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Three basic adaptive responses regulatory, acclimatory, [#permalink]
15 Mar 2005, 18:43
Three basic adaptive responsesâ€”regulatory, acclimatory, and developmentalâ€”may occur in organisms as they react to changing environmental conditions. In all three, adjustment of biological features (morphological adjustment) or of their use (functional adjustment) may occur. Regulatory responses involve rapid changes in the organismâ€™s use of its physiological apparatusâ€”increasing or decreasing the rates of various processes, for example. Acclimation involves morphological changeâ€”thickening of fur or red blood cell proliferationâ€”which alters physiology itself. Such structural changes require more time than regulatory response changes. Regulatory and acclimatory responses are both reversible.
Developmental responses, however, are usually permanent and irreversible: they become fixed in the course of the individualâ€™s development in response to environmental conditions at the time the response occurs. One such response occurs in many kinds of water bugs. Most water-bug species inhabiting small lakes and ponds have two generations per year. The first hatches during the spring, reproduces during the summer, then dies. The eggs laid in the summer hatch and develop into adults in late summer. They live over the winter before breeding in early spring. Individuals in the second (overwintering) generation have fully developed wings and leave the water in autumn to overwinter in forests, returning in spring to small bodies of water to lay eggs. Their wings are absolutely necessary for this seasonal dispersal. The summer (early) generation, in contrast, is usually dimorphicâ€”some individuals have normal functional (macropterous) wings; others have much-reduced (micropterous) wings of no use for flight. The summer generationâ€™s dimorphism is a compromise strategy, for these individuals usually do not leave the ponds and thus generally have no use for fully developed wings. But small ponds occasionally dry up during the summer, forcing the water bugs to search for new habitats, an eventuality that macropterous individuals are well adapted to meet.
The dimorphism of micropterous and macropterous individuals in the summer generation expresses developmental flexibility; it is not genetically determined. The individualâ€™s wing form is environmentally determined by the temperature to which developing eggs are exposed prior to their being laid. Eggs maintained in a warm environment always produce bugs with normal wing, but exposure to cold produces micropterous individuals. Eggs producing the overwintering brood are all formed during the late summerâ€™s warm temperatures. Hence, all individuals in the overwintering brood have normal wings. Eggs laid by the overwintering adults in the spring, which develop into the summer generation of adults, are formed in early autumn and early spring. Those eggs formed in autumn are exposed to cold winter temperatures, and thus produce micropterous adults in the summer generation. Those formed during the spring are never exposed to cold temperatures, and thus yield individuals with normal wing. Adult water bugs of the overwintering generation brought into the laboratory during the cold months and kept warm, produce only macropterous offspring.
1. The passage supplies information to suggest that which one of the following would happen if a pond inhabited by water bugs were to dry up in June?
(A) The number of developmental responses among the water-bug population would decrease.
(B) Both micropterous and macropterous water bugs would show an acclimatory response.
(C) The generation of water bugs to be hatched during the subsequent spring would contain an unusually large number of macropterous individuals.
(D) The dimorphism of the summer generation would enable some individuals to survive.
(E) The dimorphism of the summer generation would be genetically transferred to the next spring generation.
2. According to the passage, the dimorphic wing structure of the summer generation of water bugs occurs because
(A) the overwintering generation forms two sets of eggs, one exposed to the colder temperatures of winter and one exposed only to the warmer temperatures of spring
(B) the eggs that produce micropterous and macropterous adults are morphologically different
(C) water bugs respond to seasonal changes by making an acclimatory functional adjustment in the wings
(D) water bugs hatching in the spring live out their life spans in ponds and never need to fly
(E) the overwintering generation, which produces eggs developing into the dimorphic generation, spends the winter in the forest and the spring in small ponds
3. The author mentions laboratory experiments with adult water bugs (lines 63-66) in order to illustrate which one of the following?
(A) the function of the summer generationâ€™s dimorphism
(B) the irreversibility of most developmental adaptive responses in water bugs
(C) the effect of temperature on developing water-bug eggs
(D) the morphological difference between the summer generation and the overwintering generation of water bugs
(E) the functional adjustment of water bugs in response to seasonal temperature variation
4. Which one of the following best describes the organization of the passage?
(A) Biological phenomena are presented, examples of their occurrence are compared and contrasted, and one particular example is illustrated in detail.
(B) A description of related biological phenomena is stated, and two of those phenomena are explained in detail with illustrated examples.
(C) Three related biological phenomena are described, a hypothesis explaining their relationship is presented, and supporting evidence is produced.
(D) Three complementary biological phenomena are explained, their causes are examined, and one of them is described by contrasting its causes with the other two.
(E) A new way of describing biological phenomena is suggested, its applications are presented, and one specific example is examined in detail.