Metamorphosis and Alternation of Generation

Two Ways of Changing Shape for Different Stages of Life Cycle

Jun 6, 2009 John Blatchford

Animals are adapted to their environment at every stage. Some do it by gradual modification, others by making drastic changes.

The ways that caterpillars turn into butterflies and coelenterate polyps give rise to swimming jellyfish illustrate two different evolutionary approaches to the need for a sudden change in body-plan.

Caterpillar Metamorphosis and Butterfly

The caterpillar is the young stage of the butterfly; its job is to feed and grow as fast as possible.

When a caterpillar is old enough it encloses itself in a case (the pupa), and effectively dismantles its body to build the totally different adult (the butterfly).

Butterflies feed on high-energy foods to allow them to fly. Their job is to find a mate and lay eggs on the type of plant that will feed the caterpillars when they hatch.

Most butterflies have short lives and do not fly very far from the caterpillar host plant, but there are exceptions (such as the Monarch Butterfly), where some of the adults make long migrations to overwinter.

Medusa Alternation of Generation with Polyp

Another way to produce different body plans for different stages in the life cycle is found in some of the coelenterates – the alternation of generations. One generation is specialised for feeding and growing while permanently attached to the sea-bed (the hydroid), but the next generation looks completely different and swims around. When these jellyfish reproduce their offspring are not young jellyfish but young hydroids.

Alternating the generations in this way allows each to reproduce in the most efficient way.

Hydroids that have found a good place to settle down and feed grow ‘buds’ which produce medusae. Over the lifetime of a single hydroid many medusae can be grown and released into the sea, but the downside is that there is very little genetic variation.

Medusae, on the other hand, reproduce like most other animals, so there is a great deal of genetic variation between the offspring. This stage allows these coelenterates to produce offspring that are all a bit different from one another, so that evolution by natural selection can proceed.

Some animals retain their original shape and body-plan throughout life (placozoans, for example), but the vast majority have a number of stages of development, with each stage well adapted to its environment.

Examples abound, from some crustaceans that have a number of distinct larval stages, to humans, where development in the womb requires different adaptations from those needed as a baby, adolescent, or adult.

Most species change by a form of continuous modification; the changes occur by alterations to the previous body-plan. What makes metamorphosis and alternation of generation so interesting is that they are two different ways of producing totally different body-plans within the life-cycle, without gradual modifications. The caterpillar emerges from its pupal stage as a butterfly, the hydroid gives rise to medusae, and vice-versa.

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