These are rough notes from reading Fukuyama’s Evolution textbook.

Fundamentally, how organisms reproduce illustrates the complex trade-offs between self-propagation and self-maintenance.

Til death do us part

In general, there are two reasons why senescence exists: mutational accumulation and then genetic pleiotropy. The first is that if you have a mutation that increases your reproductive chances earlier and harms your survival later, it’ll have a larger net boost to average reproductive fitness and spread through a population. Genetic pleiotropy is the notion that the same genes which make organisms more fertile also lower their self-maintenance. There’s a direct-tradeoff between these two qualities. Given that is the case, then we should expect organisms that reproduce later to have lower reproduction earlier in their life which rises later.

The idea of mutational accumulation reminds me a lot of Amdahl’s Law. If you want to make large gains, then you need to optimize for the largest group of individuals. For computers, this is optimizing the most common actions, and for organisms, this would be impacting the reproduction of younger organisms. Since organisms have a higher probability of surviving to younger ages, mutations which make them more fecund at the expense of older organisms would end up being a “net benefit” to overall reproductive fitness.

How many babies?

While death is a common factor between all organisms, how we go about living our lives takes on very different flavors. Why do some organisms reproduce just once while others reproduce many times in a life? What are the advantages/trade-offs to these?

Why would an organism ever want to reproduce just once? If there is some exponential growth between body size and reproductive fitness then you can see why reproducing once would be the optimal strategy. In that case, it makes sense to hold your cards until the very end and reproduce once rather have lots of tiny batches of offspring. Because of the exponential curves, it’s better to wait til the very end. Why can’t an organism just keep reproducing then? Why hold? Reproduction is naturally a very costly event, and there’s always a trade-off between self-maintenance and reproduction.

When is it advantageous to reproduce multiple times then? You get a lot more insurance by reproducing multiple times. Whether that’s the literal insurance of having more than one child or even more indirectly through environmental changes. If your conditions are constantly changing, then having children multiple times gives you better odds of some of them surviving.

Different types of selection

Whether you have children early is also based on where on a population curve you are. If a population is near the carrying capacity, then it’s about survival and making the most out of the limited resources (K-selection). If a population is growing rapidly, then individuals are selected for having as many kids as early as possible (r-selection).

Some Discussion Qs (not exactly from book)

Why are weedy plants r-selected while large animals are K-selected?

Weedy plants quickly colonize an area before spreading their seeds to other places. In a way, they’re always having explosive population growth in a new area, so the most important thing for them is reproducing quickly. Because it takes a longer time for larger animals to develop, more focus is placed on survival and having kids survive rather than having as many as possible.

Will female wasps discriminate host plants more or less as they age?

Life history theory predicts that they should discriminate more. If they’re reproducing younger, then it’s okay to have sloppier reproductive strategies just repeated a bunch. The older the get, the more it matters to have better host plants since the cost/reproductive attempt rises.