreated into body, covered by extra flesh, and became hidden between two columns of flesh and bone (legs) • standing up, intercourse is very challenging for humans (penis length and body flexibility may help) • evolutionary pressure for males to develop longer penis (humans have relatively large penis compared to other primates - gorilla’ gorilla’s erect penis averages 2” 2”) • moreover, technique had to be modified to increase success at impregnation • led to changes in male / female sexual dynamics • societal and cultural factors overlie biological imperative Human sex appeal is most varied /complex 12