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[[Image:Twyllie_dna2.jpg|right|thumb|<center>DNA</center><center>[[Timothy Wyllie]]</center>]]

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'''Biology''' (from Greek: βίος, ''bio'', "[[life]]"; and λόγος, ''[[logos]]'', "speech" lit. "to talk about life"), also referred to as the '''biological sciences''', is the [[scientific study]] of [[life]]. Biology examines the structure, function, growth, origin, evolution, and distribution of living things. It classifies and describes organisms, their functions, how species come into existence, and the interactions they have with each other and with the [[natural environment]].  Four unifying principles form the foundation of modern biology:  [[cell theory]], [[evolution]], [[genetics]] and [[homeostasis]].

'''Biology''' (from Greek: βίος, ''bio'', "[[life]]"; and λόγος, ''[[logos]]'', "speech" lit. "to talk about life"), also referred to as the '''biological sciences''', is the [[scientific study]] of [[life]]. Biology examines the structure, function, growth, origin, evolution, and distribution of living things. It classifies and describes organisms, their functions, how species come into existence, and the interactions they have with each other and with the [[natural environment]].  Four unifying principles form the foundation of modern biology:  [[cell theory]], [[evolution]], [[genetics]] and [[homeostasis]].

===Cell theory===

===Cell theory===

The cell is the fundamental unit of life.  Cell theory states that all living things are composed of one or more cells, or the  [[secretion|secreted]] products of those cells, for example, [[animal shell|shell]] and [[bone]].  Cells arise from other cells through [[cell division]], and in multicellular organisms, every cell in the organism's body is produced from a single cell in a fertilized egg.  Furthermore, the cell is considered to be the basic part of the pathological processes of an organism. {{cite journal|author=Mazzarello, P|title=A unifying concept: the history of cell theory|journal=Nature Cell Biology|volume=1|pages=E13-E15|date=1999|doi=10.1038/8964}}

The cell is the fundamental unit of life.  Cell theory states that all living things are composed of one or more cells, or the  [[secretion|secreted]] products of those cells, for example, [[animal shell|shell]] and [[bone]].  Cells arise from other cells through [[cell division]], and in multicellular organisms, every cell in the organism's body is produced from a single cell in a fertilized egg.  Furthermore, the cell is considered to be the basic part of the pathological processes of an organism.

=== Evolution ===

=== Evolution ===

A group of organisms shares a [[common descent]] if they share a common [[ancestor]]. All [[organism]]s on the [[Earth]] both living and extinct have been or are descended from a common ancestor or an ancestral [[gene pool]]. This last universal common ancestor of all organisms is believed to have appeared about [[Timeline of evolution|3.5 billion years ago]]. Biologists generally regard the universality of the [[genetic code]] as definitive evidence in favor of the theory of universal common descent (UCD) for all [[bacterium|bacteria]], [[archaea]], and [[eukaryote]]s (see: [[origin of life]]).

A group of organisms shares a [[common descent]] if they share a common [[ancestor]]. All [[organism]]s on the [[Earth]] both living and extinct have been or are descended from a common ancestor or an ancestral [[gene pool]]. This last universal common ancestor of all organisms is believed to have appeared about [[Timeline of evolution|3.5 billion years ago]]. Biologists generally regard the universality of the [[genetic code]] as definitive evidence in favor of the theory of universal common descent (UCD) for all [[bacterium|bacteria]], [[archaea]], and [[eukaryote]]s (see: [[origin of life]]).

Evolution does not always give rise to progressively more complex organisms. For example, the process of [[dysgenic]]s has been observed among the human population.<ref>{{cite journal | last =Lynn | first =Richard | coauthors =Van Court, Marilyn | title =New evidence of dysgenic fertility for intelligence in the United States (Intelligence | volume =32 | issue =2 | pages =p. 193 issn 0160-2896}}

Evolution does not always give rise to progressively more complex organisms. For example, the process of [[dysgenic]]s has been observed among the human population.

 

=== Gene theory ===

=== Gene theory ===

Classification is the province of the disciplines of [[systematics]] and [[alpha taxonomy|taxonomy]]. Taxonomy places organisms in groups called [[taxa]], while systematics seeks to define their relationships with each other. This classification technique has evolved to reflect advances in [[cladistics]] and [[genetics]], shifting the focus from physical similarities and shared characteristics to [[phylogenetics]].

Classification is the province of the disciplines of [[systematics]] and [[alpha taxonomy|taxonomy]]. Taxonomy places organisms in groups called [[taxa]], while systematics seeks to define their relationships with each other. This classification technique has evolved to reflect advances in [[cladistics]] and [[genetics]], shifting the focus from physical similarities and shared characteristics to [[phylogenetics]].

Traditionally, living things have been divided into five kingdoms:<ref>{{cite book|last=Margulis|first=L|coauthors=Schwartz, KV|authorlink=Lynn Margulis|title=Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth  ISBN 978-0716731832

Traditionally, living things have been divided into five kingdoms (Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth  ISBN 978-0716731832)

However, many scientists now consider this five-kingdom system to be outdated. Modern alternative classification systems generally begin with the [[three-domain system]]:<ref>{{cite journal | author = Woese C, Kandler O, Wheelis M | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eukarya. [http://www.pnas.org/cgi/reprint/87/12/4576] Proc Natl Acad Sci U issn 0027-8424

However, many scientists now consider this five-kingdom system to be outdated. Modern alternative classification systems generally begin with the [[three-domain system]][http://www.pnas.org/cgi/reprint/87/12/4576]  

These domains reflect whether the cells have nuclei or not, as well as differences in the cell exteriors.

These domains reflect whether the cells have nuclei or not, as well as differences in the cell exteriors.

Although the concept of ''biology'' as a single coherent field arose in the 19th century, the biological sciences emerged from [[history of medicine|traditions of medicine]] and [[natural history]] reaching back to [[Galen]] and [[Aristotle]] in ancient Greece.  During the Renaissance and early modern period, biological thought was revolutionized by a renewed interest in [[empiricism]] and the discovery of many novel organisms. Prominent in this movement were [[Vesalius]] and [[William Harvey|Harvey]], who used experimentation and careful observation in physiology, and naturalists such as [[Carolus Linnaeus|Linnaeus]] and [[Georges-Louis Leclerc, Comte de Buffon|Buffon]] who began to [[Scientific classification|classify the diversity of life]] and the [[fossil record]], as well as the development and behavior of organisms.  [[Microscopy]] revealed the previously unknown world of microorganisms, laying the groundwork for [[cell theory]].  The growing importance of [[natural theology]], partly a response to the rise of [[mechanical philosophy]], encouraged the growth of natural history. (Ernst Mayr, The Growth of Biological Thought, ISBN 978-0674364462,  Magner, LN|title=A History of the Life Sciences, ISBN 978-0824708245

Although the concept of ''biology'' as a single coherent field arose in the 19th century, the biological sciences emerged from [[history of medicine|traditions of medicine]] and [[natural history]] reaching back to [[Galen]] and [[Aristotle]] in ancient Greece.  During the Renaissance and early modern period, biological thought was revolutionized by a renewed interest in [[empiricism]] and the discovery of many novel organisms. Prominent in this movement were [[Vesalius]] and [[William Harvey|Harvey]], who used experimentation and careful observation in physiology, and naturalists such as [[Carolus Linnaeus|Linnaeus]] and [[Georges-Louis Leclerc, Comte de Buffon|Buffon]] who began to [[Scientific classification|classify the diversity of life]] and the [[fossil record]], as well as the development and behavior of organisms.  [[Microscopy]] revealed the previously unknown world of microorganisms, laying the groundwork for [[cell theory]].  The growing importance of [[natural theology]], partly a response to the rise of [[mechanical philosophy]], encouraged the growth of natural history. (Ernst Mayr, The Growth of Biological Thought, ISBN 978-0674364462,  Magner, LN|title=A History of the Life Sciences, ISBN 978-0824708245

Over the 18th and 19th centuries, biological sciences such as [[botany]] and [[zoology]] became increasingly professional [[scientific discipline]]s.  [[Lavoisier]] and other physical scientists began to connect the animate and inanimate worlds through physics and chemistry.  Explorer-naturalists such as [[Alexander von Humboldt]] investigated the interaction between organisms and their environment, and the ways this relationship depends on geography&mdash;laying the foundations for [[biogeography]], [[ecology]] and [[ethology]].  Naturalists began to reject [[essentialism]] and consider the importance of [[extinction]] and the [[history of evolutionary thought|mutability of species]].  [[Cell theory]] provided a new perspective on the fundamental basis of life.  These developments, as well as the results from [[embryology]] and [[paleontology]], were synthesized in {{aps|Charles Darwin}}s theory of [[evolution]] by [[natural selection]].  The end of the 19th century saw the fall of [[spontaneous generation]] and the rise of the [[germ theory of disease]], though the mechanism of [[biological inheritance|inheritance]] remained a mystery. (Futuyma, D, Evolution, ISBN 978-0878931873, Biology in the Nineteenth Century: Problems of Form, Function and Transformation|date=1978|publisher=Cambridge University Press, ISBN 978-0521292931

Over the 18th and 19th centuries, biological sciences such as [[botany]] and [[zoology]] became increasingly professional [[scientific discipline]]s.  [[Lavoisier]] and other physical scientists began to connect the animate and inanimate worlds through physics and chemistry.  Explorer-naturalists such as [[Alexander von Humboldt]] investigated the interaction between organisms and their environment, and the ways this relationship depends on geography&mdash;laying the foundations for [[biogeography]], [[ecology]] and [[ethology]].  Naturalists began to reject [[essentialism]] and consider the importance of [[extinction]] and the [[history of evolutionary thought|mutability of species]].  [[Cell theory]] provided a new perspective on the fundamental basis of life.  These developments, as well as the results from [[embryology]] and [[paleontology]], were synthesized in [[Charles Darwin]]'s theory of [[evolution]] by [[natural selection]].  The end of the 19th century saw the fall of [[spontaneous generation]] and the rise of the [[germ theory of disease]], though the mechanism of [[biological inheritance|inheritance]] remained a mystery. (Futuyma, D, Evolution, ISBN 978-0878931873, Biology in the Nineteenth Century: Problems of Form, Function and Transformation|date=1978|publisher=Cambridge University Press, ISBN 978-0521292931

In the early 20th century, the rediscovery of [[Gregor Mendel|Mendel's]] work led to the rapid development of [[genetics]] by [[Thomas Hunt Morgan]] and his students, and by the 1930s the combination of [[population genetics]] and natural selection in the "[[Modern evolutionary synthesis|neo-Darwinian synthesis]]".  New disciplines developed rapidly, especially after [[James D. Watson|Watson]] and [[Francis Crick|Crick]] proposed the structure of [[DNA]]. Following the establishment of the [[Central Dogma]] and the cracking of the [[genetic code]], biology was largely split between ''organismal biology''&mdash;the fields that deal with whole organisms and groups of organisms&mdash;and the fields related to ''[[cell biology|cellular]] and [[molecular biology]]''.  By the late 20th century, new fields like [[genomics]] and [[proteomics]] were reversing this trend, with organismal biologists using molecular techniques, and molecular and cell biologists investigating the interplay between genes and the environment, as well as the genetics of natural populations of organisms. (Allen, GE, Life Science in the Twentieth Century, ISBN 978-0521292962, Fruton, JS, Proteins, Enzymes, Genes: The Interplay of Chemistry and Biology, ISBN 978-0300076080, Morange, M & Cobb, A History of Molecular Biology, ISBN 978-0674001695, Smocovitis, VB, Unifying Biology, 978-0691033433

In the early 20th century, the rediscovery of [[Gregor Mendel|Mendel's]] work led to the rapid development of [[genetics]] by [[Thomas Hunt Morgan]] and his students, and by the 1930s the combination of [[population genetics]] and natural selection in the "[[Modern evolutionary synthesis|neo-Darwinian synthesis]]".  New disciplines developed rapidly, especially after [[James D. Watson|Watson]] and [[Francis Crick|Crick]] proposed the structure of [[DNA]]. Following the establishment of the [[Central Dogma]] and the cracking of the [[genetic code]], biology was largely split between ''organismal biology''&mdash;the fields that deal with whole organisms and groups of organisms&mdash;and the fields related to ''[[cell biology|cellular]] and [[molecular biology]]''.  By the late 20th century, new fields like [[genomics]] and [[proteomics]] were reversing this trend, with organismal biologists using molecular techniques, and molecular and cell biologists investigating the interplay between genes and the environment, as well as the genetics of natural populations of organisms. (Allen, GE, Life Science in the Twentieth Century, ISBN 978-0521292962, Fruton, JS, Proteins, Enzymes, Genes: The Interplay of Chemistry and Biology, ISBN 978-0300076080, Morange, M & Cobb, A History of Molecular Biology, ISBN 978-0674001695, Smocovitis, VB, Unifying Biology, 978-0691033433