Chapter 7 Summary

CONCEPT 7.1 Life history patterns vary within and among species.

Life histories are diverse, varying between individuals of the same species as well as between species. The source of this variation may be genetic or environmental.
Organisms may reproduce sexually or asexually, and sometimes the same organism can do both.
Most organisms have complex life cycles with multiple stages that differ in size, morphology, and habitat.

CONCEPT 7.2 Reproductive patterns can be categorized along several continua.

Semelparous species reproduce only once in a lifetime, while iteroparous species reproduce multiple times.
The terms r-selection and K-selection describe two ends of a continuum of reproductive patterns based on population growth rates.
Grime’s triangular model categorizes plant life histories by the degree of competition, disturbance, and stress in the habitat type to which they are adapted.
Charnov’s life history cube attempts to remove the effects of size and time in order to compare life histories across a broader diversity of organisms.

CONCEPT 7.3 There are trade-offs between life history traits.

There is a trade-off between offspring size and number, such that organisms tend to produce larger numbers of relatively small offspring or smaller numbers of relatively large offspring.
There is a trade-off between investment in current reproduction and investment in future reproductive bouts.
As organisms age, they experience a decline in their physiological capabilities, termed senescence, the onset of which can set an upper age limit for reproductive events.

CONCEPT 7.4 Organisms face different selection pressures at different life cycle stages.

The small sizes of early life cycle stages make them vulnerable to predation and food shortages.
Small life cycle stages are well suited to serve some important functions, such as dispersal and diapause.
Complex life cycles allow life histories to evolve in response to differences in selection pressures on different sizes and stages.