BIOL300 Evolution - Macroevolutionary Strategies Notes

Macroevolutionary Strategies

The lecture discusses life history strategies within the context of evolution.
The professor jokes about being old but reassures that experience still has value.

The lecture begins by questioning why slower reproduction or fewer offspring might be favored when faster reproduction with more offspring seems advantageous.
The central question arises: Why would a shorter lifespan ever be favored, given that it reduces reproductive opportunities and, therefore, fitness?
Senescence, the deterioration into senility and death, should theoretically not evolve, yet it does.
Another question: Why do individuals live past their reproductive age, when, after reproducing, they should die quickly to free up resources for offspring?

Iteroparity: Reproducing multiple times during a lifetime.
- Example: Rainbow trout
Semelparity: Reproducing only once.
- Favored if the probability of surviving past reproduction is low.
- Also favored if there is a correlation between body size and reproductive output.
- Example: Salmon, which invest all resources into a single reproductive event, making it incredibly costly.
Reproductive budget: This refers to the tradeoffs between growth and survival versus fecundity.
- Semelparous species have more offspring in a single reproductive event by sacrificing survival.
- Iteroparous species have fewer offspring over a longer time, balancing survival and reproduction.

r-selected species: Species characterized by early reproduction, high fecundity and reproductive effort, small offspring, and a short lifespan.
K-selected species: Species characterized by delayed start of reproduction, low fecundity per reproductive episode, large offspring, and long lives.
Species experiencing higher extrinsic death risk (mortality), such as due to predation, usually evolve fast life histories. Conversely, species with lower mortality rates usually have slow life histories.
The expectation of survival strongly influences reproductive pace over the lifespan.
There is a tradeoff between the quantity and quality of offspring.

Example: Trinidad guppies
- Lower stretches of streams have more predators; upper stretches have fewer.
- When fewer predators are present, guppies survive longer, and their reproduction slows.
- This illustrates how life history schedules evolve in response to ecological pressures in real-time.

Aging raises the question of why organisms cannot live long or stay young forever, considering that aging is detrimental and reduces reproduction.
Prokaryotes and some single-cell eukaryotes seem to avoid aging, unlike sexual eukaryotes, which age and die.

Environmental factors, such as predation, disease, accidents, and starvation, generally end life.
Alleles supporting early maturation and reproduction tend to be selected for, while alleles supporting longevity are selected against.
Mutation accumulation hypothesis for senescence:
- Late-acting genes accumulate mutations because they are less subject to purifying selection.
- Late-age related alleles are not selected out of populations.
- Selection shadow: a late-age period where natural selection is weak.
Leaving the population through death may reduce competition with offspring, benefiting them through selection.

Human somatic (body) cells senesce (degrade with age) more slowly than gametes, meaning reproduction stops before the body completely deteriorates.
This phenomenon is less apparent in males, as gamete (sperm) production continues.

The question arises: Why not stay reproductive throughout life (e.g., why menopause)?
Grandmother hypothesis: It is better to redirect effort to existing offspring than to producing new offspring, illustrating tradeoffs and kin selection.
This phenomenon may also occur in orcas and baleen whales but is primarily unique to humans.

Why would slower reproduction or fewer offspring ever be favored?
- There are tradeoffs (costs and benefits) associated with fast reproduction, such as impacts on survival and offspring quality.
Why would a shorter lifespan ever be favored?
- Longer lifespans are lost if they are not needed; late-acting lethal or semi-lethal genes increase in frequency by mutation and genetic drift if late reproduction is not needed for fitness.
Why would an individual live past its reproductive age?
- If it continues to contribute to offspring fitness, as seen via in the grandmother effect.