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life_history.txt
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life_history.txt
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UNIT 4: Life history
----------------------------------------------------------------------
TSEC Introduction
{\bf Life history} refers to patterns of how organisms allocate
resources to key components underlying reproductive success:
POLL free_text_polls/kjRkTLMDSwSqi6g Give a one-word example of such a component.
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
----------------------------------------------------------------------
RSLIDE Oaks and dandelions
BC
SIDEFIG images/oak.jpg
NC
SIDEFIG images/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 free_text_polls/RxLfPtycVi4bTyp 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
----------------------------------------------------------------------
RSLIDE Strategies
BCC
SIDEFIG images/giraffes.png
NCC
SIDEFIG images/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
----------------------------------------------------------------------
RSLIDE Within species
FIG images/mallard_nest.jpg
----------------------------------------------------------------------
RSLIDE Between populations
FIG images/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
----------------------------------------------------------------------
RSLIDE Tradeoffs
FIG images/bamboo.jpg
CHANGE CC: add tradeoff with photosynthesis
----------------------------------------------------------------------
RSLIDE Tradeoffs
FIG images/peacock.jpg
----------------------------------------------------------------------
Optimization frontiers
We expect tradeoffs because:
organisms have limited \textbf{resources}
organisms are under natural selection in a complex world
----------------------------------------------------------------------
RSLIDE Optimization frontiers
FIG life_history/frontier.Rout-0.pdf
----------------------------------------------------------------------
Optimization frontiers
FIG life_history/frontier.Rout-1.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-1.pdf
EC
----------------------------------------------------------------------
Evolution and optimization
We often think of organisms as making ``choices" that maximize their
evolutionary fitness.
What is really happening here?
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
----------------------------------------------------------------------
RSLIDE Programmed optimization
FIG images/moth_light.jpg
----------------------------------------------------------------------
RSLIDE Programmed optimization
HIGHFIG images/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
----------------------------------------------------------------------
RSLIDE Semele
HIGHFIG images/semele.jpg
----------------------------------------------------------------------
RSLIDE Cole's paradox
FIG images/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 images/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 ``Closing the loop": $f$ is not seeds per plant, it's
plants per plant; not as high as you think
ANS Population regulation: the long-term average value of
$\lambda$ is 1, so increasing $f$ by 1 is a \emph{lot}
ANS Risky environments: long-lived organisms can deal better with
variation in offspring success.
----------------------------------------------------------------------
RSLIDE Responses to Cole
SIDEFIG images/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 free_text_polls/5vyO0qrVgRGFWHm What are potential advantages of producing fewer offspring with the same amount of energy?
ANS Greater chance of survival (or reproductive success)
ANS Dispersal
CHANGE CC: add higher chance to reproduce again
----------------------------------------------------------------------
RSLIDE How many offspring?
BCC
SIDEFIG images/acorns.jpg
NCC
SIDEFIG images/sequoia_cones.jpg
EC
----------------------------------------------------------------------
RSLIDE How many offspring?
FIG images/mosquito_laying.jpg
----------------------------------------------------------------------
RSLIDE How many offspring?
FIG images/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
----------------------------------------------------------------------
RSLIDE Dispersal investment
FIG images/mulberries.jpg
----------------------------------------------------------------------
RSLIDE Dispersal investment
FIG images/maple_fruit.jpg
----------------------------------------------------------------------
RSLIDE Dispersal investment
FIG images/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
----------------------------------------------------------------------
RSLIDE Maples and marigolds
BC
SIDEFIG images/maple.jpg
NC
SIDEFIG images/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 multiple_choice_polls/oJIl9jyOs45P3ti In [indiv/ha]? Which has a higher carrying capacity [indiv/ha]? maples, marigolds
POLL multiple_choice_polls/jg5HEJysPyiJUWS 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
----------------------------------------------------------------------
RSLIDE Strategies
BCC
SIDEFIG images/giraffes.png
NCC
SIDEFIG images/cutters.jpg
EC
----------------------------------------------------------------------
RSLIDE Example: trees
FIG images/larches.jpg
----------------------------------------------------------------------
RSLIDE Open environment
FIG images/old_field.jpg
----------------------------------------------------------------------
RSLIDE Stable environment
FIG images/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 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
CHANGE CC: add agriculture/human activities as disturbance
----------------------------------------------------------------------
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 locusts, century plants
Every species life history has specific, important {\em details}
But general principles are very important to guide our
understanding
CHANGE CC: change locust into cicadas
----------------------------------------------------------------------
RSLIDE Biology is complicated
BC
WIDEFIG images/sea_turtle.jpg
NC
WIDEFIG images/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 free_text_polls/zw4SJE9HcxG3rZP When conditions are bad ($\R<1$),
should organisms be fast or slow to maximize $\lambda$? When conditions are
bad (R<1), should organisms be fast or slow, to maximize λ?
ANS Slow!
CHANGE CC: change poll into multiple choices
----------------------------------------------------------------------
Changing life history
Some 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 free_text_polls/uXfLWxzcQeTNO9E 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
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
CHANGE CC: you were thinking of adding a reminder in slide about putting away phones after polls? and maybe give a species name example for animals that reach maturity faster under good conditions
----------------------------------------------------------------------
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 free_text_polls/RrvAaSlcPCYevro What are some advantages of $r$ strategists?
What are some advantages of r strategists?
ANS $r$ strategists are likely to respond better to human attempts
to manipulate the environment to their advantage
POLL free_text_polls/Ac5nrU1aNcHWopy What are some advantages of $K$ strategists?
What are some advantages of K strategists?
ANS $K$ strategists may be more sustainable to grow for a long
time in a stable environment
CHANGE CC: rephrase the 1st question + advantages r: add grow faster
CHANGE CC: stop here on Feb 28
----------------------------------------------------------------------
TSEC Bet hedging
In a risky world, you never want to put all 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 By successful organism, I mean surviving organism
----------------------------------------------------------------------
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:
$x * y * z = m * m * m$
CHANGE CC: not sure the example of stealing money was very clear
----------------------------------------------------------------------
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 free_text_polls/mUguRKsPmb9MaZh What is the long term average growth rate of plant Q?
ANS The geometric mean of 6, 0.6 and 0.06: $\lambda=0.6$
CHANGE CC: not a change but a note: students did not know how to calculate the GM
----------------------------------------------------------------------
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 free_text_polls/KkFgfiBDNr8CAtX What is the average growth rate of plant D?
ANS Average survival is the mean of 0.2, 0.02, and 0.002
ANS Growth is the arithmetic mean of 3, 0.3 and 0.03: $\lambda=1.11$
CHANGE CC: apparently you went a little bit too fast to explain how you did the calculations
----------------------------------------------------------------------
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 much greater
Therefore, organisms benefit from averaging within generations,
rather than between generations
----------------------------------------------------------------------
RSLIDE Comparing averages
FIG life_history/meancomp.Rout-0.pdf
----------------------------------------------------------------------
RSLIDE 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
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
CHANGE CC: staying home: ANS of a student: group protection
CHANGE CC: dispersal: ANS students: decrease inbreeding, find a better place
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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
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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
CHANGE CC: not sure why this slide
CHANGE CC: stop here on March 2nd
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\clearpage
TSEC Sex ratios
POLL free_text_polls/XVdaqUEFBnNJXxX Should organisms allocate more resources to producing males or 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
CHANGE CC: Comparative aspects better explain for the poll
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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
CHANGE CC: Add more explanation for the ANS of balance arg
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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 images/elephant_seal_couple.jpg
EC
CHANGE CC: Add poll for the question
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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
CHANGE CC: Better explanation after a question (future should explain how you get each number from the beginning)
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Sex ratio and balance
Imagine a population where organisms use the same amount of
resources to produce male or female offspring
Thus, the \emph{number} of offspring I can make does not depend on
sex
POLL free_text_polls/w1XdvYUv8V4ycCh If everyone else is making more males than females, what should I do?
ANS Make more females, because that will increase my average
fitness
How will this population evolve in the long term?
ANS More and more females