Some conditions that accurately predict the future may not always occur

We have discussed responses to environmental signals that always occur. However, many other changes in an organism's environment are uncertain. Predators may or may not be active in an organism's environment. An individual may live under crowded or uncrowded conditions. The sexes and ages of its associates may change. Nevertheless, if such changes have occurred frequently during the evolution of a species, evolved developmental plasticity may allow individuals to respond to them. For example, as we saw at the beginning of this chapter, individuals of some fish species change sex in response to alterations in their social environment. The social environment, including the sexes of others in the social group, determines which is the most adaptive sex for an individual to have at a particular time.

Similarly, individuals that could sense the presence of predators in their environment and change their development so as to become less likely to be eaten by them would be more likely to survive than individuals whose development did not respond to the presence of predators. Thus, developmental responses to predators have evolved in numerous species. For example, when water fleas (Daphnia cucullata) encounter predatory larvae of the fly Chaeoborus, the "helmets" on the top of their heads grow to twice their normal size (Figure 21.11). The fly larvae can ingest Daphnia with large helmets only with difficulty. Helmet induction also occurs if Daphnia are exposed to water in which the fly larvae have been swimming. Moreover, the offspring that are developing in the abdomens of mothers with induced large helmets are born with large helmets. There is a trade-off, however: Daphnia with large helmets produce fewer eggs than do Daphnia with small helmets. Otherwise, we would expect all individuals to develop large helmets.

Tadpoles of the spadefoot toad (Scaphiopus couchii), which breeds in ephemeral ponds in the arid southwestern United States, respond developmentally if their pond begins to dry up while they are growing. At the time she lays her eggs, a mother toad cannot know how long the pond will persist, because that depends on unpredictable future rainfall. If the pond dries up completely before the tadpoles' development has been completed, they will die. Some spadefoot tadpoles respond to crowding in a shrinking pond by developing a wider mouth and powerful jaw muscles. They complete their development rapidly before the pond dries up by eating other tadpoles.

Light exerts a powerful influence on plant development. Low light conditions stimulate the elongation of cells, so that plants growing in the shade become spindly (Figure 21.12). It is obvious why this response is adaptive: A spindly plant is more likely to reach a patch of brighter light than a plant that remains compact. And because they have meristems, plants can continue to respond to light as long as they grow.

Daphnia Developmental Plasticity

21.11 Predator-Induced Developmental Plasticity in Daphnia

This scanning electron micrograph shows the predator-induced form of Daphnia (left), with an enlarged helmet, and the normal form of the crustacean (right).These two individuals are genetically identical from a single asexually produced clone.

21.11 Predator-Induced Developmental Plasticity in Daphnia

This scanning electron micrograph shows the predator-induced form of Daphnia (left), with an enlarged helmet, and the normal form of the crustacean (right).These two individuals are genetically identical from a single asexually produced clone.

Light Conditions Plants
21.12 Light Seekers The bean plants on the left were grown experimentally under low-light conditions.The plants cells have elongated in response to low light, and the overall plant has become spindly. The control plant on the right was grown in normal light conditions.

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  • flavia fallaci
    How plants respond light?
    7 years ago

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