Transcript Evolution of life histories

Tradeoffs Among Life History Traits
There are tradeoffs among different life history traits and
among the costs associated with them.
Without tradeoffs the logical direction for species to evolve
would be more of everything: more offspring, larger offspring
size, better survivorship and longer life.
The hypothesis of reproductive cost explains the tradeoffs
between increased reproduction and other aspects of the life
history. By extension, it also applies to other possible
tradeoffs.
Tradeoffs may occur in the behaviour, physiology, and
strategies of individuals, but may also involve balances
between parents and offspring, called `inter-generational', or
occur at the population level.
The idea of tradeoffs as an important life history argument
may be only 20 – 30 years old, but it is not new. From R.A.
Fisher's classic book The Genetical Theory of Natural
Selection (1930):
"It would be instructive to know not only by what
physiological mechanisms a just apportionment is made
between the nutriment devoted to the gonads and that
devoted to the rest of the parental organism, but also
what circumstances in the life-history and environment
would render profitable the diversion of a greater or
lesser share of the available resources towards
reproduction."
Traits among which compromises may be struck would
include at least the following:
Individual traits:
a) current reproduction (measured as mx)
b) future reproduction (residual reproductive value or
Vx)
c) parental survival (measured by lx or px)
d) parental growth
e) parental condition
Inter-generation traits:
f) offspring growth
g) offspring condition
h) offspring survival
Population level traits:
i) number of offspring
j) size of offspring
All strategies are compromises between lx and mx. Stated
more explicitly – the foundation of life history theory is the
hypothesis of reproductive cost. Any increase in present
reproduction (at least in an iteroparous species that hopes to
reproduce again) is associated with a decrease in future (or
alternatively residual) reproductive value, i.e. the expected
contribution to future generations resulting from all future
bouts of reproduction.
That decrease could be due to either reduced fecundity in the
future, or to reduced survivorship.
This can be represented in an equation for reproductive value
having two components: current reproduction and the value
of future reproduction (Vx) in a population which is changing
in size.
Va  ma  (e
 ra
/ la 

e
x  a 1
 rx
l x mx
It is this negative correlation between reproductive effort in
the present and the value of future reproduction which makes
the optimization of timing and intensity of reproductive effort
possible.
If there weren't a negative correlation, then species would
maximize both current and future reproduction, as well as
beginning as early as possible and lasting as long through life
as possible.
The idea of reproductive cost is explained using the principle
of allocation:
An organism has a limited set of basic functions or
needs which must be met: these functions are usually classed
as maintenance, growth and reproduction.
1. Maintenance refers to baseline metabolism plus costs
borne to ensure survival, e.g. water regulation,
resistance to predation, competition or disease, etc. It
does not include any component of increase.
2. Growth refers to the energetic cost of increase.
3. Reproduction refers to costs of producing gametes, of
acquiring mates (including development of displays,
behaviours, etc.), of bearing the young, and of parental
care.
There are energetic costs associated with each function, and
clearly energy allocated to one function is not available for the
others. Thus there are reciprocal adjustments among
functions, and evolution should favor adjustments which
maximize fitness.
Variations in patterns of allocation among these functions may
be genetic or phenotypic, and may occur in many factors
important to fitness, including age of first reproduction,
temporal pattern of reproduction (the shape of the mx curve;
semelparity versus iteroparity, and the size of mx.
We will explore data that examines evidence of tradeoffs and
adaptations evident in many of these variables.
Effects of age of first reproduction
There is an approximately inverse relationship between rmax
and generation time (see the table below), which, if perfect,
would have indicated a constant R0 over a wide variety of
taxa. It's also clear that instantaneous rates of increase would
increase in organisms with shorter generation times where R0
is constant.
Even though the simple formula (ln R0/G) calculates only an
approximate r, it is apparent that a smaller generation time G
would increase the rate of increase.
Values for rmax, the generation time G, and their product as an
indication of potential growth rate among a variety of animal
species:
Taxon Species
Bacteria E. coli
rmax
60.
G
.014
rxG
.84
Protazoa Paramecium aurelia 1.24
P. caudatum
.94
.40
.30
.508
.282
Insecta
Tribolium confusum 12
Calandra oryzae
.11
Eurostis hilleri
.01
Magicicada septendecim .001
80.
58.
110.
6050.
9.60
6.38
1.1
6.05
Mammalia Rattus norvegicus .015
Microsus agrestis
.013
Canis domesticus
.009
Homo sapiens
.0003
150.
171.
1000.
7000.
2.25
2.22
9.0
2.1 .
There is a compromise between fitness gained by reproducing
earlier and fitness lost through a reduced number of broods
(survivorship declines) or reduced success of broods (through
reduced size or reduced offspring survivorship).
In plants another factor can be important. Age related changes
in fecundity are particularly important in plants, and therefore
important in evolving compromises. Most long-lived plants
have indefinite growth. Seed production in a wide variety of
herbs and trees increases steadily with age.
If present reproduction reduced growth, but fecundity
increases with increasing size and age resulting from growth,
then the need to optimize the compromise is clear.
For a delay in reproduction to be advantageous, the age-related
gain in fecundity must outweigh the mortality risks and
increased generation time.
There is no explicit mathematics which can pre-determine the
optimum . There is, however, a framework that at least states
what should be maximized by selection. It is simply the sum
of current and residual reproductive values (which is the total
reproductive value at age x). That can be depicted in a graph.
The x axis is age; the y and z axes are for current reproduction
and residual reproductive value. The sum is the distance out
from the age axis. When that reaches a maximum, evolution
should select for initiating reproduction if a species is
semelparous. Otherwise, for iteroparous species, the graph
indicates the optimum allocation between current effort and
energy retained for future reproduction.
Here are the graphs for a) a semelparous species and b) an
iteroparous one:
Heavy line indicates observed
strategy, here all energy reserved
As residual reproductive value until
Current benefit exceeds value of
retention for later reproduction.
Semelparous species
Here the heavy line indicates partial
commitment to current reproduction and
partial retention for future reproduction.
In explanation of the meaning of these curves…
If the curve for any age is concave, then the optimum strategy
is either to hold all energy for residual reproductive value (i.e.
don't reproduce now at all) or to throw all available energy
into reproduction, holding none back for later bouts. The
concave curve will either be furthest from the age axis at its
intersection with the current reproduction or the residual value
axis. That is a semelparous strategy.
If the curve is convex, then some intermediate allocation of
energy to current reproduction is optimum, accompanied by
the holding of a fraction of energy for future reproduction.
That is an iteroparous approach.
Accepting cost, the next question to evaluate is how big that
cost is, i.e.
The Numbers and Sizes of Offspring
Clutch size varies enormously among species. Assume that
evolution should have optimized clutch size. We need to
understand how it is regulated.
There are obvious possible conflicts between parent and
offspring about number and size. Who controls the
reproductive output? Do parents usually initiate more
offspring than they can finish producing, then selectively
abort some?
This happens in plants. Does it also happen in animals? How
is size balanced against number?
From the parental perspective the optimum is to produce as
many as can be provisioned well enough to have a good
chance to survive to reproduce. From the offspring
perspective a much smaller number might be preferred.
Should a parent make a few large offspring or make numerous
smaller ones, using the same energy, and at the same time in
the life history etc. This is part of parent-offspring conflict.
The parent, all else being equal, would increase fitness by
making larger numbers; the offspring would be better off if
they were larger, and fewer were produced.
Offspring or seed size is likely critical to individual fitness,
and evolutionists argue that it should be strongly optimized by
selection, i.e. there should be little additive genetic variation
left.
Plants disperse offspring, and seed size influences
dispersal distance. As a result, there's the potential for
interesting compromises between seed size and seed number,
a condition more evident as compromise in plant life history
than in the cases of animals.
What evidence do we have that there is little variation left in
seed size?
A plant stressed by competition (or abiotic conditions) seems
first to drastically reduce allocation (absolute amount of
biomass, at least) to reproduction and the number of seeds
produced before additionally reducing seed or fruit size. The
relative plasticity in components of reproduction is indicated
in wheat (data from Harper), when ratios of performance at
high and low densities are compared:
ears per plant
total seeds per plant
grains per ear
mean grain weight
Ratio:
56
833
1.43
1.04
How do plants achieve this?
Harper (1977) points out an observation of some importance,
since studied also by Stephenson (1984). Most plants initiate
many more flowers (and seeds) than they develop, aborting
the excess. Thus the number aborted can be adjusted at the
times of energetic drain (flower development, seed
maturation) with much less error than inherent in a time lag
process which would make these decisions at the outset of
reproduction (initiation) or alternatively building a fixed
number into the genotype.
In a study of goldenrods (Werner and Platt. 1976) found
variation in size, but almost absolute complimentarity between
size and number, i.e.
Number x Weight = K
This implies a strict allocation pattern, but flexibility ( based
in the differing ecology of the species) in the balance between
size and number.
The graph that follows shows a log-log plot of the number of
propagules per basal stem against the mean weight of the
propagules. It follows a straight line quite well. That is what
the basic equation portrays. The regression equation which fits
this line is:
log N = 5.29 - 1.19 log W
The constant 1.19 does not differ significantly from 1.
It is interesting and important to recognize that goldenrods in
differing habitats differ significantly in size.
How can they allocate the same total biomass to seeds (called
achenes), varying only the balance between size and number,
when they are different sized plants?
They must have evolved adjustments to the balance in
allocation among growth, maintenance, and reproduction.
Here we’ll look at compromises between achene size and
number.
Later we’ll look at biomass allocations, and the compromises
between growth and reproduction.
Look again at the graph…
Prairie and old field populations were studied. The prairie
populations came from northwestern Iowa; the old field
populations came from the Kellogg Biological Station, near
Kalamazoo, MI; oak woods populations were also from
western Michigan.
Old field populations produce the largest numbers of smallest
seeds (this environment corresponds to dry, open,
disturbed sites). Prairie populations are intermediate in both
size and number, and oak woods populations produce smaller
numbers of larger seeds.
For individual species from old field and prairie (where we
can find the same species in both habitats)...
Species
Old field
Wt.
#
loading
S. nemoralis
26.7 2300 7.319
S. missourensis 17.6 4200 2.862
S. speciosa
19.5 9100 5.345
S. canadensis 27.3 13000 3.385
S. graminifolia 24.5 17700 3.92
Wt.
104.
39.3
146.3
58.3
10.6
Prairie
#
200
1100
500
1100
7800
loading
9.968
4.485
13.193
8.965
1.509
Achene weights are in g. Loading is a ratio of achene weight
to the area of the dispersal accessory structure, called a
plummule. It should be closely correlated to dispersal distance.
Weights are higher from prairie samples for each species
except S. graminifolia. The same is true for wing loading.
Numbers are lower for each species.
S. nemoralis
S. rigida
S. speciosa
S. missouriensis
S. graminifolia
Something else should have been evident…
The product of weight and number was, according to the
larger scale hypothesis, supposed to be a constant. Is it?
Species
Old field
Wt.
#
total
Wt.
S. nemoralis
26.7 2300 61,410 104.
S. missourensis 17.6 4200 73,920
39.3
S. speciosa
19.5 9100 177,450 146.3
S. canadensis 27.3 13000 354,900
58.3
S. graminifolia 24.5 17700 433,650 10.6
Prairie
#
total
200 20,800
1100
43,230
500
73,150
1100 64,130
7800 82,680
Clearly not. But it is also evident that there is a gradient
among these species.
These differences are due to the species being distributed
along a soil moisture gradient. S. nemoralis, typically found in
open and [relatively] disturbed sites, occurs in both old fields
and prairies, but it occupies the driest sites in both places. In
both habitats it produces the smallest total biomass of achenes.
The other species are arrayed along a gradient in soil moisture
which, on the prairie, corresponds to walking down from the
drier ridge tops into the moist valleys between.
There are 2 parallel conditions intertwined in the comparisons
in the size-number data. Successional status (diversity, intraand interspecific competition) clearly influences size-number
tradeoffs, and we can hypothesize that the diverse, climax
prairie should expose component species to higher biotic
stress.
However, there is also water stress caused by the moisture
gradient. We can, at least in a primitive way, separate those
factors. How does the allocation to seeds (the product of size
and number for each species) vary along the moisture gradient
in each habitat?
If the slopes of these two lines were identical, we could
dismiss competitve stress as a significant factor. However,
the slopes differ markedly.
The slope is much lower on the prairie (2966 µg/% moisture)
than in old field (21,117 µg/% moisture), indicating that the
biotic stress on the prairie has already limited the potential
response to improving moisture conditions.
Allocation to Reproduction: When & How Much
The question is usually phrased in some way to ask how to
schedule allocation to reproduction through the lifespan of the
organism.How much should be allocated at each age?
We’ve already seen a set of graphs that showed theoretically
how to optimize when and how much according to
semelparous and iteroparous strategies.
Why should a species evolve to be iteroparous versus
semelparous?
If being iteroparous is advantageous, how is lifetime
reproductive effort scheduled?
Semelparity versus Iteroparity
It’s almost Shakespearean…
Whether to reproduce only once (i.e. semelparity or big-bang
reproduction), or whether to reproduce more moderately but
repeatedly (i.e. iteroparity).
Reproducing early, and using the big- bang approach, the
organism need not worry about possible effects on adult
survivorship. Caution can be “thrown to the winds”. The
generation time, G, will be considerably shortened, and R0
can be increased. This should clearly increase 'r'.
However, even if every litter were somewhat smaller, the
iteroparous species can have many such litters, and a much
larger number of offspring (R0) over a life cycle.
The answer (which is the better strategy) is not clear.
A comparison of strategies is part of Cole's (1954) classic
paper, and comes to what, at the outset, seems to be a shocking
conclusion. Here are the parameters of Cole’s comparison:
1) Assume that the semelparous species has perfect
survivorship to age 1 year, then reproduces, producing a
litter of size b.
2) Assume that the iteroparous species has perfect
survivorship, not only until it begins reproducing at age 1,
but also thereafter; it produces a litter of size b annually.
Now compare the population growth...
First the semelparous species. The exponential growth model:
Nt = N0 ert
if t = 1 year, and each female has a litter of size b, then the
growth equation can be re-written:
Nt/N0 = er = b
or
rs = ln (b)
The calculations for the iteroparous species involve another
infinite sum, with the approximate result that:
ri = ln (b+1)
Cole's way of stating this comparative result is clear and
concise:
“for an annual species, the absolute gain in intrinsic
population growth which could be achieved by
changing to the perennial reproductive habit
would be exactly equivalent to adding one
individual to the average litter size.”
Not only that, but the gain, logically, is a function of litter size
and the age of first reproduction, which aren't considered in
this basic comparison. The gain from changing to iteroparity
(when the litter size of the semelparous species is equal)
increases both with delay in first reproduction (i.e. increasing
) and with decreasing litter size.
Consider why…
If it takes one extra in the litter for a semelparous species to
keep up, that represents a bigger proportional increase in
reproductive effort when litter size is small than when it's
large.
Now consider the effects of changing . The general idea can
best be seen by comparing population size over time in
semelparous and iteroparous populations.
First in the comparison that produced the original result:
Time
0
1
2
3
4
Semelparity 1
b
b2
b3
b4
Iteroparity 1
b+1
b2 +2b+1
b3+3b2+3b+1 (b+1)4
Now, what happens if  were 2…
Time
0
1
2
Semelparity 1
b
Iteroparity 1
b+1
3
4
b2
2b+1
b2+4b+1
The advantage at every age beyond 2 is much larger. The
advantage increases as  increases.
If alpha increased from 1 to 5 for both the iteroparous and
semelparous species being compared, then by the time
grandchildren are born in the semelparous species, the
iteroparous parent will have produced offspring at ages 6,7,8
and 9, each time with a litter of size b. That's clearly a bigger
advantage than accrued when alpha was 1. Cole showed the
general result in a graph:
Most semelparous species tend to have short pre-reproductive
periods. That minimizes the advantage that might be gained
by becoming iteroparous. A few examples make the basic
point. Annual plants (many weeds) and almost all insects
complete their life cycles in a single year, and most which do
not complete life cycles in 2 years instead (e.g. biennial plants
such as weedy thistles, teasel, onions, garlic).
Semelparous species also typically have very large litter sizes.
Many produce 106 eggs or more, e.g. Musca domestica, the
common housefly, oysters, or the salmon.
The problem can also be approached from the opposite point
of view. It is clearly advantageous, from an evolutionary view,
to increase the intrinsic rate of increase 'r', 'all else being
equal'. A change in 'r' could result from becoming iteroparous
while maintaining litter size, or from an increase in the
semelparous litter size.
Since either change could be equally effective, we can look at
the change in semelparous litter size required to achieve the
same 'r' as would be reached by switching to iteroparity while
just maintaining litter size.
Since Cole worked it out so nicely, I’ll use it even though it is
an animal example…
The change in litter size required can be dramatic. Take the
lowly tapeworm as an example:
The approximate daily litter size of the mature tapeworm is
100,000 eggs.
Including larval development time, the maturation time, or
alpha, is approximately 100 days.
If the tapeworm were to become indefinitely iteroparous, we
find that the equivalent semelparous litter size to achieve the
same growth rate would be about 800,000 (or an increase in
litter size by a factor of 8). The species is not, of course,
indefinitely iteroparous, and the required semelparous litter (or
litter size factor) size is not quite so large.
There are real situations which suggest a considerable
advantage (in an evolutionary sense) to iteroparity.