An organism has limited time, energy and material resources. Investing more in one biological function can reduce what remains for others. These constraints create life-history trade-offs.

Important life-history traits include:

• age at first reproduction;
• number and size of offspring;
• frequency of reproduction;
• investment in parental care;
• growth rate and adult size;
• survival and lifespan.

For example, producing many small offspring can increase the number released into the environment but reduce investment in each offspring. Producing fewer large offspring can improve survival per offspring but lowers offspring number.

Similarly, reproducing early can increase the chance of leaving descendants before death, but allocating resources to early reproduction can reduce growth or future reproductive output.

Natural selection acts on these combinations through their effects on lifetime reproductive success. There is no universally optimal life history: the favored balance depends on mortality patterns, resource predictability, competition and other environmental conditions.

This is why life histories should be understood as continuous combinations of trade-offs, not as two rigid categories into which every species must fit.

A useful causal chain is

$$\boxed{\text{limited resources} \rightarrow \text{allocation trade-offs} \rightarrow \text{different survival/reproductive schedules} \rightarrow \text{fitness differences}.}$$

Life-history theory connects organismal allocation decisions to the demographic schedules that determine population growth.