Biology  5/e   Raven/Johnson
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Reproductive isolation: There are many different mechanisms that prevent hybridization between species. These mechanisms are classified, in general, on the basis of whether they prevent the formation of a zygote. Prezygotic isolating mechanisms include geographic isolation, ecological isolation, temporal isolation, behavioral isolation, mechanical isolation, and prevention of gamete fusion. Postzygotic isolating mechanisms include failure of the hybrid zygote to develop normally and hybrids being less fit or even sterile. Most courtship behavior is species-specific and helps maintain reproductive isolation between species.

Neurons and interneurons: Neurons are the cells of the nervous system that are specialized for transmitting nerve impulses or electrical signals. Receptor cells respond to stimuli from the animal's external and internal environment and trigger sensory neurons that carry signals from the receptors to the central nervous system. Motor neurons carry signals from the central nervous system to effectors such as muscles and glands. Interneurons connect the sensory and motor neurons and comprise the central nervous system. They process all the incoming signals and determine appropriate responses. Animals could not behave without their neurons and nervous systems. They would not be able to perceive stimuli nor respond to them.

Memory and learning: The exact mechanisms of how memory and learning occur are still not known. It appears, however, that short-term memory works by short-term excitation of specific neurons (i.e., an electrical basis), whereas long-term memories involve actual structural changes in the neural connections in parts of the brain. There is no single part of the brain in which memories appear to reside.

Sensing the environment: Animals have numerous sensory systems that are specialized for perceiving different stimuli or aspects of the environment. Each system has the same basic components: receptor cells for perceiving the information, sensory neurons to carry the nerve impulses to the brain, and a brain to process the incoming information and formulate an appropriate response. There must also be motor neurons to carry the brain's commands to appropriate effectors. Different receptor cells are specialized to respond to such diverse stimuli as temperature, mechanical forces, chemicals, sound waves, light, electricity, and magnetism. Without sensory systems, animals could not respond to environmental stimuli; in other words, they could not behave.

Hormonal control of physiological processes: Hormones are chemical messengers that are produced in one part of the body by endocrine glands and have their effects on target cells in different parts of the body. Many hormones are under the control of the central nervous system via the hypothalamus and its releasing hormones. Most physiological processes are controlled by varying levels of specific hormones. Fine-tuned control is maintained by feedback mechanisms. The hormonal system provides slower, but longer lasting effects on body functionings than does the nervous system. Hormone levels influence an animal's behavior as well as its physiology.

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