Life History Variation

Populations evolve to maximise their Darwinian fitness — essentially, their reproductive success, often measured by a high intrinsic rate of natural increase (the r value). This fitness is not absolute; it is shaped by the specific habitat in which a population lives. Under a given set of selection pressures — the challenges and opportunities of that environment — organisms evolve toward the most efficient reproductive strategy for that setting.

There is no single "best" way to reproduce. Instead, life history traits vary dramatically across species. Some organisms breed only once in their lifetime — classic examples are the Pacific salmon fish and bamboo. Others breed many times over their lifespan, as seen in most birds and mammals.

Similarly, the number and size of offspring vary. Some species produce a very large number of small-sized offspring, like oysters and pelagic fishes. Others produce a small number of large-sized offspring, like birds and mammals.

So which strategy is better for maximising fitness? Ecologists argue that these life history traits have evolved in response to the constraints imposed by both the abiotic (non-living) and biotic (living) components of the habitat. For instance, in a highly unpredictable or resource-poor environment, producing many small offspring may be advantageous because at least some will survive. In a stable, resource-rich environment, investing in a few large offspring that are better able to compete may yield higher fitness. …