Life Histories
Reading; S & S Chapter 13
Consider; The Super-Organism
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Indestructible
Immortal
Can subsist on anything
Produces millions of young
Produces young all the time
Young are indestructible
Is an excellent parent
First reproduction is 15 sec after birth
• This super-organism is impossible because of the
metabolic, developmental, and ecological tradeoffs
inherent to living things.
– Growing quickly means that there is no energy for
early reproduction
• Extended parental care means having fewer
offspring.
• Producing a clutch with many offspring means that
each one must be smaller because there is only
so much metabolic energy set aside for
reproduction.
• Setting aside more energy means not reproducing
for a year or more.
– Ultimately, reproducing means not living as long,
because the act of reproducing drains energy, and
subjects the organism to risks.
An Economic Analogy
• Imagine you had 1000 dollars to spend on
reproduction.
– Large offspring cost more than small offspring, but
large offspring are more likely to survive.
– Parental care costs, but it means your offspring are
more likely to survive.
– If you don’t spend it now, you can put it away and
earn interest on the money, but you might be eaten or
killed before you can withdraw it…
• How would you spend the 1000?
• How do you get the most return from your investment?
Reproductive Value
• Organisms allocate their resources in such a way as
to maximize their lifetime reproductive success.
– Lifetime reproductive success is the total number of
offspring an organism produces over the course of their
lifetime, (accounting for the proportion of an individual’s
genes its offspring shares)
• Of course, that is only measurable after an
organism’s life is OVER. At any given time during
the life of an organism the expected amount of
reproductive success in an organism’s future is
called its reproductive value.
• At any given time during an organism’s life, the
amount of reproduction they have in front of
them can be expressed as follows;
• Vx= {i=x to infinity}S(ly/lx)my
• Where Vx= the reproductive value of the organism
• my =the expected reproduction at time interval y
• ly/lx=the probability of the organism living to time
interval y
– Thus, to maximize its own fitness, an organism will be
selected to have those life-history attributes that maximize
its reproductive value. This may involve a comprimise
between reproducing now and reproducing in the future:
• Vx=mx+ {I=x+1 to infinity} S(ly/lx)my
– Where mx is contemporary reproductive output, and the
second term is future reproduction.
– Depending upon the ecology of the organism: energy
expended reproducing now, mx, might affect the ability to
reproduce in the future (my), or survive to the future (ly).
– Optimal fitness involves maximizing Vx.
– Which attributes maximize Vx depend upon the ecology of
the organism.
This is an optimality
model
it assumes that
organisms
will evolve to
ultimately
maximize their fitness,
given the constraints
under which they
operate.
In theory, it is a perfect
model.
In practice?
It is difficult to test
But qualitatively, it
seems to hold up
pretty well.
Some Tradeoffs in Life-History Evolution
• Life histories evolve. Traits that affect the life history of
an organism have an enormous potential to affect fitness.
• The intensity and type of selective pressure on a life
history character depends upon the particular ecological
circumstances of the organism
• MacArthur and Wilson coined the terms “r selection” and
“K selection” to describe two general, and opposite trends
in life history evolution.
– R strategists are selected for rapid population growth
– K strategists are selected for rapid competitive ability
• Though thought-provoking and useful, this description
deals is stereotpyes. Most organisms have a mixture of R
and K selected traits.
– (Also, check out the R, C, S system in your textbook)
• Some Life-History Tradeoffs
• Some of these have been studied extensively, some are
thought to represent evolutionary tradeoffs based on our
current ideas of how their evolution works.
• Early reproduction vs. amassing resources to reproduce
later
• A few, large eggs vs. many smaller ones
• Parental care vs. reproducing more often or having more
offspring
• Sexual or asexual reproduction
• Male or female offspring
• Use all your energy in one bout of reproduction
(semalparous) or hold back, and live to reproduce later
(iteroparous).
Clutch Size/Parental Care
• One crucial life-history tradeoff concerns the
balance between clutch size (animals) or seed
set (plants) vs. the size of each offspring.
– Assume that the organism has a set amount of
resources that they are able to devote to a bout of
reproduction.
– Producing more offspring means producing
smaller offspring.
• For a wide variety of organisms, smaller offspring have
reduced survivorship relative to larger ones.
• It also limits options regarding parental care.
• Lack of parental care also restricts survivorship.
Is this true?
• It depends upon how you frame the question.
» The Australian plague locust, Chorotoicities
terminifera provides no parental care. It produces
about five clutches of fifty eggs each.
» The European earwig, Forficulata auricularia
provivides extensive parental care, it produces about
seven clutches of fifty eggs each.
» Just comparing species is not much of a test,
however, the two species have different
ecologies.
• It is better to ask whether, in the context of a given
species, a larger clutch size means reduced survivorship.
The Lack Optimum
• For a wide variety of birds, and a parasitoid wasp, it is
fairly well established that having more offspring
translates into reduced survivorship for the offspring, OR,
reduced opportunities to reproduce later.
• The idea of optimum clutch size was suggested by David Lack, who
suggested that birds that lay the optimum have the highest fitness.
– Bird clutch sizes vary a great deal among species, and for some
species (but not others), they vary among species.
– Birds do not survive without parental care, though the amount of
parental care varies among species. For a given bout of
reproduction, it is reasonable to expect that more eggs means 1)
there is a little less for everyone, or 2) the parents have to bust a
nut caring for the extra offspring.
• Thus, a bird faces a “decision” every time they build a nest.
Lay too few, and their fitness is lower than it could have
been. Lay too many, and their fitness is lower as well.
• The Lack optimum has been tested many times.
– In some studies, the larger broods produce more
offspring (ie. Lessels 1991).
– In others, larger broods suffer increased mortality and
yielded fewer offspring.
– In one particularly interesting study of collared
flycatchers (Gustaffson and Sunderland), larger clutches
yielded more young, but those offspring had reduced
survivorship, and the parents that took care of those
young had reduced future reproduction (they busted a
nut).
• Parasitoid wasps face a very
similar “decision” when they
decide to lay eggs.
• Ian Hardy studied the bethylid
parasitoid Goniozus
nephantidus.
• This species attacks leaf-rolling
caterpillars. It finds them in
their rolled leaves and paralyzes
them with its venom.
• Once it has tracked one down,
and has it at its mercy, it must
decide how many eggs to lay.
http://wwwhortnet.co.nz
• Hardy manipulated the number of eggs on the hosts.
– Too few eggs, and the immune systems of the hosts actually
destroy the parasitoids
– Too many eggs, and the female offspring (because of their
ecology, those are the ones that really affect the fitness of
their mom), established that laying too many eggs led to
females that were very small and had very low fitness.
• Optimum clutch size depends upon host size, however,
larger hosts can support larger clutches.
• If you encounter an already-parasitized host, it is
sometimes beneficial to “screw” the original egg layer by
laying a few of your own. This impairs their fitness, but
helps your own (better than laying no eggs).
• Thus, the Lack Optimum is fairly well established as an
ecological and evolutionary phenomenon, but, like everything in
the field, it isn’t as simple as the model originally proposed.
Clutch size vs. Latitude
• For birds and lizards, there is an interesting ecological
relationship between clutch size and latitude.
– Northern species lay more eggs per clutch than equatorial
species.
– This pattern can be extended to insects, such as milkweed
bugs (total number of eggs, not clutch size), pitcher plant
mosquitoes, and the first clutch of eggs laid by queen ants.
• This pattern probably reflects the underlying effects of
resource availability on life-history evolution.
– High latitudes; winters kill many individuals, in spring, there
is less competition and more food to go around
– Low latitudes; competitive environments, where high parental
investment in a few offspring is favored.
Sex
• Another crucial life history decision concerns
whether or not to reproduce sexually, and if so,
whether or not to have sex with one’s self.
• Check your your textbook, pp. 231
– This ties in with ecology a great deal as well, because
the evolutionary utility of sexual reproduction is largely
to produce variable offspring.
– If the organism is very well-adapted for the
environment, this can be a disadvantage, because sex
breaks up potentially useful combinations of alleles.
– In changeable or uncertain environments, sexual
reproduction can be the key to the continued survival of
a genetic lineage.
• A nearly ubiquitous in species that can either
reproduce asexually, or sexually, is to reproduce
asexually as long as conditions are good, and then,
when conditions become unfavorable, to switch to
sexual reproduction.
• This strategy is seen in aphids, many protozoa and
algae, and many fungi.
• For example, coprophagic fungi such as Psilocibin
cubansis, grow asexually as a mycellium within
patches of manure. Once the food supply is exhausted,
the mycellium will produce fruiting bodies that shed
sexually-derived spores into the environment. A
cowpie does not last long, and no two are precisely
alike, so this strategy optimizes the long-term survival
of an individual’s alleles.
• Although the vast majority of animals reproduce
sexually every generation, there are animals that
exhibit this life history pattern as well.
• Asexual reproduction is particularly common among
so called “r selected” animals.
– Presumably, the asexual option conveys a fitness advantage
when colonizing an empty patch of environment.
• The freshwater crustacean, Bythotrephes, is such a
species.
– It is an invasive exotic. One reason it has become so
common is its high reproductive rate.
http://www.seagrant.umn.edu/exotics
female
male
The female broods from one to ten eggs in a pouch on its back
(note the limited parental care). This takes about 10 days. Sex is
determined environmentally, so under normal situations, the eggs
develop directly into asexual females, which reproduce
parthenogenically.
When water gets cold, and conditions are bound to change because of
the onset of winter, sexual males and females hatch from eggs.
Asexual loop
Sexual loop
Reproduce Now? Or Do It Later?
• This is probably the
most ubiquitous lifehistory tradeoff
organisms face.
• DO IT NOW!
– Reproducing now means that an organism does
not incur the chance of dying before they get
another opportunity to reproduce.
– In growing offspring, there is a “compound
interest effect”, an organism’s offspring will
produce their own offspring sooner if
reproduction is early. Thus, rapidly growing
populations favor early reproduction.
• Mom told me to wait…..DO IT LATER
– Reproducing later means that the organism may
be able to amass more resources, so that the bout
of reproduction, when it finally occurs, is more
successful.
• For a wide variety of organisms, delayed
reproduction means increased body size, and
increased body size means greater reproduction.
• Example;
– Gizzard Shad reproducing at two years of age
produce 59,000 eggs. Those delaying
reproduction until 3 years of age produce
379,000 eggs.
– Interestingly, the population is polymorphic.
About 15% of the population reproduce at 2
years of age, and about 80% reproduce at 3
years of age.
Question;
If humans began fishing a population of
shad, so that the chance of survival
between 2 and three years were
dramatically reduced, how would you
expect the population to evolve?
• David Reznick studied life history evolution in
guppies.
– Guppies grow fastest before they begin reproduction.
Larger guppies produce bigger broods of young.
– He hypothesized that if there was a high risk of predation
early on, but that risk disappeared later in life, then larger
individuals and delayed reproduction should be favored,
because by delaying reproduction, a nice reproductive
payoff was in store for the predation-free older guppies if
they delayed reproduction and amassed resources.
– Conversely, if the risk of predation was high across-theboard, then there was no advantage to delayed
reproduction, so early reproduction should be favored.
• Reznick went to Trinidad and located a variety
of river systems.
– Guppies can’t always go upstream from waterfalls,
so there were areas where guppies lived
downstream, but not upstream. The larger predators
don’t get over waterfalls at all.
– Reznick divided the habitats into two varieties;
“high predation” and “low predation”.
– “Low Predation” areas have only a killifish that
preys on juvenile individuals but can’t eat the big
ones.
– “High Predation” areas have the killifish, and also
have three big predators that eat large guppies.
• Experiment;
– He introduced guppies from high predation
environments downstream, into low predation
environments upstream.
– The downstream environments served as a control.
– He predicted; later age at first reproduction, lower
reproductive allocation
• Experiment;
– He introduced large predators to a low-predation
environment upstream.
– He predicted that the guppies upstream would evolve
earlier reproduction, increased reproductive
allocation.
• The results of his experiment matched his predictions
perfectly.
– Over the course of 7 years, the guppies introduced from
high predation environments to low predation
environments evolved to reproduce later and be larger as
adults, with less reproductive allocation.
– The areas where predators were introduced saw a
decrease in the size of adult guppies, and an evolutionary
increase in reproductive allocation.
– The introductions were done in 1981, the evolution
continues.
• Conclusion. The life history model is supported. Also
interesting is the demonstration that evolution can happen
so quickly.
Check it out
http://www.esof2004.org/pdf_ppt/session_material/reznick.pdf