Level Name Study of:
Molecules Molecular biology Biological molecules
in cells
Molecular genetics DNA and its use
in cells
Cells Cytology Cell structure
and function
Tissues Histology Tissue structure
and function Organs and Anatomy Structures of organs individuals Physiology Functions of cells, tissues, and organs Genetics Inheritance patterns Species Virology Viruses and groups Microbiology Microorganisms of species Bacteriology Bacteria Mycology Fungi Botany Plants Zoology: Animals Malacology Molluscs Entomology Insects Ichthyology Fishes Herpetology Reptiles and amphibians Ornithology Birds Mammalogy Mammals Physical Humans anthropology Interactions Ethology Animal behavior Ecology Interactions among organisms Parasitology Interactions between parasites and hosts Epidemiology Spread of parasites Population biology Populations Evolution Population genetics Genetic variation in populations Taxonomy Classification Systematics Evolutionary basis of classification Evolutionary biology Evolutionary processes
design) or adaptation to their environments and can only be understood in terms of evolutionary ancestry. This applies to some of their structural and functional characteristics (see vestigial characteristics) and particularly to much of the DNA (see noncoding DNA). This is what evolutionary geneticist Theodosius Dobzhansky meant by his famous state- ment, “Nothing in biology makes sense except in the light of evolution” (see Dobzhansky, Theodosius). Furthermore, evolution has given an organizing principle to biology, which might otherwise have continued to be a cataloguing of types of organisms and their structures and functions.
Although biologists study life, they cannot precisely define it. Ernst Mayr (see Mayr, Ernst), perhaps the most prominent biologist of modern times, indicated that a life- form must have the following capacities:
• For metabolism, in which energy is bound and released, for example, to digest food molecules
• For self-regulation, whereby the chemical reactions of metabolism are kept under control and in homeostasis, for example, to maintain relatively constant internal conditions of temperature, or moisture, or chemical composition • To respond to environmental stimuli, for example, moving
toward or away from light
• To store genetic information that determines the chemical reactions that occur in the organism, for example, DNA, and to use this information to bring about changes in the organism • For growth • For differentiation, for example to develop from an embryo into a juvenile into an adult • For reproduction • To undergo genetic change which, in a population, allows evolution to occur While most of these characteristics may not be much in dispute, it is impossible for scientists to imagine all the pos- sible forms that they could take. In addition, no life-form carries out all of these activities all of the time or under all conditions. These considerations become important in two respects. First, would it be possible to recognize a life-form on another planet? Scientists may not be able to witness putative life-forms carrying out metabolic and other activi- ties (see Mars, life on). Second, at what point might scien- tists be able to construct a mechanical life-form? Although computerized robots cannot grow or reproduce themselves, they can construct new components and whole new robots. Many computer algorithms already utilize natural selection to generate improvements in structure and function (see evo- lutionary algorithms). Should robots, or even computer programs, be considered life-forms?
Fundamental assumptions of biology include the following: • The physical and chemical components and processes of
organisms are the same as those of the nonliving world. That is, organisms are constructed of the same kinds of atoms (though not necessarily in the same relative amounts) as the nonliving world; and the laws of physics and chemis- try are the same inside an organism as outside. The alterna-
tive to this view, vitalism, claimed that organisms were made out of material that is different from the nonliving world. This view was widespread along with many biblical views of the natural world until the 19th century, despite the fact that the Bible says “Dust thou art and to dust thou shalt return.” The German chemist Friedrich Wöhler put an end to vitalism in 1815 when he synthesized urea, a biological molecule, from ammonia, an inorganic molecule. Some peo- ple continue to believe, or hope, that there is some further essence within organisms, at least within humans, that is not shared with the nonliving environment, but no evidence of such an essence has been found. Many of the processes that occur in organisms are more complex than those in the non- living world; in addition, when complex molecules interact, they can produce emergent properties (see emergence) that could not have been predicted from a study of the atoms themselves. But this is no different from what happens in the nonliving world. One cannot explain water in terms of the properties of hydrogen and oxygen; the properties of water are emergent; yet nobody claims that water molecules vio- late the laws of physics and chemistry, or that a nonmaterial essence is needed to make water what it is.
• Explanations of biological phenomena can be made at two levels. Consider, for example, why a mockingbird sings. First, scientists can explain the immediate physical and chemical causes of the singing. The pineal gland in the mockingbird’s head senses the increase in the length of the day; this indicates that spring is coming. In response to this information, the mockingbird’s brain produces enhanced levels of the hormone melatonin, which acti- vates the nerve pathways that cause the production of sound. The brain uses both instinctive and learned infor- mation to determine which songs the mockingbird sings. The brain also stimulates the bird to leap and dance. This complex network of immediate causation, which involves sunlight, a gland, the brain, a hormone, the voice box, and muscles, is the proximate causation of the mocking- bird’s singing. Second, scientists can explain the advan- tage that the ancestors of the mockingbird obtained from undertaking such behavior. Singing and dancing was a territorial display that allowed dominant male mock- ingbirds to keep other male mockingbirds away and to attract female mockingbirds. Natural selection in the past favored these activities and selected the genes that now determine this behavior. The evolutionary causation is the ultimate causation of the mockingbird’s singing (see behavior, evolution of).
• Structure and function are interconnected in organisms. That is, the structures do what they look like, and they look like what they do. Both the xylem cells of plants and the blood vessels of animals conduct fluid, and they have a long, cylindrical structure. They act, and look, like pipes. A student may memorize anatomical structures, or physi- ological processes, but will understand biology only when bringing the two together.
• Large organisms have less external surface area relative to their volume than do small organisms (see allometry). biology
Since organisms must bring molecules in from, and eject molecules out into, their environments through surfac- es large organisms must have additional surface area to compensate for this relatively lesser surface area. Small animals can absorb the oxygen that they need through their external surfaces, but large animals need additional surface areas (either gills on the outside, or lungs on the inside) that absorb oxygen. The surface area of human lungs, convoluted into millions of tiny sacs, is as great as that of a tennis court. In small organisms, molecules can diffuse and flow everywhere that is necessary, since they do not need to go very far; larger organisms need circula- tory systems.
The following is a brief outline of some of the areas of study within biology:
I. Autecological context. This is the ecological interaction of an individual organism with its nonliving environment (aut- comes from the Greek for self). Organisms must interact with the energy and matter of their environments. This results in the flow of energy and the cycling of matter.
A. Energy flows from the Sun, through the systems of the Earth, and then is lost in outer space. Some of this energy empowers climate and weather and keeps organisms warm. Photosynthetic organisms (see photosynthesis, evolu- tion of) absorb a small amount of the energy and store it in sugar and other complex organic molecules. Other organisms obtain energy by eating photosynthetic organisms, or one another. In this way, energy passes through the food web of organisms. Eventually all organisms die, and the decomposers release the energy into the environment, where it eventually goes into outer space.
B. Matter cycles over and over on the Earth. Photo- synthetic organisms obtain small molecules, such as carbon dioxide from the air and nitrates from the soil, from which they make complex organic molecules. Other organisms obtain molecules by eating photosynthetic organisms, or one another. In this way, atoms pass through the food web of organisms. Eventually all organisms die and the decomposers release the atoms into the environment, where they are used again by photosynthetic organisms.
C. Energy can flow, and matter can cycle, on a dead planet, but on the living Earth, these processes are almost completely different than they would be on a lifeless planet. Two examples are oxygen and water. Photosynthesis pro- duces oxygen gas, which is highly reactive. An atmosphere contains oxygen gas only if it is continually replenished. Therefore the presence of oxygen gas in a planetary atmo- sphere is evidence that there is life on the planet. Water cycles endlessly, through evaporation from oceans, condensation in clouds, and precipitation onto the ground. Forests slow down the rain and allow it to percolate into the soil. In this way, forests prevent the floods and mudslides that would occur on a bare hillside and recharge the groundwater. Trees release water vapor into the air, creating more clouds than would form over a lifeless landscape.
II. Chemistry of life. Organisms consist largely of carbon, hydrogen, oxygen, nitrogen, phosphorus, and a few other kinds of atoms. Most of the other chemical elements play no part in organisms except as contaminants. Carbon, hydrogen, oxygen, nitrogen, and phosphorus are the principal compo- nents of biological molecules, which include: carbohydrates and fats, which often store chemical energy; nucleic acids, which store genetic information; and proteins, which often control the chemical reactions of organisms. Proteins release genetic information from nucleic acids, allowing that infor- mation to determine the chemical reactions (metabolism) of the organism.
III. Cells and tissues. All life processes occur within cells. Tis- sues are groups of similar cells. Cells can replicate their genet- ic information, which allows one cell to become two (cell division). Cell division allows three things:
A. Old or damaged cells can be replaced by new ones (maintenance).
B. Cells or organisms can produce new cells or organ- isms (reproduction).
C. A single cell can grow into an embryo, which grows into a new organism (development). The use of genetic infor- mation changes during development, which allows a small mass of similar cells (the embryo) to develop into a large mass of many different kinds of cells (the juvenile and adult). IV. Organs. Large organisms (mostly plants and animals) need organs to carry out basic processes necessary to their survival. Plants grow by continually adding new organs (new leaves, stems, and roots) and shedding some old organs (dead leaves). Plants can lose organs and keep on living. Animal growth, however, involves the growth of each organ, the loss of any of which may be fatal to the animal.
A. Exchange. Both plants and animals have surfaces through which food molecules enter and waste molecules leave the organism. In plants, most of these surfaces are exter- nal (thin leaves, fine roots), while in animals they are internal (in the intestines, lungs, and kidneys).
B. Internal movement. Both plants and animals have internal passageways that allow molecules to move from one place to another within the body. In plants this movement is mostly one direction at a time (water from the roots to the leaves, sugar from the leaves to the roots), while large ani- mals have internal circulation of blood.
C. Internal coordination. Both plants and animals have structures that support them and functions that allow their organs to work together. Hormones carry messages from one part of a plant to another, allowing it to coordinate its growth. The responses to hormones are mostly on the cellu- lar level in plants. Hormones also carry messages from one part of an animal to another, but in addition animals have nerves that allow internal coordination, for example, main- taining homeostasis of body temperature and balance during movement. The responses to nerves and to hormones in ani- mals can be on the cellular level or involve the movements of muscles and bones.
D. Response to environment. Both plants and animals have mechanisms that detect environmental stimuli, and pro- cesses that allow a response to them. In plants stimuli are usu- ally detected by molecules such as pigments, and hormones allow the responses, such as bending toward light. Animals have sensory organs and a central nervous system that figures out the appropriate response to the stimuli. V. Sexual reproduction. Nearly all organisms have sexual life cycles. Certain organs produce cells with only half the genetic information that normal cells contain (see meiosis). In many plants and animals, these reproductive cells come in two sizes: The large female ones are either megaspores or eggs, while the small male ones are either microspores or sperm. The male cells of one individual may fuse with the female cells of another, a process called fertilization. Fertilization produces a zygote, which may be sheltered inside of a seed, an eggshell, or a womb (see sex, evolution of; life history, evolu- tion of). Sexual reproduction is not necessary for survival but generates genetic diversity.
VI. Inheritance patterns. Because the traits of different parents can be shuffled into new combinations by sexual reproduction, new combinations of traits can occur in each generation (see Mendelian genetics).
VII. Populations and evolution. All the interacting organ- isms of one species in one location constitute a population. Populations can grow rapidly, but limited resources prevent them from doing so forever. Because some individuals in a population reproduce more successfully than others, natural selection occurs and the population evolves (see natural selection). If the population separates into two populations, they can evolve into two species (see speciation).
VIII. Diversity. Evolution has produced millions of species. Biologists have attempted to classify organisms on the basis of their evolutionary diversification rather than merely on the differences in their appearance. One such classification is:
A. Domain Archaea: bacteria-like cells that, today, sur- vive in extreme environments (see archaebacteria).
B. Domain Eubacteria: bacteria-like cells that may use sunlight (photosynthetic bacteria) or inorganic chemicals (chemosynthetic bacteria) as energy sources for producing organic molecules or may obtain energy from organic mol- ecules in living or dead organisms (heterotrophic bacteria) (see bacteria, evolution of). Early in evolutionary history, some heterotrophic bacteria invaded larger cells, and today they are the mitochondria that release energy from sugar in nearly all larger cells. Early in evolutionary history, some photosynthetic bacteria invaded larger cells, and today they are the chloroplasts that carry out photosynthesis in some larger cells (see symbiogenesis).
C. Domain Eukarya: organisms composed of complex cells with DNA in nuclei (see eukaryotes, evolution of). The evolutionary linages of the eukaryotes are still being worked out. From within these lineages evolved:
1. The plant kingdom. Land plants are descendants of green algae (see seedless plants, evolution of; gymno- sperms, evolution of; angiosperms, evolution of).
2. The fungus kingdom consists of decomposers and pathogens that absorb food molecules.
3. The animal kingdom. Animals, descendants of one of the lineages of protozoa (see invertebrates, evolution of; fishes, evolution of; amphibians, evolution of; rep- tiles, evolution of; birds, evolution of; mammals, evo- lution of).
IX. Synecological context. This is the ecological interaction of organisms with one another (syn- comes from the Greek for together).
A. General ecological interactions. Evolution has refined the interactions that broad groups of organisms have with one another (see coevolution): for example, herbivores that eat plants and plants that defend themselves from herbivores; predators that eat prey, and prey that defend themselves from predators; pollinators and flowering plants. Many com- plex animal behavior patterns have evolved, particularly in response to sexual selection.
B. Symbiotic ecological interactions. Symbiosis results from the very close interaction of two species, in which at least one of the species depends upon the other. These inter- actions have resulted from coevolution.
1. Parasitism occurs when a parasite harms the host. 2. Commensalism occurs when a commensal has no effect on the host.
3. Mutualism occurs when both species benefit. Mutualism is so widespread that, in some cases, different spe- cies of organisms have actually fused together and formed new kinds of organisms.
4. Natural selection favors the evolution of hosts that resist parasites and sometimes favors the evolution of parasites that have only mild effects on their hosts. Natural selection can favor the evolution of parasitism into commen- salism, and commensalism into mutualism.
C. Ecological communities are all of the interacting spe- cies in a location. They generally form clusters, based upon temperature and moisture conditions in different parts of the world: for example, tundra, forests, grasslands, deserts, lakes, shallow seas, deep oceans. Each community contains micro- habitats with smaller communities within them. D. Ecological communities continually undergo change. On a large scale, continents drift (see continental drift), mass extinctions occur, and climates fluctuate (see ice ages; Snowball Earth). On a small scale, disturbances such as fires are followed by stages of regrowth called ecological succession. Billions of years of these changes have produced an entire evolutionary history, as described throughout this book, and the world as humans know it today.
The Earth is filled with living organisms. But is the Earth itself alive? It does not match all of the characteristics listed above to qualify as an organism, but it does have some pro- cesses that produce a semblance of homeostasis, which is not readily understandable by the operations of the organisms on biology
its surface (see Gaia hypothesis). At present, most biologists consider the earth to be the home of biology, rather than a biological being.
Further Reading
Keller, Evelyn Fox. Making Sense of Life: Explaining Biological Development with Models, Metaphors, and Machines. Cam- bridge, Mass.: Harvard University Press, 2002.
Mayr, Ernst. This Is Biology: The Science of the Living World. Cam- bridge, Mass.: Harvard University Press, 1997.
———. What Makes Biology Unique?: Considerations on the Auton- omy of a Scientific Discipline. New York: Cambridge University Press, 2004.
Vandermeer, John. Reconstructing Biology: Genetics and Ecology in the New World Order. New York: John Wiley and Sons, 1996.
biophilia
Biophilia, a term invented by evolutionary biolo- gist E. O. Wilson (see Wilson, Edward O.), refers to a uni- versal love (-philia) of nature and life (bio-). It may have a genetic component, as well as a learned component that is acquired by children during exposure to the outdoors.The feeling of biophilia, contends Wilson, was as much the product of natural selection as any other aspect of sociobiology such as religion or the fear of strang- ers. Biophilia provided a fitness advantage for humans and their evolutionary ancestors. The enjoyment of birds, trees, and mammals encouraged learning about them. By carefully watching the other species, primitive humans could learn to