5.1.2: Life Histories and Natural Selection

Learning Objectives

• Describe how life history patterns are influenced by natural selection.
• Compare and contrast semelparity and iteroparity.

Life History Patterns

Energy is required by all living organisms for their growth, maintenance, and reproduction; at the same time, energy is often a major limiting factor in determining an organism’s survival. Plants, for example, acquire energy from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich seeds to produce the next generation. All species have an energy budget: they must balance energy intake with their use of energy for metabolism, reproduction, and energy storage.

Reproductives strategies

Population size can be influenced by the time reproduction begins, how often the organism reproduces, and how many offspring the organism produces in one reproductive period. The timing of reproduction in a life history also affects species survival. Organisms that reproduce early on have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that begin to reproduce later in life often have greater fecundity (ability to produce many offspring), but they risk that they will not survive to reproductive age.

These different energy strategies and tradeoffs are key to understanding the evolution of each species as it maximizes its fitness and fills its niche. In terms of energy budgeting, some species “blow it all” and use up most of their energy reserves to reproduce early before they die. Other species delay having reproduction to become stronger, more experienced individuals and to make sure that they are strong enough to provide parental care if necessary. 

For example, an oak tree grows much slower than an annual grass. Though this is a high investment, oaks are well adapted to expected stressors such as fires or drought. On the other hand, alders and willows grow and colonize open riparian areas very quickly. 

Single versus Multiple Reproductive Events

Some life history traits, such as fecundity, timing of reproduction, and parental care, can be grouped together into general strategies that are used by multiple species. Semelparity occurs when a species reproduces only once during its lifetime and then dies, as do annual plants. Such species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive. Some bamboo exhibit a semelparity life strategy: they flower once and then die. The century plant, or agave, also exhibits this strategy, reproducing once towards the end of its life (Figure \(\PageIndex{1}\)). Iteroparity describes a reproductive strategy where the species reproduces repeatedly during its life. Many plants reproduce using this strategy.

Figure \(\PageIndex{1}\): Left: Some bamboo produce large seed clusters (Credit: Vijayanrajapuram; CC BY-SA 4.0); Right: Agave (Century plant) lives for 10-30 years and reproduces only once before it dies (Credit: Alvesgaspar; CC BY-SA 4.0). Both are examples of semelparity.

Contributors and Attributions

Curated and authored by Kammy Algiers using Life Histories and Energy Budgets from General Biology by Boundless (licensed CC-BY-SA)