Essentials of Biology 4th Edition
The Diversity of Life
Life on Earth takes on a staggering variety of forms, often with appearances and behaviors that may be strange to humans. As we will see in this chapter, one of the ways that biologists classify life is by species. So how many species are there on the planet? The truth is, we really don’t know. While most scientists believe that there are probably as many as 15 million species on the planet, some estimate that there may be over 100 million diﬀerent species. So far, around 2 million species have been identiﬁed, and most of those are insects.
However, new species are being discovered all the time. Some of those recent additions are shown here. The world’s smallest reptile, Brookesia micra, found on Madagascar, is less than 1 inch long. The sneezing monkey, Rhino-pithecus strykeri, was discovered in northeast Myanmar. While very rare, it is easy to locate in the forest, since it sneezes when it rains. Not all new species are rare—some have simply remained isolated from human contact, such as the Nepalese autumn poppy (Meconopsis autumnalis), which lives only at elevations about 10,000 feet in the Himalayas. As we will learn in this chapter, although life is diverse, it also shares a number of important characteristics.
As you read through this chapter, think about the following questions:
1. What are the general characteristics that separate life from nonliving things?
2. How do species ﬁt into the biological levels of organization?
3. What are some of the challenges facing science today?
The Characteristics of Life
Upon completion of this section, you should be able to
1. Explain the basic characteristics that are common to all living organisms.
2. Distinguish between the levels of biological organization.
3. Summarize how the terms homeostasis, metabolism, and adaptation all relate to living organisms.
4. Contrast chemical cycling and energy ﬂow within an ecosystem.
As we observed in the chapter opener, life is diverse (Fig. 1.1). Life may be found everywhere on the planet, from thermal vents at the bottom of the ocean to the coldest reaches of Antarctica. Biology is the scientiﬁc study of life. Biologists study not only life’s diversity but also the characteristics that are shared by all living organisms. These characteristics include levels of organi-zation, the ability to acquire materials and energy, the ability to maintain an internal environment, the ability to respond to stimuli, the ability to reproduce and develop, and the ability to adapt and evolve to changing conditions. By studying these characteristics, we gain insight into the complex nature of life, which helps us distinguish living organisms from nonliving things. In the next sections, we will explore these character istics in more detail.
Life Is Organized
The complex organization of life begins with atoms, the basic units of matter. Atoms combine to form small molecules, which join to form larger molecules within a cell, the smallest, most basic unit of life. Although a cell is alive, it is made from nonliving molecules (Fig. 1.2).
The majority of the organisms on the planet are single-celled. Plants, fungi, and animals are multicellular and are composed of many types of cells. In multicellular organisms, similar cells combine to form tissues. Tissues make up organs, as when various tissues combine to form a heart or a leaf. Organs work together in organ systems; for example, the heart and blood vessels form the cardiovascular system. Various organ systems work together within complex organisms.
The organization of life extends beyond the individual organism. All the members of a species (organisms with a similar structure that are capable of interbreeding) within a particular area belong to a population. When populations interact, such as the humans, zebras, and trees in Figure 1.2, they form a community. At the ecosystem level, communities interact with the physical environment (soil, atmosphere, etc.). Collectively, the ecosystems on the planet are called the biosphere, the zone of air, land, and water at the
surface of the Earth where living organisms are found.
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