In the last lecture, in presenting the concept of allocation, I introduced the idea that evolved strategies must involve trade-offs, among allocations to different functions and among approaches to reproduction for example.

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In the last lecture, in presenting the concept of
allocation, I introduced the idea that evolved
strategies must involve trade-offs, among
allocations to different functions and among
approaches to reproduction for example. Today we
will examine some of those trade-offs
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• There are a number of tradeoffs that life history
  ecology has studied
• Maturation time (? or age of first reproduction)
  versus survivorship. In the extreme youve
  already seen r versus K strategies. Its really
  a continuum

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2. A tradeoff between fecundity and survival
if a female produces more offspring, each
energetically costly, that decreases the
amount of energy she can put into storage
to help her get through hard times. Reduced
energy going towards maintenance and
survivorship means higher mortality and, on
average, a shorter lifespan. It also means
that, even if the organism does survive, it will
have less energy to devote to future bouts
of reproduction (and likely fewer
offspring. Check out the model of the balance
between gain in reproductive output and reduction
in survivorship on pp. 210-211 of the text.
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3. There is a tradeoff between growth and
fecundity. This should follow logically from
the other tradeoffs energy spent on
reproduction clearly cannot be spent on
growth. This tradeoff is important to those
species that show indeterminate growth.
Rather than reaching an individual organism
carrying capacity (an equilibrium adult
size), these organisms continue to grow (though
usually at a slow rate) even as adults. Fish
are good examples. Why keep growing? Because
a female fish produces a number of eggs that is
proportional to her body size larger means more
eggs and more young.
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4. Tradeoffs between size and number of offspring
We know that evolution selects the
strategy that produces the highest lifetime
reproductive success, but how? Optimization.
One well worked-out example is clutch size in
birds. David Lack studied clutch size in
European starlings Clutch size in the starling
varies over a maximum range from 1 to 10 eggs.
How do we determine whether there is an optimal
clutch size and what that size is? Is a larger
number of eggs better?
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Success means producing young who themselves
survive to reproduce. So, we begin with how many
of the clutch of eggs successfully hatch. Then
we ask how many of the hatched eggs are
successfully fledged. Finally, we ask how many
survive the winter to reach reproductive
maturity.
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First, hatching success. It isnt perfect, but
very close, and increases linearly with the
number of eggs in the clutch. Just less than 1
egg hatches from a 1 egg clutch, and
almost exactly 4.5 hatch from a 5 egg clutch.
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Following hatching, the nestlings are fed by the
parents. Lack believed that feeding was the key,
either due to food availability or to a lack of
time to find and gather food for the young. The
number of young surviving to be fledged decreases
with clutch size.
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The number of young fledged (recruits) is the
product of the number hatched times the
proportion surviving the nestling period. Heres
an example from Lacks data Clutch
size hatched prop. fledged recruits 1
1 1.0 1 2 1.8
0.95 1.71 3 2.7 0.85 2.29 4
3.6 0.75 2.7 5 4.5
0.6 2.7 6 5.3 0.45 2.38 The
number of recruits peaks at either 4 or 5
eggs. Overwinter mortality turned out to be a
simple of fledglings.
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What this shows is that there is no point in
cheating. Laying extra eggs (beyond the 4 or 5
egg optimum) does not lead to leaving behind a
larger number of recruits. That would correspond
to increased fitness. There is additional
energetic cost to producing more eggs, but no
added benefit. Instead, optimization occurs at
some intermediate number of eggs that is most
successful at producing recruits. Now the
background for the first clicker question
Starlings typically produce a second clutch of
eggs each breeding season. However, it is late
enough that the amount of food and time to
harvest it to feed nestlings are lower.
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There are a number of reasons for these
results. When a female lays more eggs, each
offspring is likely to be smaller. Smaller
offspring are less likely to survive the
nestling period, and much less likely to survive
the winter. Here are nestling weights
at 15 days of age Clutch size nestling
weight 2 88 g 5 77.6 g 7 71.4
g A general principle An important tradeoff is
that between size and number of offspring.
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Assuming that there is an optimum size for this
second clutch, what number of eggs do you think
is optimal?

1. 1 or 2 eggs
2. 3 or 4 eggs
3. 5 or 6 eggs
4. 7 or 8 eggs

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In the example of the starlings, because parents
in many bird species are severely limited by the
amount of time they have to feed the young It
is possible to test for optimality in clutch size
experimentally. Perrins used captive swifts and
placed different numbers of eggs in their
nests. The clutch sizes he observed, at least in
the optimal number, match observations in the
field...
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The trade-offs between offspring size and number
are also evident in plants. Goldenrods (genus
Solidago) are among the most widely studied
plants. Different species grow in woods,
climax grassland, and old fields. Their
reproductive patterns clearly indicate both
patterns in allocation and in size-number trade-of
fs. First, allocation differences among species
growing in different habitats, and even
individuals of the same species growing in those
habitats... In a comparison of goldenrods by
Abrahamson Gadgil
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Wet site a moderately open Meadow Woods
site even at the edge, its shady Dry site
very open and sunny
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• Species from the woods site, shaded, have less
  energy to
• allocate to reproduction than those from a wetter
  site, and
• much less than those from a dry site.
• The dry site is open, plants are typically
  smaller, but get
• full sun.
• The wet site is more densely vegetated, but with
  plants of
• similar stature as the goldenrods.
• However, this does not indicate trade-offs
  between offspring
• size and number. For that data we look instead to
  a
• comparative study by Werner Platt...

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These goldenrods were all growing on Cayler
Prairie, Iowa. They are ordered from dry to wet
areas on the prairie. They dont follow the
pattern perfectly (S. speciosa messes it up),
but, in general, plants producing larger seeds
make less. Species Wt. Wt. x
S. nemoralis 104.
200 20,800 S. missourensis 39.3
1100 43,230 S. speciosa 146.3
500 73,150 S. canadensis 58.3
1100 64,130 S. graminifolia 10.6
7800 82,680
A more general pattern was also described among
goldenrods in a variety of differing habitats
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The slope of the line in the previous figure is
statistically indistinguishable from -1, i.e. the
product of seed size and seed number is a
constant. This suggests that the allocation of
energy to reproduction in all these closely
related species is the same. Even if it isnt the
same, we would expect similarity among closely
related species as a result of their descent from
a common ancestor.
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Optimal clutch size is a consequence of natural
selection, and reflects the environment in which
a population lives. Though clutch size is a
term usually referring to birds (or other species
that lay eggs), the principle applies to all
types of reproductive output. Clutch size will
vary among species as a result of
evolved optimization. Even for the same species,
optimal clutch size will vary from year to year
and place to place due to environmental
variation. Clutches that are either too large
or too small are not favoured because they result
in fewer recruits to the next generation produced
successfully.
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Thus far, we have considered evolution of optima
as if the selected phenotypes were fixed, that
there is a one-to-one mapping from genotype to
phenotype. Is that really true? There is another
form of adaptation (and it, too, results from
selection) Phenotypic plasticity The expression
of the genotype is the phenotype. Environmental
conditions affect that expression, and variation
in the expressed phenotype under differing
environmental conditions is called phenotypic
plasticity. The capacity to vary phenotype can be
(and is) selected.
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The relationship between phenotype and some
environmental factor that affects its expression
is called a reaction norm.
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If two populations within a species have long
lived under differing environmental conditions,
their reaction norms may differ. Fig. 10.6 shows
you that reaction norms for Michigan and Alaska
populations of Papilio (swallowtail butterflies)
differ. The Michigan population shows a
larger reaction to increasing temperature than
the Alaskan population. That indicates the
selective impact of exposure to different
environments over a long (evolutionary) time.
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When reaction norms cross (genotype A does better
than B in one environment, the reverse is true in
a second environment), this difference indicates
the presence and impact of a genotype-environment
interaction. The modular nature of plant
architecture makes the occurrence of both
phenotypic plasticity and genotype-environment
interactions common in plants through
alteration in the number and size of modules.
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You can investigate the differences in
populations and their response to environmental
conditions by means of reciprocal transplant
experiments
The observed initial observation is in the first
box the two populations have different observed
phenotypes in their home environments.
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If the phenotypes are invariant in the reciprocal
transplant experiment (the 2nd box), then the
differences observed are entirely genetic. If
both populations show the same (or parallel)
reaction norms, then the observed difference is
solely due to phenotypic plasticity. Both
populations are responding in the same way. If
the reaction norms have different slopes, then
species respond in different ways. The fact they
respond indicates the presence of phenotypic
plasticity the difference in response indicates
evolved genotypic difference.
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Reaction norms are evident in the developmental
response to differing environments. One of the
good examples is in maturation of species
measured by their age and size at reaching sexual
maturity
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And heres how a reaction norm is evident as a
response to food conditions (environmental
quality) in metamorphosis from tadpole to frog
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Which of the following correctly describes frog
metamorphosis evident in the reaction norm?

1. Frogs metamorphose at younger age in a high
   food environment
2. Frogs metamorphose at a smaller size in a high
   food environment
3. Frogs metamorphose when they are older in a high
   food environment
4. Frogs metamorphose at a larger size in a high
   food environment.
5. Both 1 and 4 are correct
6. Both 2 and 3 are correct