NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000.

  • By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.
Cover of Developmental Biology

Developmental Biology. 6th edition.

Show details

Chapter 2Life cycles and the evolution of developmental patterns

The view taken here is that the life cycle is the central unit in biology. … Evolution then becomes the alteration of life cycles through time, genetics the inheritance mechanisms between cycles, and development all the changes in structure that take place during one life cycle.

J. T. Bonner (1965)*

It's the circle of life

And it moves us all.

Tim Rice (1994)**


Bonner, J. T. 1965. Size and Cycle: An Essay on the Structure of Biology. Princeton University Press, Princeton, NJ, p. 3.


“The Circle of Life.” Music by Elton John. Lyrics by Tim Rice. Copyright 1994 Wonderland Music Company, Inc. All rights reserved. Used by permission.

Traditional ways of classifying catalog animals according to their adult structure. But, as J. T. Bonner (1965) pointed out, this is a very artificial method, because what we consider an individual is usually just a brief slice of its life cycle. When we consider a dog, for instance, we usually picture an adult. But the dog is a “dog” from the moment of fertilization of a dog egg by a dog sperm. It remains a dog even as a senescent dying hound. Therefore, the dog is actually the entire life cycle of the animal, from fertilization through death.

The life cycle has to be adapted to its environment, which is composed of nonliving objects as well as other life cycles. Take, for example, the life cycle of Clunio marinus, a small fly that inhabits tidal waters along the coast of western Europe. Females of this species live only 2–3 hours as adults, and they must mate and lay their eggs within this short time. To make matters even more precarious, egg laying is confined to red algae mats that are exposed only during the lowest ebbing of the spring tide. Such low tides occur on four successive days shortly after the new and full moons (i.e., at about 15-day intervals). Therefore, the life cycle of these insects must be coordinated with the tidal rhythms as well as the daily rhythms such that the insects emerge from their pupal cases during the few days of the spring tide and at the correct hour for its ebb (Beck 1980; Neumann and Spindler 1991).

One of the major triumphs of descriptive embryology was the idea of a generalizable life cycle. Each animal, whether an earthworm, an eagle, or a beagle, passes through similar stages of development. The major stages of animal development are illustrated in Figure 2.1. The life of a new individual is initiated by the fusion of genetic material from the two gametes—the sperm and the egg. This fusion, called fertilization, stimulates the egg to begin development. The stages of development between fertilization and hatching are collectively called embryogenesis.

Figure 2.1. Developmental history of a frog.

Figure 2.1

Developmental history of a frog. The stages from fertilization through hatching (birth) are known collectively as embryogenesis. The region set aside for producing germ cells is shown in bright purple. Gametogenesis, which is completed in the sexually (more...)


  • The Circle of Life: The Stages of Animal Development
  • The Frog Life Cycle
  • The Evolution of Developmental Patterns in Unicellular Protists
  • Multicellularity: The Evolution of Differentiation
  • Developmental Patterns among the Metazoa
  • Principles of Development: Life Cycles and Developmental Patterns
  • References

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, Sinauer Associates.
Bookshelf ID: NBK10044


  • Cite this Page

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...