Biology
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Biology is the "science of life." It is the study of living and once-living things, from submicroscopic structures in single-celled organisms to entire ecosystems with billions of interacting organisms; it further ranges in time focus from a single metabolic reaction inside a cell to the life history of one individual and on to the course of many species over eons of time. Biologists study the characteristics and behaviors of organisms, how species and individuals come into existence, and their interactions with each other and with the environment. The purview of biology extends from the origin of life to the fundamental nature of human beings and their relationship to all other forms of life.
Biology, or "life science," offers a window into fundamental principles shared by living organisms. These principles reveal a harmony and unity of the living world operating simultaneously among a great diversity of species and even in the midst of competition both between and within species for scarce resources. The overlying harmony is seen at each level, from within a cell to the level of systems in individuals (nervous, circulatory, respiratory, etc.), the immediate interactions of one organism with others, and on to the complex of organisms and interactions comprising an ecosystem with a multitude of ecological niches each supporting one species. Such harmony is manifested in many universally shared characteristics among living beings, including interdependence, a common carbon-based biochemistry, a widespread pattern of complementary polarities, sexual reproduction, and homeostasis.
As the science dealing with all life, biology encompasses a broad spectrum of academic fields that have often been viewed as independent disciplines. Among these are molecular biology, biochemistry, cell biology, physiology, anatomy, developmental biology, genetics, ecology, paleontology, and evolutionary biology. While competition among individuals expressing genetic variability has generally been identified as a key factor in evolutionary development, the pivotal roles of cooperation[1] and long-term symbiosis or symbiogenesis (Margulis and Sagan 2002) in living systems have emerged in the late twentieth century as essential complementary focal points for understanding both the origin of species and the dynamics of biological systems.
Principles of biology
While biology is unlike physics in that it does not usually describe biological systems in terms of objects that exclusively obey immutable physical laws described by mathematics, it is nevertheless characterized by several major principles and concepts, which include: universality, evolution, interactions, diversity, and continuity.
Universality: Cells, biochemistry, energy, development, homeostasis, and polarity
See also: Life
Living organisms share many universal characteristics, including that they are composed of cells; pass on their heredity using a nearly universal genetic code; need energy from the environment to exist, grow, and reproduce; maintain their internal environment; and exhibit dual characteristics or complementary polarities. This are the common set of characteristics identified by biologists that distinguish living organisms from nonliving things.
With the exception of viruses, all organisms consist of cells, which are the basic units of life, being the smallest unit that can carry on all the processes of life, including maintenance, growth, and even self-repair. Some simple life forms, such as the paramecium, consist of a single cell throughout their life cycle and are called unicellular organisms. Multicellular organisms, such as a whale or tree, may have trillions of cells differentiated into many diverse types each performing a specific function.
All cells, in turn, are based on a carbon-based biochemistry, and all organisms pass on their heredity via genetic material based on nucleic acids such as DNA using a nearly universal genetic code. Every cell, no matter how simple or complex, utilizes nucleic acids for transmitting and storing the information needed for manufacturing proteins.
Every living being needs energy from the environment in order to exist, grow, and reproduce. Radiation from the sun is the main source of energy for life and is captured through photosynthesis, the biochemical process in which plants, algae, and some bacteria harness the energy of sunlight to produce food. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on Earth. There are also some bacteria that utilize the oxidation of inorganic compounds such as hydrogen sulfide or ferrous iron as an energy source. An organism that produces organic compounds from carbon dioxide as a carbon source, using either light or reactions of inorganic chemical compounds as a source of energy, is called an autotroph. Other organisms do not make their own food but depend directly or indirectly on autotrophs for their food. These are called heterotrophs.
In development, the theme of universal processes is also present. Living things grow and develop as they age. In most metazoan organisms the basic steps of the early embryo development share similar morphological stages and include similar genes.
All living organisms, whether unicellular or multicellular, exhibit homeostasis. Homeostasis is the property of an open system to regulate its internal environment so as to maintain a stable condition. Homeostasis can manifest itself at the cellular level through the maintenance of a stable internal acidity (pH); at the organismal level, warm-blooded animals maintain a constant internal body temperature; and at the level of the ecosystem, for example when atmospheric carbon dioxide levels rise, plants are theoretically able to grow healthier and thus remove more carbon dioxide from the atmosphere. Tissues and organs can also maintain homeostasis.
In addition, living beings share with all existent beings the quality of dual characteristics or complementary polarities. One common pair of dual characteristics is the quality of positivity and negativity: Just as sub-atomic particles have positive (electron) and negative (proton) elements that interrelate and form atoms, living beings commonly exhibit positive and negative characteristics. Most animals reproduce through relationships between male and female, and higher plants likewise have male and female elements, such as the (male) stamen and (female) pistil in flowering plants (angiosperms). Lower plants, fungi, some of the protists, and bacteria likewise exhibit reproductive variances, which are usually symbolized by + and - signs (rather than being called male and female), and referred to as "mating strains" or "reproductive types" or similar appellations.
Another more philosophical concept is the universal dual characteristic of within each organism of the invisible, internal character or nature and the visible aspects of matter, structure, and shape. For example, an animal will exhibit the internal aspects of life, instinct, and function of its cells, tissues, and organs, which relate with the visible shape made up by those cells, tissues, and organs.
Sexual reproduction is a trait that is almost universal among eukaryotes. Asexual reproduction is not uncommon among living organisms. In fact, it is widespread among fungi and bacteria, many insects reproduce in this manner, and some reptiles and amphibians. Nonetheless, with the exception of bacteria (prokaryotes), sexual reproduction is also seen in these same groups. (Some treat the unidirectional lateral transfer of genetic material in bacteria, between donors (+ mating type) and recipients (- mating type), as a type of sexual reproduction.) Evolutionary biologist and geneticist John Maynard Smith maintained that the perceived advantage for an individual organism to pass only its own entire genome to its offspring is so great that there must be an advantage by at least a factor of two to explain why nearly all animal species maintain a male sex.
Another characteristic of living things is that they take substances from the environment and organize them in complex hierarchical levels. For example, in multicellular organisms, cells are organized into tissues, tissues are organized into organs, and organs are organized into systems.
In addition, all living beings respond to the environment; that is, they react to a stimulus. A cockroach may respond to light by running for a dark place. When there is a complex set of response, it is called a behavior. For example, the migration of salmon is a behavioral response.
Evolution: A common organizing principle of biology
See also: Evolution
A central, organizing concept in biology is that all life has descended from a common origin through a process of evolution. Indeed, eminent evolutionist Theodosius Dobzhansky has stated that "Nothing in biology makes sense except in the light of evolution." Evolution can be considered a unifying theme of biology because the concept of descent with modification helps to explain the common carbon-based biochemistry, the nearly universal genetic code, and the similarities and relationships among living organisms, as well as between organisms of the past with organisms today.
Evolutionary theory actually comprises several distinct components. Two of the major strands are the theory of descent with modification, which addresses the "pattern" of evolution, and the theory of natural selection, which addresses the "process" of evolution. Charles Darwin established evolution as a viable theory by marshaling and systematizing considerable evidence for the theory of descent with modification, including evidence from paleontology, classification, biogeography, morphology, and embryology. The mechanism that Darwin postulated, natural selection, aims to account for evolutionary changes at both the microevolutionary level (i.e., gene changes on the populational level) and the macroevolutionary level (i.e., major transitions between species and origination of new designs). Experimental tests and observations provide strong evidence for microevolutionary change directed by natural selection operating on heritable expressed variation, while evidence that natural selection directs macroevolution is limited to fossil evidence of some key transition sequences and extrapolation from evidences on the microevolutionary level. (Alfred Russel Wallace is commonly recognized as proposing the theory of natural selection at about the same time as Darwin.)
The evolutionary history of a species—which tells the characteristics of the various species from which it descended—together with its genealogical relationship to every other species is called its phylogeny. Widely varied approaches to biology generate information about phylogeny. These include the comparisons of DNA sequences conducted within molecular biology or genomics, and comparisons of fossils or other records of ancient organisms in paleontology. Biologists organize and analyze evolutionary relationships through various methods, including phylogenetics, phenetics, and cladistics. Major events in the evolution of life, as biologists currently understand them, are summarized on an evolutionary timeline.
Interactions: Harmony and bi-level functionality
Every living thing interacts with other organisms and its environment. One of the reasons that biological systems can be difficult to study is that there are so many different possible interactions with other organisms and the environment. A microscopic bacterium responding to a local gradient in sugar is as much responding to its environment as a lion is responding to its environment when it is searching for food in the African savanna. Within a particular species, behaviors can be cooperative, aggressive, parasitic, or symbiotic.
Matters become more complex still when two or more different species interact in an ecosystem, studies of which lie in the province of ecology. Analysis of ecosystems shows that a major factor in maintaining harmony and reducing competition is the tendency for each species to find and occupy a distinctive niche not occupied by other species.
Overlying the interactions of organisms is a sense of unity and harmony at each level of interaction. On the global level, for example, one can see the harmony between plant and animal life in terms of photosynthesis and respiration. Plants, through photosynthesis, use carbon dioxide and give off oxygen. While they also respire, plants' net input to the globe is considerably more oxygen than they consume (with algae in the ocean being a major source of planetary oxygen). Animals, on the other hand, consume oxygen and discharge carbon dioxide.
On the trophic level, the food web demonstrates harmony. Plants convert and store the sun's energy. These plants serve as food for herbivores, which in turn serve as food for carnivores, which are consumed by top carnivores. Top carnivores (and species at all other trophic levels), when dead, are broken down by decomposers such as bacteria, fungi, and some insects into minerals and humus in the soil, which is then used by plants.
On the level of individuals, the remarkable harmony among systems (nervous, circulatory, respiratory, endocrine, reproductive, skeletal, digestive, etc.) is a wonder to behold. Even within a cell, one sees remarkable examples of unity and harmony, such as when a cell provides a product to the body (such as a hormone) and receives oxygen and nourishment from the body. So remarkable is the harmony evident among organisms, and between organisms and the environment, that some have proposed a theory that the entire globe acts as if one, giant, functioning organism (the Gaia theory). According to well-known biologist Lynn Margulis and science writer Dorion Sagan (Microcosmos, 1997), even evolution is tied to cooperation and mutual dependence among organisms: "Life did not take over the globe by combat, but by networking."
An underlying explanation for such observed harmony is the concept of bi-level functionality, the view that every entity exists in an integral relation with other entities in ways that permit an individual entity to advance its own multiplication, development, self-preservation, and self-strengthening (a function for the individual) while at the same time contribute toward maintaining or developing the larger whole (a function for the whole). These functions are not independent but interdependent. The individual's own success allows it to contribute to the whole, and while the individual contributes something of value to the larger entity, assisting the larger entity in advancing its own function, the larger entity likewise provides the environment for the success of the individual.
For example, in the cells of a multicellular organism, each cell provides a useful function for the body as a whole. A cell's function may be to convert sugar to ADP energy, attack foreign invaders, or produce hormones. A cell in the epithelial tissue of the stomach may secrete the enzyme pepsin to help with digestion. The cell's function of providing pepsin to the body is harmonized with the body's needs for maintenance, development, and reproduction. The body, on the other hand, supports the individual cell and its function by providing food, oxygen, and other necessary materials, and by transporting away the toxic waste materials. Each cell actually depends on the other cells in the body to perform their functions and thus keep the body in proper functioning order. Likewise, a particular taxonomic group (taxa) not only advances its own survival and reproduction, but also provides a function for the ecosystems of which it is part, such as the ocelot species helping to regulate prey populations and thus help ecosystems to maintain balance. An ecosystem provides an environment for the success of this taxonomic group and thus its contribution to the ecosystem. In essence, this explanation holds that while animals and plants may seem to struggle against one another for existence, in reality they do not. Rather, they all contribute to the whole, in harmony.
Human beings, the most complex of all biological organisms, likewise live in a biosphere that is all interrelated and is necessary for physical life. Thus, it becomes essential that human beings, as the most powerful of all life forms and in many ways an encapsulation of the whole (a "microcosm of creation" according to a theological perspective[2]), understand and care for the environment. In religious terms, this is sometimes referred to as the "third blessing," the role of humankind to love and care for creation. The science of biology is central to this process.
The science of physics offers complementary rationales both for explaining evolutionary development and also for urging humans to love and care for the biosphere. This striking advance in physics arises through the extension of the second law of thermodynamics to apply to "open" systems, which include all forms of life. The extended second law states simply that natural processes in open systems tend to dissipate order as rapidly as possible. From this perspective, evolution of life's successively more ordered and complex systems occurs because the greater a system's order and complexity, the greater its capacity to dissipate order. Human beings, as the planet's dominant and most complex species, face a thermodynamic imperative to apply themselves toward establishing an even greater level of order and dynamic complexity on the planet. Achieving such greater order would likely require that humans learn to live together in peace while living in synergy with the biosphere.
Diversity: The variety of living organisms
See also: Diversity of Life
Despite the underlying unity, life exhibits an astonishing wide diversity in morphology, behavior, and life histories. In order to grapple with this diversity, biologists, following a conventional western scientific approach and historically unaware of the profound interdependence of all life on the planet, attempt to classify all living things. This scientific classification should reflect the evolutionary trees (phylogenetic trees) of the different organisms. Such classifications are the province of the disciplines of systematics and taxonomy. Taxonomy puts organisms in groups called taxa, while systematics seeks their relationships.
Until the nineteenth century, living organisms were generally divided into two kingdoms: animal and plant, or the Animalia and the Plantae. As evidence accumulated that these divisions were insufficient to express the diversity of life, schemes with three, four, or more kingdoms were proposed.
A popular scheme, developed in 1969 by Robert Whitaker, delineates living organisms into five kingdoms:
In the six-kingdom classification, the six top-level groupings (kingdoms) are:
- Archaebacteria, Monera (the bacteria and cyanobacteria), Protista, Fungi, Plantae, and Animalia.
These schemes coexist with another scheme that divides living organisms into the two main divisions of prokaryote (cells that lack a nucleus: bacteria, etc.) and eukaryote (cells that have a nucleus and membrane-bound organelles: animals, plants, fungi, and protists).
In 1990, another scheme, a three-domain system, was introduced by Carl Woese and has became very popular (with the "domain" a classification level higher than kingdom):
The three-domain system is a biological classification that emphasizes his separation of prokaryotes into two groups, the Bacteria and the Archaea (originally called Eubacteria and Archaebacteria). When recent work revealed that what were once called "prokaryotes" are far more diverse than suspected, the prokaryotes were divided into the two domains of the Bacteria and the Archaea, which are considered to be as different from each other as either is from the eukaryotes. Woese argued based on differences in 16S ribosomal RNA genes that these two groups and the eukaryotes each arose separately from an ancestral progenote with poorly developed genetic machinery. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms. The groups were also renamed the Bacteria, Archaea, and Eukaryota, further emphasizing the separate identity of the two prokaryote groups.
There is also a series of intracellular "parasites" that are progressively less alive in terms of being metabolically active:
Continuity: The common descent of life
See also: Descent with Modification
A group of organisms is said to have common descent if they have a common ancestor. All existing organisms on Earth are descended from a common ancestor or ancestral gene pool. This "last universal common ancestor," that is, the most recent common ancestor of all organisms, is believed to have appeared about 3.5 billion years ago. (See: Origin of life.)
The notion that "all life [is] from [an] egg" (from the Latin "Omne vivum ex ovo") is a foundational concept of modern biology, it means that there has been an unbroken continuity of life from the initial origin of life to the present time. Up into the nineteenth century it was commonly believed that life forms can appear spontaneously under certain conditions ( abiogenesis).
The universality of the genetic code is generally regarded by biologists as strong support of the theory of universal common descent (UCD) for all bacteria, archaea, and eukaryotes.
Scope of biology
Academic disciplines
Biologists study life over a wide range of scales: Life is studied at the atomic and molecular scale in molecular biology, biochemistry, and molecular genetics. At the level of the cell, life is studied in cell biology, and at multicellular scales, it is examined in physiology, anatomy, and histology. Developmental biology involves study of life at the level of the development or ontogeny of an individual organism.
Moving up the scale toward more than one organism, genetics considers how heredity works between parent and offspring. Ethology considers group behavior of organisms. Population genetics looks at the level of an entire population, and systematics considers the multi-species scale of lineages. Interdependent populations and their habitats are examined in ecology.
Two broad disciplines within biology are botany, the study of plants, and zoology, the study of animals. Paleontology is inquiry into the developing history of life on earth, based on working with fossils, and includes the main subfields of paleobotany, paleozoology, and micropaleontology. Changes over time, whether within populations (microevolution) or involving either speciation or the introduction of major designs (macroevolution), is part of the field of inquiry of evolutionary biology. A speculative new field is astrobiology (or xenobiology) which examines the possibility of life beyond the Earth.
Biology has become such a vast research enterprise that it is not generally studied as a single discipline, but as a number of clustered sub-disciplines. Four broad groupings are considered here. The first broad group consists of disciplines that study the basic structures of living systems: cells, genes, and so forth; a second grouping considers the operation of these structures at the level of tissues, organs and bodies; a third grouping considers organisms and their histories; and a final constellation of disciplines focuses on the interactions. It is important to note, however, that these groupings are a simplified description of biological research. In reality, the boundaries between disciplines are very fluid and most disciplines borrow techniques from each other frequently. For example, evolutionary biology leans heavily on techniques from molecular biology to determine DNA sequences that assist in understanding the genetic variation of a population; and physiology borrows extensively from cell biology in describing the function of organ systems.
Ethical aspects
As in all sciences, biological disciplines are best pursued by persons committed to high ethical standards, maintaining the highest integrity and following a good research methodology. Data should be interpreted honestly, and results that do not fit one's preconceived biases should not be discarded or ignored in favor of data that fits one's prejudices. A biologist who puts her or his own well-being first (money, popularity, position, etc.), runs the risk of faulty or even fraudulent research. But even well-meaning biologists have gone off course in trying to fit research findings to personal biases.
Also overlying work in many biological fields is the more specific concept of bioethics. This is the discipline dealing with the ethical implications of biological research and its applications. Aspects of biology raising issues of bioethics include cloning, genetic engineering, population control, medical research on animals, creation of biological weapons, and so forth.
Structure of life
See also: Molecular biology, Cell biology, Genetics, and Developmental biology
Molecular biology is the study of biology at the molecular level. The field overlaps with other areas of biology, particularly genetics and biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, especially by mapping the interactions between DNA, RNA, and protein synthesis and learning how these interactions are regulated.
Cell biology studies the physiological properties of cells, as well as their behaviors, interactions, and environment; this is done both on a microscopic and molecular level. Cell biology researches both single-celled organisms like bacteria and specialized cells in multicellular organisms like humans.
Understanding the composition of cells and how cells work is fundamental to all of the biological sciences. Appreciating the similarities and differences between cell types is particularly important to the fields of cell and molecular biology. These fundamental similarities and differences provide a unifying theme, allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types.
Genetics is the science of genes, heredity, and the variation of organisms. In modern research, genetics provides important tools in the investigation of the function of a particular gene (e.g., analysis of genetic interactions). Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules.
Genes encode the information necessary for synthesizing proteins, which in turn play a large role in influencing the final phenotype of the organism, although in many instances do not completely determine it.
Developmental biology studies the process by which organisms grow and develop. Originating in embryology, today, developmental biology studies the genetic control of cell growth, differentiation, and "morphogenesis," which is the process that gives rise to tissues, organs, and anatomy. Model organisms for developmental biology include the round worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, the zebrafish Brachydanio rerio, the mouse Mus musculus, and the small flowering mustard plant Arabidopsis thaliana.
Physiology of organisms
See also: Physiology, Anatomy
Physiology studies the mechanical, physical, and biochemical processes of living organisms, by attempting to understand how all the structures function as a whole. The theme of “structure to function” is central to biology.
Physiological studies have traditionally been divided into plant physiology and animal physiology, but the principles of physiology are universal, regardless of the particular organism being studied. For example, what is learned about the physiology of yeast cells can also apply to other cells. The field of animal physiology extends the tools and methods of human physiology to non-human animal species. Plant physiology also borrows techniques from both fields.
Anatomy is an important part of physiology and considers how organ systems in animals such as the nervous, immune, endocrine, respiratory, and circulatory systems function and interact. The study of these systems is shared with the medically oriented disciplines of neurology, immunology, and the like. The field of health science deals with both human and animal health.
Diversity and evolution of organisms
See also: Evolutionary biology, Botany, Zoology
Evolutionary biology is concerned with the origin and descent of species, and their change over time, i.e., their evolution. Evolutionary biology is an inclusive field because it includes scientists from many traditional taxonomically oriented disciplines. For example, it generally includes scientists who may have a specialist training in particular organisms such as mammalogy, ornithology, or herpetology, but uses those organisms as systems to answer general questions in evolution. It also generally includes paleontologists who use fossils to answer questions about the mode and tempo of evolution, as well as theoreticians in areas such as population genetics and evolutionary theory. In the 1990s, developmental biology made a re-entry into evolutionary biology from its initial exclusion from the modern synthesis through the study of evolutionary developmental biology. Related fields which are often considered part of evolutionary biology are phylogenetics, systematics, and taxonomy.
The two major traditional taxonomically oriented disciplines are botany and zoology. Botany is the scientific study of plants. It covers a wide range of scientific disciplines that study the growth, reproduction, metabolism, development, diseases, and evolution of plant life. Zoology is the discipline that involves the study of animals, which includes the physiology of animals studied under various fields, including anatomy and embryology. The common genetic and developmental mechanisms of animals and plants is studied in molecular biology, molecular genetics, and developmental biology. The ecology of animals is covered under behavioral ecology and other fields.
Classification of life
The dominant classification system is called Linnaean taxonomy, which includes ranks and binomial nomenclature. How organisms are named is governed by international agreements such as the International Code of Botanical Nomenclature (ICBN), the International Code of Zoological Nomenclature (ICZN), and the International Code of Nomenclature of Bacteria (ICNB). A fourth Draft BioCode was published in 1997 in an attempt to standardize naming in the three areas, but it has not yet been formally adopted. The International Code of Virus Classification and Nomenclature (ICVCN) remains outside the BioCode.
Interactions of organisms
See also: Ecology, Ethology, Behavior
Ecology studies the distribution and abundance of living organisms, and the interactions between organisms and their environment. The environment of an organism includes both its habitat, which can be described as the sum of local abiotic factors like climate and geology, as well as the other organisms which share its habitat. Ecological systems are studied at several different levels—from individuals and populations to ecosystems and the biosphere level. Ecology is a multi-disciplinary science, drawing on many other branches of science.
Ethology studies animal behavior (particularly of social animals such as primates and canids), and is sometimes considered as a branch of zoology. Ethologists have been particularly concerned with the evolution of behavior and the understanding of behavior in terms of evolutionary thought. In one sense, the first modern ethologist was Charles Darwin, whose book The Expression of the Emotions in Animals and Men influenced many ethologists.
History of the word "biology"
The word "biology" derives from Greek and is generally rendered as "study of life." Specifically, it is most commonly referenced as deriving from the Greek words βίος (bios), translated as "life," and "λόγος (logos), a root word that may be translated as "reasoned account," "logic," "description," "word," or "human knowledge."
The suffix "-logy" is common in science, in such words as geology, ecology, zoology, paleontology, microbiology, and so forth. This suffix is generally translated as "the study of." Notably, the term ology is considered a back-formation from the names of these disciplines. Many references trace such words as "-logy" and "ology" from the Greek suffix -λογια (-logia), speaking, which comes from the Greek verb λεγειν (legein), to speak. The word ology is thus misleading as the “o” is actually part of the word stem that receives the -logy ending, such as the bio part of biology.
The word "biology" in its modern sense seems to have been introduced independently by Gottfried Reinhold Treviranus (Biologie oder Philosophie der Lebenden Natur, 1802) and by Jean-Baptiste Lamarck (Hydrogéologie, 1802). The word itself is sometimes said to have been coined in 1800 by Karl Friedrich Burdach, but it appears in the title of Volume 3 of Michael Christoph Hanov's Philosophiae Naturalis Sive Physicae Dogmaticae: Geologia, Biologia, Phytologia Generalis et Dendrologia, published in 1766.
Notes
- ↑ The West Group, Cooperation Between Species — Mutualism. Retrieved May 28, 2008.
- ↑ C. P. E. Burns, "Altruism in nature as manifestation of divine energia," Zygon 41(2006): 125-137.
ReferencesISBN links support NWE through referral fees
- Margulis, L., and D. Sagan. Microcosmos: Four Billion Years of Evolution from our Microbial Ancestors. University of California Press, 1997.
- Margulis, L., and D. Sagan. Acquiring Genomes: A Theory of the Origins of Species. Basic Books, New York, 2002.
- Swenson, Rod. Order, evolution, and natural law: Fundamental relations in complex system theory. In Cybernetics and Applied Systems, edited by C.V. Negoita, 125-147. Marcel Dekker, New York, 1992.
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