Life History Evolution

Patterns of reproduction
Semelparity - Produce all offspring in one brief interval.
Iteroparity - Spread out reproduction over a long time.

Which of these strategies evolves is determined by adult survivorship and by fertility as a function of age.

Nomenclature
l_x = Probability of surviving to age x
p_x = Probability of surviving from age x to age x+1.
m_x = Expected number of offspring produced at age x.
These are often presented in the form of a "life table"

x 0 1 2 3 4

l_x l₀ l₁ l₂ l₃ l₄

m_x m₀ m₁ m₂ m₃ m₄

Total fitness = (l_xm_x)
where the summation is over the entire lifecycle.

High adult survivorship
(meaning that the probability of an adult surviving from one time interval to the next is relatively high.)
Often advantageous to spread out reproduction or even delay sexual maturity.
Spreading out reproduction (iteroparity) allows greater investment in each offspring.

If adult survivorship is high and m_x increases with adult age, then it is often advantageous to delay the onset of reproduction.

Low adult survivorship
Often advantageous to produce all offspring at once (semelparity) and early.

Example: Guppies in Trinidad
Some live in areas where there is a predator that preys on adults as well as young.
These:

Reproduce earlier
Grow to smaller size
Produce more offspring at first reproduction

Others live in areas with no adult predators, these:

Reproduce later
grow to larger size
Produce fewer offspring at first reproduction

The actual differences are around 20%

These observations are consistent with our theory, but confirmation requires experimental tests

Experimental evidence When guppies from pools with adult predators are transplanted to pools where such predators are absent, they rapidly evolve the same life history and morphological traits that we see in populations that have never been subjected to predation.
Thus has also been demonstrated in laboratory experiments, where it was further shown that the result is reversible.

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Note on evolutionary rates.
This was one of a number of recent experiments that showed that the rate of evolution in nature can be much higher than suggested by the fossil record.

An old measure of the rate of evolution is the "darwin".
One darwin = a change by a factor of 2.718 in 1,000,000 years.

Fast evolution, estimated from the fossil record, is on the order of 3 darwins.
The rate of evolution in the guppy study was around 40,000 darwins.
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An interesting pattern appears when we plot age specific survivorship (p_x) against age.
For any multicellular organism, if we remove any predators and provide ample resources, the age specific survivorship curve looks something like this:

Note that even under the best of circumstances, p_x drops to zero abruptly at an age that is generally characteristic of the species (i.e. the maximum lifespan).
This phenomenon is referred to as senescence, and it is universal among complex organisms.

Senescence seems odd, because it would seem to be adaptive to keep living and reproducing as long as possible.

Two factors are necessary to understand the evolution of senescence.

1) After the age of first reproduction, selection on survivorship becomes weaker with increasing age.

Consider, as a simple example, an organism that lives to age 5, with l_x = 1 up to age 5 and m_x = 1 from age 1 to age 5. The life table for such an organism looks like this:

x 0 1 2 3 4 5

p_x 1 1 1 1 1 0

l_x 1 1 1 1 1 1

m_x 0 1 1 1 1 1

Here, fitness = (l_xm_x) = 5

Now imagine a mutation that reduces survivorship (p_x) at a particular age to 0.5. Even if this mutation is expressed only at one age, it will cause the value of l_x to be lower at all subsequent ages, since l_x is a cumulative function of p_x. If this mutation is expressed at age 0 (before reproduction starts) then we have:

x 0 1 2 3 4 5

p_x 0.5 1 1 1 1 0

l_x 1 0.5 0.5 0.5 0.5 0.5

m_x 0 1 1 1 1 1

Here, fitness = (l_xm_x) = 2.5
So the mutation reduces fitness by 50%.

Now consider what would happen if the mutation were expressed at age 4:

x 0 1 2 3 4 5

p_x 1 1 1 1 0.5 0

l_x 1 1 1 1 1 0.5

m_x 0 1 1 1 1 1

Here, fitness = (l_xm_x) = 4.5

So the reduction in fitness, in this case, is only 10%.
Selection against this particular mutation is thus 5 times stronger when it is expressed at the beginning of the lifecycle as when it is expressed at the end of the lifecycle.

This is a purely mathematical fact that follows from the arithmetic of population growth.

The second key to understanding the evolution of senescence is an empirical observation:

2) There are tradeoffs between early reproduction and later survival.

This has been demonstrated in laboratory experiments using Bacteria, Drosophila, Nematodes, and Mice.
The exact mechanism is still not completely understood, but such tradeoffs appear in every organism studied.

The experiments show:
1) Selecting for high rates of early reproduction reduces later survivorship.
2) Selecting for long life reduces early reproduction.

There is thus a negative genetic covariance between early reproduction and lifespan.

Such tradeoffs could be due to either genetic or physiological factors.

Experiments in Drosophila show that, in lines that have been selected for high early reproduction (and that therefore exhibit short lifespan), introducing a gene that only blocks oogenesis (and therefore precludes reproduction) leads to an instant increase in lifespan.
Thus, in this case, it appears that there is an actual physiological connection between reproduction and lifespan.

Combining this tradeoff with the rule that selection on survivorship decreases with age: Most mutations that increase fecundity will also reduce later survivorship, and such mutations will generally be favored by selection because there is a relatively small fitness cost associated with reducing late survivorship.

Evolution of sex

Nearly all types of multicellular organisms reproduce sexually at least occasionally.

This is noteworthy, since we should expect there to be a very general short term advantage to asexual reproduction.
In a species with separate sexes, an asexual female who produces only daughters will tend to have twice as many copies of her genes passed on to future generations than would a sexual female.
This is assuming that the sexual individual is able to find a mate and reproduce at all.

Also, In a stable environment, an individual who has survived to reproductive age probably has a genome that is well adapted to her environment. Sexual reproduction will reshuffle this genome with that of someone else, which is likely to produce a less well adapted genome in the offspring.

Advantage of sex: Response to directional selection

Large populations of sexually reproducing organisms can respond to directional selection faster than can equal sized populations of asexual organisms.

This is because sex can bring together advantageous mutations that initially appear in different individuals.

For example, if mutations A and B are both advantageous individually, and particularly so when combined (AB individuals are the most fit of all), then as both A and B increase in the population, sex will quickly produce some AB individuals.
By contrast, in an asexual population, the combination AB will appear only if a B mutation occurs in an individual that already has A (or vice versa). This will tend to take longer, since it is unlikely to happen until either A or B has become very common.

Note that this property of sex will be less advantageous in small populations, since then it is unlikely that mutations A and B will both be present in the population at the same time until one is fixed.

Experimental studies with the green alga Chlamydomonas confirm that large sexual populations respond most quickly to directional selection, and that this advantage is lost in small populations.

Adaptation to an unpredictable environment.
If the environment that one's offspring are going to encounter is highly variable and unpredictable, then it can be advantageous to produce highly variable offspring (so that there is a chance that at least some of them will be well adapted to the environment that they encounter).

This is consistent with the observation that many organisms that can reproduce either sexually or asexually only do sex when they are about to produce offspring that will disperse to different habitats. Jul 8, 2021

x	0	1	2	3	4
l_x	l₀	l₁	l₂	l₃	l₄
m_x	m₀	m₁	m₂	m₃	m₄