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life_history.txt
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UNIT 5: Life history
----------------------------------------------------------------------
TSEC Introduction
{\bf Life history} refers to patterns of how organisms allocate
resources to key components underlying reproductive success:
POLL Give a one-word example of a fundamental component of success.
ANS Survival
ANS Growth
ANS Reproduction
ANS Dispersal
----------------------------------------------------------------------
Diversity
Differing life-history \textbf{strategies} are part of the reason
for the remarkable diversity of life
Organisms that are too similar are not expected to co-exist
One will out-compete the other
But two organisms may be able to exploit the same resources using
different life-history strategies
----------------------------------------------------------------------
PSLIDE Oaks and dandelions
BC
SIDEFIG webpix/oak.jpg
NC
SIDEFIG webpix/dandy_flower.jpg
EC
----------------------------------------------------------------------
Oaks and dandelions
We can think of acorns as machines for making more acorns, and
dandelion seeds as machines for making more dandelion seeds
Both have access to very similar biochemical machinery.
Both use the same resources.
ANS Water, sunlight, nutrients
POLL What are some differences?
ANS Oak trees are bigger
ANS Oak trees wait longer to reproduce
ANS Oak trees reproduce many times
ANS Oak trees put much more energy into each seed
ANS Dandelion seeds are dispersed by wind, acorns by animals
----------------------------------------------------------------------
PSLIDE Strategies
BCC
SIDEFIG webpix/giraffes.png
NCC
SIDEFIG webpix/cutters.jpg
EC
----------------------------------------------------------------------
Scales of competition
Organisms compete with other individuals of the same species
They also compete with other species
We think about life history on different scales
Evolution within populations
Competition between populations
----------------------------------------------------------------------
PSLIDE Within species
FIG webpix/mallard_nest.jpg
----------------------------------------------------------------------
PSLIDE Between populations
FIG webpix/tree_competition.jpg
----------------------------------------------------------------------
TSEC Tradeoffs
Some evolutionary changes simply help organisms function better
Hemoglobin is highly evolved to bind and release oxygen
Most have advantages and disadvantages
Building a strong immune system may reduce growth rates
A leaf that produces a lot of energy at high light may not be
able to produce any at low light
A \textbf{tradeoff} occurs when improvements in one area come at a
cost of disadvantages in another area
----------------------------------------------------------------------
PSLIDE Tradeoffs
FIG webpix/bamboo.jpg
----------------------------------------------------------------------
PSLIDE Tradeoffs
FIG webpix/peacock.jpg
----------------------------------------------------------------------
Optimization frontiers
We expect tradeoffs because:
organisms have limited \textbf{resources}
organisms are under natural selection in a complex world
----------------------------------------------------------------------
PSLIDE Optimization frontiers
FIG life_history/frontier.Rout-0.pdf
----------------------------------------------------------------------
PSLIDE Optimization frontiers
FIG life_history/frontier.Rout-1.pdf
----------------------------------------------------------------------
Optimization frontiers
FIG life_history/frontier.Rout-2.pdf
----------------------------------------------------------------------
Optimization frontiers
BC
Under natural selection, we expect organisms to be near the frontier
of high fitness
While they're near this frontier, it will be hard to improve one
quality without a tradeoff that hurts another quality
NC
SIDEFIG life_history/frontier.Rout-2.pdf
EC
----------------------------------------------------------------------
Evolution and optimization
We often think of organisms as making ``choices" that maximize their
evolutionary fitness.
Do oaks choose how big their acorns should be?
Then what's going on?
ANS Natural selection is selecting random variants
ANS On average, variants which survive are better at producing
offspring over the long-term than those which don't survive
----------------------------------------------------------------------
Programmed optimization
Organisms pursue very sophisticated strategies to optimize fitness
But they don't know they're doing this
Plants sensing water environments
Moths circling light bulbs
People pursuing sexual opportunities
----------------------------------------------------------------------
PSLIDE Programmed optimization
FIG webpix/moth_light.jpg
----------------------------------------------------------------------
PSLIDE Programmed optimization
HIGHFIG webpix/teenage_couple.jpg
----------------------------------------------------------------------
Tradeoff: Quick maturation vs.~large final size
A key component of a life history is how quickly an organism matures
Organisms that mature quickly can reproduce quickly
Organisms that mature slowly have more time to get large, or build
lasting structures, before they reproduce
they typically reproduce more (or for a longer time period) in
the long run
or allocate more energy to each offspring, giving the offspring a
better chance to be successful
----------------------------------------------------------------------
Tradeoff: large reproductive output vs.~longevity
Survival-reproduction balance: at a given time, organisms face a
tradeoff between:
energy spent on producing offspring
produce more offspring, or give more resources to helping
each get started in life
energy reserved for survival and future offspring
spend less energy reproducing this year, but live for longer
----------------------------------------------------------------------
Semelparity
The extreme case of this balance is called {\bf semelparity}: the
life-history strategy of reproducing only once
Many organisms are semelparous
We can imagine that converting all your resources to reproduction
once you start could be very efficient
Many organisms are {\bf iteroparous}: they reproduce many times
----------------------------------------------------------------------
EXTRA PSLIDE Semele
HIGHFIG webpix/semele.jpg
----------------------------------------------------------------------
PSLIDE Cole's paradox
FIG webpix/tomato.jpg
----------------------------------------------------------------------
Cole's paradox
BC
Why are many organisms iteroparous?
If $\lambda = f + p$, surely it is easier to increase $f$ by
spending on reproduction, than to increase $p$, which can never be
larger than 1.
Raising $p$ from 0 to 1 becoming \emph{immortal} instead of annual,
is only as good as increasing $f$ by 1
NC
SIDEFIG webpix/tomato.jpg
EC
----------------------------------------------------------------------
Responses to Cole
What are some reasons why it makes evolutionary sense for organisms
to be iteroparous, in light of Cole's arguments?
ANS $f$ is not seeds per plant, it's
plants per plant; Remember to close the loop
ANS Population regulation: the long-term average value of
$\lambda$ is 1, so increasing $f$ by 1 is actually a \emph{lot}
ANS Risky environments: long-lived organisms can deal better with
variation in offspring success.
COMMENT Parenthood (taking care of offspring) not where we're going,
but is a relevant point
----------------------------------------------------------------------
PSLIDE Responses to Cole
SIDEFIG webpix/tomato.jpg
----------------------------------------------------------------------
PSLIDE Responses to Cole
SIDEFIG webpix/jungle.jpg
----------------------------------------------------------------------
Tradeoff example: many offspring vs.~high-quality offspring
Apart from how much energy to put into offspring now vs.~later,
organisms can make many or few offspring, using a given amount of
energy
What is a vivid example of ecologically similar organisms that
produce wildly different numbers of offspring?
ANS Oaks vs.~pines
ANS Tsetses vs. mosquitoes
POLL What are potential advantages of producing fewer offspring with the same amount of energy?
ANS Greater chance of survival (or reproductive success)
ANS Dispersal
ANS More energy left over?
ANS No (see question)
----------------------------------------------------------------------
PSLIDE How many offspring?
BCC
SIDEFIG webpix/acorns.jpg
NCC
SIDEFIG webpix/sequoia_cones.jpg
EC
----------------------------------------------------------------------
PSLIDE How many offspring?
FIG webpix/mosquito_laying.jpg
----------------------------------------------------------------------
PSLIDE How many offspring?
FIG webpix/tsetse_laying.jpg
----------------------------------------------------------------------
Tradeoff: direct investment vs.~dispersal investment
Investment in reproduction may not go directly to the
offspring, but instead to mechanisms to help the offspring disperse.
Why is this particularly important in plants?
ANS Parent-assisted dispersal is often their only chance to move.
What are some example mechanisms?
ANS Edible fruits
ANS Helicopter attachments
ANS Exploding seed pods
----------------------------------------------------------------------
PSLIDE Dispersal investment
FIG webpix/mulberries.jpg
----------------------------------------------------------------------
PSLIDE Dispersal investment
FIG webpix/maple_fruit.jpg
----------------------------------------------------------------------
PSLIDE Dispersal investment
FIG webpix/sandbox_fruit.jpg
----------------------------------------------------------------------
TSEC The $r$ vs.~$K$ metaphor
Regulated growth provides a powerful metaphor for life-history
tradeoffs involving growth vs. competitive ability
Recall $r$ and $K$ from our regulated population models.
ANS $r$ is the per-capita rate of growth, units ...
ANS [1/t]
ANS $K$ is the stable, equilibrium level that we expect a
population to reach, units ...
ANS [pop] or [pop density]
----------------------------------------------------------------------
DEFHEAD $r$ vs.\ $K$ strategies
We call organisms that tend to out-perform other species at low
densities ``$r$-strategists''
They do well in recently disturbed, uncrowded environments
We call organisms that tend to out-perform other species at high
densities ``$K$-strategists''
They do well in stable, crowded environments
----------------------------------------------------------------------
DEFHEAD $r$-strategists
All organisms tend to do well in uncrowded environments, but
$r$-strategists are selected to do better than other species
They are selected for a high rate of exponential growth during the
relatively short time that the environment is uncrowded
Why do we call them $r$-strategists, and not $\R$-strategists?
ANS Because they are selected to maximize $\rmax$, the
\emph{rate} of exponential growth
ANS A species with a high value of $\Rmax$, but a slow life
cycle, may not have enough time to capitalize on the opportunity
----------------------------------------------------------------------
DEFHEAD $K$-strategists
$K$-strategists are selected to do well in crowded environments
$K$ measures the maximum density at which a species can ``make a
living'' -- by keeping $\R=1$
Comparing $K$ between species can be tricky
----------------------------------------------------------------------
PSLIDE Maples and marigolds
BC
SIDEFIG webpix/maple.jpg
NC
SIDEFIG webpix/marigold.jpg
EC
----------------------------------------------------------------------
Measuring $K$
Which is the $K$ strategist: maple trees or marigolds?
ANS Maple trees do better at competing under stable conditions
ANS Marigolds are faster at invading new environments
Which has a higher value of $\rmax$?
ANS Marigolds
Which has a higher value of $K$?
POLL In [indiv/ha]? Which has a higher carrying capacity [indiv/ha]? maples; marigolds
POLL In [kg/ha]? Which has a higher carrying capacity [kg/ha]? maples; marigolds
To compare species, we attempt to measure $K$ in units that reflect
the effect of crowding on the competitive environment
biomass; area covered; resource consumed
----------------------------------------------------------------------
PSLIDE Strategies
BCC
SIDEFIG webpix/giraffes.png
NCC
SIDEFIG webpix/cutters.jpg
EC
----------------------------------------------------------------------
PSLIDE Example: trees
FIG webpix/larches.jpg
----------------------------------------------------------------------
PSLIDE Open environment
FIG webpix/old_field.jpg
----------------------------------------------------------------------
PSLIDE Stable environment
FIG webpix/old_growth_opal.jpg
----------------------------------------------------------------------
Example: trees
Assuming there is a tradeoff between $\rmax$ and $K$, would you
expect individuals with high $\rmax$, or high $K$, to do well:
In an empty, suitable habitat after a fire, flood, clearcut or other
major {\bf disturbance}?
ANS High $\rmax$ leads to faster exponential growth
In a crowded, stable old-growth forest?
ANS High $K$ means you can continue doing well when the forest
is already too crowded for others
----------------------------------------------------------------------
DEFHEAD $r$ vs.~$K$ strategists
All species are selected for characteristics relating to both $\rmax$
and $K$
But it is often useful to compare species based on which they
emphasize more heavily
There will often be tradeoffs between $\rmax$ and $K$
Species that specialize in colonizing disturbed environments are
thought of as $r$ strategists
Apple trees are often the first to reproduce in abandoned
fields
Species that specialize in stable environments are thought of as
$K$ strategists
Hemlock trees do best in stable, closed forests
----------------------------------------------------------------------
Life-history characteristics
Compared to $K$ strategists, $r$ strategists should:
Have relatively fast life cycles
Reach maturity earlier
Allocate more resources to reproduction (and thus reproduce
more and survive less)
Produce more offspring, with less resources for each
This allows high growth rates in the absence of competition
In crowded conditions, these ``quick" offspring may
be out-competed by offspring with more resources
Be more aggressive about dispersal.
ANS They need to find the next empty, suitable habitat before
this one gets too crowded
----------------------------------------------------------------------
Biology is complicated
The $r$-$K$ dichotomy is useful for thinking about strategies, but
organisms don't always fit it perfectly
Some species live long, but don't invest a lot in each offspring
Sea turtles, pine trees
Some species mature slowly but reproduce only once
17-year cicadas, century plants
Every species life history has specific, important {\em details}
But general principles are very important to guide our
understanding
----------------------------------------------------------------------
PSLIDE Biology is complicated
BC
WIDEFIG webpix/sea_turtle.jpg
NC
WIDEFIG webpix/century_plant.jpg
EC
----------------------------------------------------------------------
Changing conditions
Recall, $\lambda$ is usually between 1 and $\R$, gets closer to 1
when the life cycle is
ANS slower
When conditions are good ($\R>1$), should organisms be fast
or slow to maximize $\lambda$?
ANS Fast
POLL When conditions are bad ($\R<1$),
should organisms be fast or slow to maximize $\lambda$? Should organisms be fast or slow to maximize λ under bad conditions? fast; slow; it depends
ANS Slow!
ANS Decrease more slowly during the bad times
----------------------------------------------------------------------
Changing life history
Many organisms have evolved to change their life history patterns in
response to good or bad conditions
ANS Move slow when things are bad, and fast when things are good
POLL What are some examples? Can you think of an example of changing life-history patterns?
ANS Many animals reach sexual maturity faster under good
conditions: horses, elephants
ANS Trees may survive longer under bad conditions (by growing
slowly and not allocating energy to reproduction)
ANS Bacteria enter ``stationary state'' when conditions are
bad -- don't reproduce or grow at all, but may survive for a long
time
----------------------------------------------------------------------
Applications
How would $r$ and $K$ strategists differ in their response to human
activities/disturbance?
ANS $r$ strategists will generally deal with disturbance better
What are advantages of $r$ or $K$ strategists for human production (eg.
biofuels, agriculture, drug production etc..)?
POLL What are some advantages of $r$ strategists?
What are some advantages of r strategists?
ANS grow faster
ANS likely to respond well to disturbance
POLL What are some advantages of $K$ strategists?
What are some advantages of K strategists?
ANS may be more sustainable to grow for a long
time in a stable environment
----------------------------------------------------------------------
TSEC Bet hedging
In a risky world, you never want to “put all of your eggs in the same
basket”
If all your offspring are born into similar conditions, they can all do
well together -- or they can all die together
Strategies that \emph{usually} do well aren't good enough
The species we see now have survived for billions of years (if we
include ancestral species, who also had to survive)
Floods, fires, ice ages, disease outbreaks
All ``successful" organisms have strategies for spreading risk
ANS In fact, \emph{every} organism must have successfully spread
risk to survive to where you saw it
----------------------------------------------------------------------
Averaging
Mathematically, we can think about bet-hedging strategies in terms
of averages
Arithmetic means are means with respect to addition:
$x + y + z = m + m + m$
Geometric means are means with respect to multiplication:
$x * y * z = m * m * m$
----------------------------------------------------------------------
Averaging
A population has a different growth rate ($\lambda$) each year. The
long term growth rate would be the same if it grew by what constant
amount each year?
ANS The geometric mean growth rate
A farmer harvests dandelion seeds from 5 different fields. Each
field produces a different number of seeds. The harvest would be the
same if each field produced what constant amount?
ANS The arithmetic mean seed production
----------------------------------------------------------------------
Example: plant Q
Plant Q is an annual plant.
Each successful adult produces 30 offspring on average
In a good year, 20% of these offspring survive to reproduce; in a
normal year 2% of the offspring survive to reproduce; in a bad year
0.2% of the offspring survive to reproduce
The three kinds of year are equally likely
POLL What is the long term average growth rate of plant Q?
ANS The geometric mean of 6, 0.6 and 0.06: $\lambda=0.6$
ANS Effective survival is the geometric mean of 0.2, 0.02 and 0.002
----------------------------------------------------------------------
Plant D
Plant D is similar to plant Q, except that it produces seeds that
disperse over great distances
Because it has to invest in dispersal mechanisms, it only produces
half as many seeds.
The seeds of the new variety do just as well as those of plant Q,
but they disperse so far (in this hypothetical example) that 1/3 of
them experience good, normal and bad conditions every year.
POLL What is the average growth rate of plant D?
ANS Average survival is the arithmetic mean of 0.2, 0.02, and 0.002
ANS Growth is the arithmetic mean of 3, 0.3 and 0.03: $\lambda=1.11$
----------------------------------------------------------------------
Averaging
Variation between organism generations is multiplicative; we
understand its effect using the geometric mean
ANS Because we multiply per-capita success in each generation to
find out what happens to the population
Variation within a generation is additive; we understand its effect
using the arithmetic mean
ANS Because lifetime reproductive success is calculated by adding
components from different places or time periods
The arithmetic mean is greater than the geometric mean. When
variation is high, it can be \emph{much} greater
Therefore, organisms benefit from averaging within generations,
rather than between generations
----------------------------------------------------------------------
PSLIDE Comparing averages
FIG life_history/meancomp.Rout-0.pdf
----------------------------------------------------------------------
PSLIDE Comparing averages
FIG life_history/meancomp.Rout-1.pdf
----------------------------------------------------------------------
Comparing averages
DOUBLEPDF life_history/meancomp.Rout
----------------------------------------------------------------------
Dispersal, spreading risk over space
As an organism, do I want my offspring to grow up where I grew up,
or to disperse?
Advantages of staying home
ANS Dispersal is costly
ANS Home is apparently a good place to survive
ANS the parent survived and is reproducing
ANS Support from kin group
Advantages of dispersal
ANS Reduce competition between offspring
ANS Distribute risk -- if you don't disperse, \emph{all}
of your offspring could die if there is a disturbance
ANS Reduce inbreeding
ANS May find a better place
----------------------------------------------------------------------
Spreading risk over time
Organisms that disperse spread their risk across space
But some disturbances (bad weather, disease outbreaks) may cover
very large areas
Many organisms also have mechanisms for spreading risk over time
Iteroparity
Delayed development: many semelparous organisms have mechanisms
that allow a fraction of their offspring to remain {\bf dormant}
(ie., wait) before developing
----------------------------------------------------------------------
Why is it called bet hedging?
Bet hedging means reducing your risk, or not betting everything you
have on any one choice, even if it's a good choice.
ANS If you don't disperse in space, or spread out risk in time,
you are ``betting" all of your offspring on a single environment
ANS If you bet on many different environments, you are reducing your
risk, or ``hedging''
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TSEC Sex ratios
POLL To maximize fitness, should organisms allocate more resources to producing males or females? | Organisms can maximize fitness by allocating:
equally; more to males; more to females
ANS They should allocate more resources to females because it is
females that limit the growth rate
ANS They should allocate the same amount of resources to males
and females because males and females contribute the same amount
of fitness to the next generation
ANS They should allocate more resources to males, because males
have greater potential reproductive success
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The balance argument
In a sexual population, half of all the alleles in each generation
come from males, and half from females
Therefore, the total fitness of males and the total fitness of
females in the population is equal
Therefore, individuals should allocate resources equally to
offspring of each type
ANS If the population on average is allocating more to one type,
individuals who allocate more to the other type would do better
than average
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Example: elephant seals
BC
Male elephant seals can control large territories and mate with very
large numbers of females
Females produce at most 12 offspring over the course of their lives
And do all of the work of raising them
To maximize their fitness, should female elephant seals produce more
male offspring, or more female offspring?
NC
SIDEFIG webpix/elephant_seal_couple.jpg
EC
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Elephant seal details
Imagine a population where 90% of elephant seals born are males.
A certain ``generation" of 400 elephant seals produces 600 successful
offspring (counting in a reasonable, closed-loop way).
What is the average fitness of the males and the females in this
generation?
ANS Half of the genes, and half of the fitness comes from 360
males; half from 40 females
ANS Males' average fitness is 300/360=0.83; females' is 300/40 =
7.5
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Sex ratio and balance