Download To reproduce - SDSU Heart Institute

The big point of being alive is…
is…
To
reproduce
(for most organisms)
Is this clear?
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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
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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!
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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)
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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
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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
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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
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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…
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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
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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
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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
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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
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