Which of the following best describes the evolution of smell in chordates? A) Ancestrally, jawless fish had few ancestral scent genes that coded for receptors resembling a combination of water and air receptors. Subsequently these genes were duplicated and specialized through natural selection either for use in water or air. B) Ancestrally, all olfactory receptors where specialized for water as the ancestor of all vertebrates was aquatic. Only once did olfactory genes become duplicated and specialized and this was in the transition to land for receptors that work in air. C) All chordates share the same number of scent receptor genes. However, they have become variously deactivated through natural selection due to either unuse or a tradeoff for sight genes. D) None of the above.
Anatomy and Physiology of Special Sensory Organs
Sensory organs can be labeled as special sensory structures that permit sight, hearing, odor, and flavor. Sensory structures permitting proprioception, touch, thermal, and pain perception can be classified as more advanced sensory organs. The sensory neurons are trained to find out modifications in the external and internal conditions so that a person's body can react to that change. A stimulus is the first signal that is recognized by any sensory receptor of the body. Stimulus is an impulse generated when there is a change in the surroundings of a person. For example, a heated environment will alert the brain through the thermal sensory organs and generate a reflex accordingly.
Sensory Receptors
The human sensory system is one of the most complex and highly evolved structures, which processes a myriad of incoming messages. This well-coordinated system helps an organism or individual to respond to external stimuli, appropriately. The sensory receptors are an important part of the sensory system. These receptors are specialized epidermal cells that respond to external environmental stimuli. These receptors consist of structural and support cells that form the peripheral unit of the receptor and the neural dendrites which receive and detect the external stimuli.
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A) Ancestrally, jawless fish had few ancestral scent genes that coded for receptors resembling a combination of water and air receptors. Subsequently these genes were duplicated and specialized through natural selection either for use in water or air.
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B) Ancestrally, all olfactory receptors where specialized for water as the ancestor of all vertebrates was aquatic. Only once did olfactory genes become duplicated and specialized and this was in the transition to land for receptors that work in air.
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C) All chordates share the same number of scent receptor genes. However, they have become variously deactivated through natural selection due to either unuse or a tradeoff for sight genes.
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D) None of the above.
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