The amount of energy available to an organism over the course of its life is
finite
Trade-offs influence organisms'
life histories
Life span (i.e. senescence) , for example, is influenced by an organism's ability to
keep itself going at the expense of reproduction
Life history comprises :
-the pattern of development and growth -the life span -the timing and quantity of reproduction
selection's perfect organism would
-be mature at birth -continuously produce lots of high quality offspring -live forever
Female Thirps egg mite mature at birth
-mature before it hatches, mates with brother -BUT, it dies after 4 days when its offspring hatch out
Brown kiwiw
-chick almost independent in a week -BUT takes two months for two eggs, males watch them for 3 months
Biology and physics impose limits, fitness *trade-offs*
-how selection balances trade-offs in different environments leads to different *life histories*
Reproduction is a ________ resource
finite
Trade-offs during any single reproductive period :
-what is the optimal number of offspring to have? -what is the optimal parental investment for each offspring?
life history traits are optimized by
natural selection to maximize parental fitness
Adaptive life history strategy evolves as
the life history traits evolve in response to ecological conditions
Life history traits do not evolve in
isolation -are linked via energy trade-offs
Investment in each life history trait has a
benefit and a coast to the organism
For each life stage, there is an optimal
investment into a certain life history trait
Investment beyond that optimum reduces
fitness by limiting energy available for other important functions
Teleology :
idea that purpose exists in evolution in the same sense that it does for human intention
Teleological language and explanations must by
avoided when discussing evolution
Evolution has no
predesigned or intentional goal
"strategy" does not imply
a conscious choice by that organism
"Strategy" is a shortcut for
the description of complicated evolutionary processes
Adaptive strategy does not mean the best possible
-like all adaptations, life history strategy is subject to constraints and needs only to be good enough -optimal life history strategy means the best of those existing in certain population under certain environmental conditions
Trade-offs means that is is the overall strategy rather than
-a single life history trait that determines fitness
The components of the life history strategy evolve
as an integrated unit
Reproductive value (Vx) of an organism is the
expected reproductive contribution of an individual of age x to the next generation
The reproductive value is closely tied to the
organism's fitness
The reproductive value changes over the course of the _____ ______
life span
Reproductive value is determined by
the interaction of lx and bx columns
Vx often increases with
age to a maximum just as the organism enters the reproductive years
As the birth rate and survivorship decline with age, Vx ______
declines
Where on this graph would selection against lethality be most intense?
-at A because there is essentially no fitness -fitness is the area under the curve
A life history strategy refers to the fact that
the life history consists of a suite of adaptations
Life history strategy is the product of
evolution
Each organism has a finite amount of energy to
-devote to life functions such as development, maintenance, and reproduction -therefore,it must allocate that energy in ways that maximize fitness
The reproductive value measures the
expected contribution of an individual age x to the next generation
Life history is affected by both
genetic mechanisms and phenotypic plasticity
The life cycle comprises three key developmental features:
-the process by which an embryo becomes an adult -the presence of dormant stages during development -the development and constancy of the organism's sex
A simple life cycle:
-Juveniles develop from the fertilized egg, grow into adults whose gender is determined genetically, live out their lives as active adults, and eventually die -ex: humans
A complex life cycle includes
-changes in the body plan, including resting stages and change in the individual's gender -ex: amphibians, insects
Direct development:
-the adult develops directly from the fertilized egg without a larval stage
Metamorphic development entails
a larval stage that is often radically different form the adult individual
Plants and some algae- the life cycle represents alternation of generations
-Gametophyte (G) - haploid (n) -Sporophyte (S)- diploid (2n) -Gametes fuse to produce a sporophyte -a sporophyte produces spores which give rise to a gametophyte -a full cycle (G to G or S to S) represents sexual reproduction in plants
Metamorphosis costs:
-significant energy expenditure -vulnerability to predation at certain stages
Metamorphosis advantages:
-Specialization on different functions of different life stages -exploitation of different ecological niches -reduced competition among larvae and adults
Neotony is the development of
-sexual larval forms that no longer metamorphose into adults -seems to be more common in extreme environments -the selective forces leading to neotony are not fully understood
Resting stages :
-a developmental stage in which the organism is dormant, inactive and often resistant to harsh environmental conditions -ex. seeds, spores,cysts
One advantage of metamorphism is
-allows the organism to exploit important resources
Sex changes // simple life cycle
-sex is determined early in development and remains constant throughout life
Sex changes // Sequential Hermaphroditism
-sex changes during the life span -Protandrous ; an individual is first male and then female -Protogynous ; an individual is first female and then male
Metamorphosis has both costs and benefits
-requires energy and complex genetic regulation -allow the organism to exploit resource-rich environments and organize the body plan for specific functions
Resting stages allow the organism to avoid
harsh physical conditions
If male and female success varies with age, size, resources, or the social system, it may be adaptive to
change sex during the life span
Life span varies greatly among
-species -from hours to thousands of years -ex; creosate bush: an individual plant may be >10,000 years old
Senescence :
late life decline in fertility and probability of survival -variation across taxa
If there is a decrease in fitness with age, then why doesn't selection act against it?
-not enough energy -other tradeoffs
Rate-of-Living theory
-gist: bodies wear out -eventually accumulation of damage: errors in DNA replication and translation, build-up of poisonous metabolites -organisms adapted to resist as long as possible -but there isn't the variation to extend life further
Rate-of-Living theory makes two predictions
-aging rate should be correlated w/metabolic rate -should not be able to select for longer life spans -BUT both predictions do not hold general
Life span is not correlated with
metabolic rate
Austad & Fischer : wide variation in lifetime expenditure of energy
-bats have higher metabolic rates than other groups, but tend to have longer lives -marsupials have lower metabolic rates, but also have shorter life spans
Longer life can or cannot be selected for?
CAN
Luckinbill et al.: artificial selection on Drosophila for longer life
-4 lab populations. 2 selected for early reproduction, 2 longer life -collected eggs from youngest and oldest individuals -couldn't shorten lifespan much -could almost double lifespan in 13 generations
think of rate-of-living as
"the result of intrinsic physiological limits on cells and tissues"
Normal cells have limits on the number of times that they can divide
-at ends of chromosomes repeated sequences (teleomeres) -a bit of teleomeres lost with each division -some cells have lots of telomerase, such as germ line and cancer cells
is senescence simply the result of losing our teleomeres?
-e.g. dyskeratosis congenita
Jueng et al. made C. elegans over-express a Teleomere binding-protein
-lived longer -BUT lots of other studies show that the relationship is not that simple
Why do Organisms age? Hypothesis 1 Limiting soma theory:
-Limiting resources devoted to reproduction result in decreased somatic maintenance -mutation that diverts energy toward reproduction early diverts energy away from maintenance and repair
Why do Organisms age? Hypothesis 2
-The intensity of natural selection declines with age -Genes whose main effects occur after the peak in Vx are not subject to the same intensity of selection as those that occur when Vx is high or increasing -Deleterious mutations then accumulate and cause senescence
Evolutionary Theory of Aging
-deleterious mutations with effects that begin late in life are harder to remove from the population
Island opossums have fewer predators than mainland opossums thus less
-ecological selection
Predicts that island opossums should
-have longer natural lifespans -higher reproductive success later in life -slowed rate of physiological aging
Why Organisms Age? Hypothesis 3
-Pleiotropic effects of genes : the action of a single gene that affects several phenotypic traits -Genes that benefit younger individuals whose reproductive value is high will be selected for even if they have deleterious pleiotropic effects that occur in old age
Trade-offs : Antagonistic Pleiotropy
ex: called flycatcher -some individuals breed year 1, others year 2 -early breeders have smaller broods throughout life -add extra egg, broods get smaller -thus, there is a trade-off between reproducing early and late
What about menopause?
-women have steep decline in reproductive potential late in life but well before death
To main hypotheses : nonadaptive effect of modern lifestyle
-women live longer then they used to -menopause not adaptive because early humans didn't live long enough -females of some captive mammals present same pattern
Grandmother Hypothesis
-as women age, benefit/cost of children changes -divert effort from direct fitness to indirect fitness -true in elephants too
Natural selection operates primarily on
the individual
Mutations can only spread through a group by selection if
if it benefits an individual
Group selection :
-idea that traits are selected for the good of the group over the individual -not an evolutionary stable strategy (ESS) because selfish mutation can invade, and altruistic mutation cannot invade -How then can altruism invade?
Altruism can invade by
-evolved organization of cells -act to increase indirect fitness
The evolution of the life span
-interacts with pleiotropic effects
Finite energy available to an organism leads to
trade-offs in life history characters
Generally speaking "live fast die young" isnt
true in biological systems
Senescence results from
the combination of external and internal factors that increase mortality as a function of age
Life span has a genetic component, selection can
-increase the life span
Three mechanisms contribute to the evolution of senescence :
-late-effect deleterious mutations -trade-offs between reproduction & maintenance -antagonistic pleiotropy
Selection optimizes
reproductive effort
reproductive effort :
-cost that must be budgeted
Energy spent on reproduction can't be spent on
other functions
Early reproduction can _______ lifespan
shorten
How much effort to put into reproduction in a given season?
-How many offspring? -How much should be invested in each one?
If trade-offs optimized, reproduction should be
maximized -to get the most offspring into future generations
High reproduction does not always ________ fitness
increase
Examples of high reproduction not increasing fitness
-enlarged clutches of pled avocet led to poorer quality of the occupied territories and higher chick mortality, decreasing their net reproductive success -removal of an egg from the female, anole lizards, leads to a higher survival to the next reproductive event due to the increase in the females stamina, sprint speed, and growth
organisms that reproduce multiple times :
iteroparous
organisms that reproduce just once are :
semelparous
Usually very little _______ in offspring size in most species
variation
Optimal compromise between size and number of offspring Assumption 1:
There is a trade-off between size and number of offspring
Optimal compromise between size and number of offspring Assumption 2:
Above a minimum size, the probability that any individual offspring will survive is an increasing function of its size
Optimal compromise between size and number of offspring Analysis:
The parental fitness gained from a single clutch of offspring of a given size is the number of offspring in the clutch multiplied by the probability that any individual offspring will survive
Where there is variation, _________ can act
selection
Usually very little ________ in offspring size in most species
variation
hatchery chinook salmon in Canada study between offspring number and quality : SELECTION example
-Adult salmon return to hatchery, are spawned, fry released to natural rivers -individual females with bigger eggs made fewer -as eggs get bigger, a larger proportion survive -hatchery gives fry a boost; increases survivorship at smaller sizes -leads to selection for smaller eggs -this has conservation implication since hatchery fish are used to stock wild population, influences egg size
Why is there variation in life history traits? Given that:
-selection reduces genetic variation of a trait -life history (i.e. reproductive) traits are so closely related to fitness -then, shouldn't life history traits have especially low heritability?
What are the evolutionary forces that maintain genetic variation?
different genotypes/phenotypes have different fitnesses at different times
Sea squirt, semelparous and interoparous example:
-compete with another sea squirt (B. leach) late in season -semelparous form puts all its energy into one reproduction, then succumbs to competition -iteroparous reserves some energy to out-compete B.leachi
selection optimizes life histories to explain the various patterns, except when it doesn't -Pink Lady Slipper
-big showy flowers -offer no nectar -flowers are self-fertile
is the pink lady slipper orchid using an evolutionary stable strategy (ESS)?
NO
Trade-off between _______ of offspring and the __________ of their survival
-number -probability -and their future reproduction
trade-off between offspring ______ and offspring ______
-quality -number
these relationships don't exist in a
vacuum
Competitive plants; life strategies
experience low stress and low disturbance and are more limited by competition between individuals than external factors
Stress tolerant plants; life strategies
inhabit physically demanding habitats where stress is high and disturbance is low
Ruderal plans; life strategies
inhabit environments with low stress and high disturbance
Plants in which corner of Grime's triangle would be favored in a recently clear-cut forest?
-Ruderal
Plants in which corner of Grime's triangle would be favored in a barren desert?
-Stress tolerant
Heppel: a method to determine which life history trait has the greatest
-effect on the population growth rate
Elasticity:
a measure of the effect that a change in trait has on population growth rate
K selection= Quality
-Stable environment: high competition and high and stable density -iteroperity, parental care, late age in maturity, small clutches, large offspring
r selection= Quantity
-Fluctuating environment: low competition, density fluctuates (often low) -semelparity, large clutches, early age at maturity, small offspring
Evolutionary which is better, quality or quantity?
it depends
Bet-Hedging theory
-in unpredictable environments, there is great potential for catastrophic loss of an entire reproductive effort -The bet-hedging strategy reduces the magnitude of each reproductive event, thus spreading the risk over multiple events
Bet-hedging in plants
-Desert annuals produce extremely long-lived seeds -subsets of seeds germinate in different years -the proportion of seeds that germinate in any year is negatively correlated with variation in reproductive success
Bet-hedging in animals
Two populations of the black-browned albatross: -a highly variable environment of south Georgia islands -a more stable environment of the Kerguelen islands --> on South Georgia, annual reproduction is lower, but interannual survival and life span is greater
A very unpredictably variable environment tends to encourage:
- r-selection and bet-hedging
The optimal reproductive output is not necessarily the
physiologically maximum possible
The age-specific reproductive rate (bx) and the age-specific survival rate (lx) are reciprocally connected:
reproduction has a mortality cost, and the reproductive pattern is a response to the mortality schedule
The reproductive life history is a response to the species
-unique ecology
A number of conceptual frameworks such as r/K fvgcfcfcfcselection, Grime's triangle, and the bet-hedging theory, organize these interactions according to
the key ecological forces acting on reproduction
Important selective forces imposed by ecology are ______ vs ______ offspring and the ______ reproductive output vs. the ________ in reproductive output.
-quantity -quality -mean -variation