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The Main Principles of Biology

While biology is unlike physics in that it does not usually describe biological systems in terms of objects which obey immutable physical laws described by mathematics, it is nevertheless characterized by several major principles and concepts which include: universality, evolution, diversity, continuity, homeostasis and interactions. 

Universality: Biochemistry, Cells and The Genetic Code
There are many universal units and common processes that are fundamental to the known forms of life. For example all forms of life consist of cells, which in turn, are based on a common carbon-based biochemistry. All organisms pass on their heredity via the genetic material which is based upon the nucleic acid DNA using a universal genetic code. In development the theme of universal processes is also present, for example in most metazoan organisms the basic steps of the early embryo development share similar morphological stages and include similar genes.

Evolution: the Central Principle of Biology
One of the central, organizing concepts in biology is that all life has descended from a common origin through a process of evolution. Indeed, it is one of the reasons that biological organism exhibit the striking similarity of units and processes discussed in the previous section. Charles Darwin established evolution as a viable theory by articulating its driving force: natural selection. (Alfred Russell Wallace is commonly recognized as the co-discoverer of this concept). Genetic drift was embraced as an additional mechanism in the so-called modern synthesis. 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 this evolutionary timeline.

Diversity: the Variety of Living Organisms
A phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains bacteria, archaea, and eukaryotes as described initially by Carl Woese. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, presumably owing to rapid rRNA evolution. The exact relationships of the three domains are still being debated.Despite the underlying unity, life exhibits an astonishing wide diversity in morphology, behavior and life histories. In order to grapple with this diversity, biologists 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.

Traditionally, living things were divided into five kingdoms:

Monera -- Protista -- Fungi -- Plantae -- Animalia
However, this five-kingdom system is now considered by many to be outdated. More modern alternatives generally begin with the three-domain system:

Archaea (originally Archaebacteria) -- Bacteria (originally Eubacteria) -- Eukaryota
These domains reflect whether cells have nuclei or not as well as differences in cell exteriors. There is also a series of intracellular "parasites" that are progressively less alive in terms of being metabolically active:

Viruses -- Viroids -- Prions
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.

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 19th century it was commonly believed that life forms can appear spontaneously under certain conditions. The universality of the genetic code is generally regarded by biologists as definitive evidence in favor of the theory of universal common descent (UCD) for all bacteria, archaea, and eukaryotes.

Homeostasis: Adapting to Change
Homeostasis is the property of an open system to regulate its internal environment so as to maintain a stable condition, by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms. All living organisms, whether unicellular or multicellular exhibit homeostasis. 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 able to grow better and thus remove more carbon dioxide from the atmosphere. Tissues and organs can also maintain homeostasis.

Interactions: Groups and Environments
Mutual symbiosis between clownfish of the genus Amphiprion that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protects the anemone fish from its predatorsEvery 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 savannah. Within a particular species behaviors can be co-operative, aggressive; parasitic or symbiotic. Matters become more complex still when two or more different species interact in an ecosystem, and is the province of ecology.

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This article is licensed under the GNU Free Documentation License.

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