Biology is a
natural science concerned with the study of
life and living
organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy.
[1] Biology is a vast subject containing many subdivisions, topics, and disciplines. Among the most important topics are five unifying principles that can be said to be the fundamental axioms of modern biology:
[2]
- Cells are the basic unit of life
- New species and inherited traits are the product of evolution
- Genes are the basic unit of heredity
- An organism regulates its internal environment to maintain a stable and constant condition
- Living organisms consume and transform energy.
Subdisciplines of biology are recognized on the basis of the scale at which organisms are studied and the methods used to study them:
biochemistry examines the rudimentary chemistry of life;
molecular biology studies the complex interactions of systems of biological molecules;
cellular biology examines the basic building block of all life, the
cell;
physiology examines the physical and chemical functions of the
tissues,
organs, and
organ systems of an organism; and
ecology examines how various organisms interact and associate with their environment.
The term
biology in its modern sense appears to have been introduced independently by
Karl Friedrich Burdach (1800),
Gottfried Reinhold Treviranus (
Biologie oder Philosophie der lebenden Natur, 1802), and
Jean-Baptiste Lamarck (
Hydrogéologie, 1802).
[4][5] It is a
classical compound inspired by the
Greek word
βίος,
bios, "
life" and the suffix
-λογία,
-logia, "study of."
Although biology in its modern form is a relatively recent development, sciences related to and included within it have been studied since ancient times.
Natural philosophy was studied as early as the ancient civilizations of
Mesopotamia,
Egypt, the
Indian subcontinent, and
China. However, the origins of modern biology and its approach to the study of nature are most often traced back to
ancient Greece.
[6] While the formal study of medicine dates back to
Hippocrates (ca. 460 BC – ca. 370 BC), it was
Aristotle (384 BC – 322 BC) who contributed most extensively to the development of biology. Especially important are his
History of Animals and other works where he showed naturalist leanings, and later more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the
Lyceum,
Theophrastus, wrote a series of books on
botany that survived as the most important contribution of antiquity to the plant sciences, even into the
Middle Ages. Significant advances in the study and development of biology were promoted through the efforts of such
Muslim physicians as the
Afro-Arab scholar
al-Jahiz (781–869) in zoology,
[7] the
Kurdish biologist
Al-Dinawari (828–896) in botany,
[8] and the
Persian physician
Rhazes (865–925) in
anatomy and
physiology. These philosophers elaborated on, expanded, and improved the Greek biological theories and systematics. Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.
Biology began to quickly develop and grow with
Antony van Leeuwenhoek's dramatic improvement of the
microscope. It was then that scholars discovered
spermatozoa,
bacteria,
infusoria and the sheer strangeness and diversity of microscopic life. Investigations by
Jan Swammerdam led to new interest in
entomology and built the basic techniques of microscopic
dissection and
staining.
[9]
Advances in
microscopy also had a profound impact on biological thinking itself. In the early 19th century, a number of biologists pointed to the central importance of the
cell. In 1838 and 1839,
Schleiden and
Schwann began promoting the ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of
life, although they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of
Robert Remak and
Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as
cell theory.
[10]
Meanwhile, taxonomy and classification became a focus in the study of natural history.
Carolus Linnaeus published a basic
taxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced
scientific names for all his species.
[11] Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of
common descent. Though he was opposed to evolution, Buffon is a key figure in the
history of evolutionary thought; his work influenced the evolutionary theories of both
Lamarck and
Darwin.
[12]
Serious evolutionary thinking originated with the works of
Jean-Baptiste Lamarck. However, it was the British naturalist
Charles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell,
Thomas Malthus's writings on population growth, and his own morphological expertise, that created a more successful evolutionary theory based on
natural selection; similar reasoning and evidence led
Alfred Russel Wallace to independently reach the same conclusions.
[13]
The discovery of the physical representation of heredity came along with evolutionary principles and
population genetics. In the 1940s and early 1950s, experiments pointed to
DNA as the component of
chromosomes that held genes. A focus on new model organisms such as
viruses and
bacteria, along with the discovery of the double helical structure of DNA in 1953, marked the transition to the era of
molecular genetics. From the 1950s to present times, biology has been vastly extended in the molecular domain. The
genetic code was cracked by
Har Gobind Khorana,
Robert W. Holley and
Marshall Warren Nirenberg after DNA was understood to contain
codons. Finally, the
Human Genome Project was launched in 1990 with the goal of mapping the general human
genome. This project was essentially completed in 2003,
[14] with further analysis still being published. The Human Genome Project was the first step in a globalized effort to incorporate accumulated knowledge of biology into a functional, molecular definition of the human body and the bodies of other organisms.
Foundations of modern biology
Much of modern biology can be encompassed within five unifying principles: cell theory, evolution, genetics, homeostasis, and energy.
[2]
Cell theory
Main article:
Cell theory states that the
cell is the fundamental unit of
life, and that all living things are composed of one or more cells or the
secreted products of those cells (e.g.
shells). All cells arise from other cells through
cell division. In
multicellular organisms, every cell in the organism's body derives ultimately from a single cell in a fertilized
egg. The cell is also considered to be the basic unit in many pathological processes.
[15] Additionally, the phenomenon of
energy flow occurs in cells in processes that are part of the function known as
metabolism. Finally, cells contain hereditary information (
DNA) which is passed from cell to cell during cell division.
Evolution
A central organizing concept in biology is that life changes and develops through
evolution, and that all life-forms known have a
common origin. Introduced into the scientific lexicon by
Jean-Baptiste de Lamarck in 1809,
[16] evolution was established by
Charles Darwin fifty years later as a viable theory when he articulated its driving force:
natural selection.
[17][18] (
Alfred Russel Wallace is recognized as the co-discoverer of this concept as he helped research and experiment with the concept of evolution.)
[19] Evolution is now used to explain the great variations of life found on Earth.
Darwin theorized that species and breeds developed through the processes of
natural selection and
artificial selection or
selective breeding.
[20] Genetic drift was embraced as an additional mechanism of evolutionary development in the
modern synthesis of the theory.
[21]
The evolutionary history of the
species—which describes the characteristics of the various species from which it descended—together with its genealogical relationship to every other species is known as 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.
[22] Biologists organize and analyze evolutionary relationships through various methods, including
phylogenetics,
phenetics, and
cladistics. (For a summary of major events in the evolution of life as currently understood by biologists, see
evolutionary timeline.)
The theory of evolution postulates that all
organisms on the
Earth, both living and extinct, have descended from a common ancestor or an ancestral
gene pool. This last universal common ancestor of all organisms is believed to have appeared about
3.5 billion years ago.
[23] Biologists generally regard the universality and ubiquity of the
genetic code as definitive evidence in favor of the theory of universal common descent for all
bacteria,
archaea, and
eukaryotes (see:
origin of life).
[24]
Genetics
A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms
are the primary units of inheritance in all organisms. A
gene is a unit of
heredity and corresponds to a region of
DNA that influences the form or function of an organism in specific ways. All organisms, from bacteria to animals, share the same basic machinery that copies and translates DNA into
proteins. Cells
transcribe a DNA gene into an
RNA version of the gene, and a
ribosome then
translates the RNA into a protein, a sequence of
amino acids. The
translation code from RNA codon to amino acid is the same for most organisms, but slightly different for some. For example, a sequence of DNA that codes for
insulin in humans also codes for insulin when inserted into other organisms, such as plants.
[25][26]
DNA usually occurs as linear
chromosomes in
eukaryotes, and circular chromosomes in
prokaryotes. A chromosome is an organized structure consisting of
DNA and
histones. The set of chromosomes in a cell and any other hereditary information found in the
mitochondria,
chloroplasts, or other locations is collectively known as its
genome. In eukaryotes, genomic DNA is located in the
cell nucleus, along with small amounts in
mitochondria and
chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the
nucleoid.
[27] The genetic information in a genome is held within genes, and the complete assemblage of this information in an organism is called its
genotype.
[28]
Homeostasis
Main article:
Homeostasis
The
hypothalamus secretes
CRH, which directs the
pituitary gland to secrete
ACTH. In turn, ACTH directs the adrenal cortex to secrete
glucocorticoids, such as
cortisol. The GCs then reduce the rate of secretion by the hypothalamus and the pituitary gland once a sufficient amount of GCs has been released.
[29] is the ability of an
open system to regulate its internal environment to maintain stable conditions by means of multiple
dynamic equilibrium adjustments controlled by interrelated regulation mechanisms. All living
organisms, whether
unicellular or
multicellular, exhibit homeostasis.
[30]
To maintain dynamic equilibrium and effectively carry out certain functions, a system must detect and respond to perturbations. After the detection of a perturbation, a biological system normally respond through
negative feedback. This means stabilizing conditions by either reducing or increasing the activity of an organ or system. One example is the release of
glucagon when sugar levels are too low.
Energy
The survival of a living organism depends on the continuous input of
energy. Chemical reactions that are responsible for its structure and function are tuned to extract
energy from substances that act as its food and transform them to help form new cells and sustain them. In this process,
molecules of
chemical substances that constitute
food play two roles; first, they contain energy that can be transformed for biological
chemical reactions; second, they develop new molecular structures made up of biomolecules.
The organisms responsible for the introduction of energy into an ecosystem are known as producers or
autotrophs. Nearly all of these organisms originally draw energy from the sun.
[31] Plants and other
phototrophs use solar energy via a process known as
photosynthesis to convert raw materials into organic molecules, such as
ATP, whose bonds can be broken to release energy.
[32] A few
ecosystems, however, depend entirely on energy extracted by
chemotrophs from
methane,
sulfides, or other non-
luminal energy sources.
[33]
Some of the captured energy is used to produce
biomass to sustain
life and provide energy for growth and development. The majority of the rest of this energy is lost as heat and waste molecules. The most important processes for converting the energy trapped in chemical substances into energy useful to sustain life are
metabolism[34] and
cellular respiration.
Structural
Schematic of typical animal
cell depicting the various
organelles and structures.
is the study of biology at a molecular level.
[36] This field overlaps with other areas of biology, particularly with
genetics and
biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interrelationship of DNA, RNA, and protein synthesis and learning how these interactions are regulated.
Cell biology studies the structural and
physiological properties of
cells, including their
behaviors, interactions, and
environment. This is done on both the
microscopic and
molecular levels, for single-celled organisms such as
bacteria as well as the specialized cells in multicellular organisms such as
humans. Understanding the structure and function of cells is fundamental to all of the biological sciences. The similarities and differences between cell types are particularly relevant to molecular biology.
Anatomy considers the forms of macroscopic structures such as
organs and organ systems.
[37]
Genetics is the science of
genes,
heredity, and the variation of
organisms.
[38][39] Genes encode the information necessary for synthesizing proteins, which in turn play a large role in influencing (though, in many instances, not completely determining) the final
phenotype of the organism. In modern research, genetics provides important tools in the investigation of the function of a particular gene, or the 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.
Developmental biology studies the process by which organisms grow and develop. Originating in
embryology, modern developmental biology studies the genetic control of
cell growth,
differentiation, and "
morphogenesis," which is the process that progressively gives rise to
tissues,
organs, and
anatomy.
Model organisms for developmental biology include the round worm
Caenorhabditis elegans,[40] the fruit fly
Drosophila melanogaster,[41] the zebrafish
Danio rerio,[42] the mouse
Mus musculus,[43], and the weed
Arabidopsis thaliana.
[44][45] (A model organism is a
species that is extensively studied to understand particular biological
phenomena, with the expectation that discoveries made in that organism provide insight into the workings of other organisms.)
[46]
Physiological
Physiology studies the mechanical, physical, and biochemical processes of living organisms by attempting to understand how all of 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 some principles of physiology are universal, no matter what particular organism is being studied. For example, what is learned about the physiology of yeast cells can also apply to human cells. The field of animal physiology extends the tools and methods of human physiology to non-human species. Plant physiology borrows techniques from both research fields.
Physiology studies how for example nervous, immune, endocrine, respiratory, and circulatory systems, function and interact. The study of these systems is shared with medically oriented disciplines such as neurology and immunology.
Evolutionary
Evolutionary research is concerned with the origin and descent of species, as well as their change over time, and includes scientists from many taxonomically oriented disciplines. For example, it generally involves scientists who have special training in particular organisms such as mammalogy, ornithology, botany, or herpetology, but use those organisms as systems to answer general questions about evolution.
Evolutionary biology is partly based on paleontology, which uses the fossil record to answer questions about the mode and tempo of evolution,[47] and partly on the developments in areas such as population genetics[48] and evolutionary theory. In the 1980s, developmental biology re-entered evolutionary biology from its initial exclusion from the modern synthesis through the study of evolutionary developmental biology.[49] Related fields often considered part of evolutionary biology are phylogenetics, systematics, and taxonomy.
Systematics
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.
Main article:
Systematics
Traditionally, living things have been divided into five kingdoms: Monera; Protista; Fungi; Plantae; Animalia.[56]
However, many scientists now consider this five-kingdom system outdated. Modern alternative classification systems generally begin with the three-domain system: Archaea (originally Archaebacteria); Bacteria (originally Eubacteria); Eukaryota (including protists, fungi, plants, and animals)[57] These domains reflect whether the cells have nuclei or not, as well as differences in the chemical composition of the cell exteriors.[57]
Further, each kingdom is broken down recursively until each species is separately classified. The order is: Domain; Kingdom; Phylum; Class; Order; Family; Genus; Species.
There is also a series of intracellular parasites that are "on the edge of life"[58] in terms of metabolic activity, meaning that many scientists do not actually classify these structures as alive, due to their lack of at least one or more of the fundamental functions that define life. They are classified as viruses, viroids, prions, or satellites.
The scientific name of an organism is generated from its genus and species. For example, humans are listed as Homo sapiens. Homo is the genus, and sapiens the species. When writing the scientific name of an organism, it is proper to capitalize the first letter in the genus and put all of the species in lowercase. Additionally, the entire term may be italicized or underlined.[59][60]
The dominant classification system is called the Linnaean taxonomy. It 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 merging draft, BioCode, was published in 1997 in an attempt to standardize nomenclature in these three areas, but has yet to be formally adopted.[61] The BioCode draft has received little attention since 1997; its originally planned implementation date of January 1, 2000, has passed unnoticed. However, a 2004 paper concerning the cyanobacteria does advocate a future adoption of a BioCode and interim steps consisting of reducing the differences between the codes.[62] The International Code of Virus Classification and Nomenclature (ICVCN) remains outside the BioCode.
Ecology
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 clown fish from its predators
Ecological systems are studied at several different levels, from individuals and populations to ecosystems and the biosphere. The term population biology is often used interchangeably with population ecology, although population biology is more frequently used when studying diseases, viruses, and microbes, while population ecology is more commonly when studying plants and animals. As can be surmised, ecology is a science that draws on several disciplines.
Ethology studies animal behavior (particularly that of social animals such as primates and canids), and is sometimes considered a branch of zoology. Ethologists have been particularly concerned with the evolution of behavior and the understanding of behavior in terms of the theory of natural selection. In one sense, the first modern ethologist was Charles Darwin, whose book, The Expression of the Emotions in Man and Animals, influenced many ethologists to come.[65]
Biogeography studies the spatial distribution of organisms on the Earth,[66] focusing on topics like plate tectonics, climate change, dispersal and migration, and cladistics.
Branches of biology
These are the main branches of biology:[67][68]
- Aerobiology — the study of airborne organic particles
- Agriculture — the study of producing crops from the land, with an emphasis on practical applications
- Anatomy — the study of form and function, in plants, animals, and other organisms, or specifically in humans
- Astrobiology- the study of evolution, distribution, and future of life in the universe—also known as exobiology, exopaleontology, and bioastronomy
- Biochemistry — the study of the chemical reactions required for life to exist and function, usually a focus on the cellular level
- Bioengineering — the study of biology through the means of engineering with an emphasis on applied knowledge and especially related to biotechnology
- Bioinformatics — the use of information technology for the study, collection, and storage of genomic and other biological data
- Biomathematics or Mathematical Biology — the quantitative or mathematical study of biological processes, with an emphasis on modeling
- Biomechanics — often considered a branch of medicine, the study of the mechanics of living beings, with an emphasis on applied use through prosthetics or orthotics
- Biomedical research — the study of the human body in health and disease
- Biophysics — the study of biological processes through physics, by applying the theories and methods traditionally used in the physical sciences
- Biotechnology — a new and sometimes controversial branch of biology that studies the manipulation of living matter, including genetic modification and synthetic biology
- Building biology — the study of the indoor living environment
- Botany — the study of plants
- Cell biology — the study of the cell as a complete unit, and the molecular and chemical interactions that occur within a living cell
- Conservation Biology — the study of the preservation, protection, or restoration of the natural environment, natural ecosystems, vegetation, and wildlife
- Cryobiology — the study of the effects of lower than normally preferred temperatures on living beings.
- Developmental biology — the study of the processes through which an organism forms, from zygote to full structure
- Ecology — the study of the interactions of living organisms with one another and with the non-living elements of their environment
- Embryology — the study of the development of embryo (from fecundation to birth). See also topobiology.
- Entomology — the study of insects
- Environmental Biology — the study of the natural world, as a whole or in a particular area, especially as affected by human activity
- Epidemiology — a major component of public health research, studying factors affecting the health of populations
- Ethology — the study of animal behavior
- Evolutionary Biology — the study of the origin and descent of species over time
- Genetics — the study of genes and heredity
- Herpetology — the study of reptiles and amphibians
- Histology — the study of cells and tissues, a microscopic branch of anatomy
- Ichthyology — the study of fish
- Integrative biology — the study of whole organisms
- Limnology — the study of inland waters
- Mammalogy — the study of mammals
- Marine Biology — the study of ocean ecosystems, plants, animals, and other living beings
- Microbiology — the study of microscopic organisms (microorganisms) and their interactions with other living things
- Molecular Biology — the study of biology and biological functions at the molecular level, some cross over with biochemistry
- Mycology — the study of fungi
- Neurobiology — the study of the nervous system, including anatomy, physiology and pathology
- Oceanography — the study of the ocean, including ocean life, environment, geography, weather, and other aspects influencing the ocean
- Oncology — the study of cancer processes, including virus or mutation oncogenesis, angiogenesis and tissues remoldings
- Ornithology — the study of birds
- Population biology — the study of groups of conspecific organisms, including
- Paleontology — the study of fossils and sometimes geographic evidence of prehistoric life
- Pathobiology or pathology — the study of diseases, and the causes, processes, nature, and development of disease
- Parasitology — the study of parasites and parasitism
- Pharmacology — the study and practical application of preparation, use, and effects of drugs and synthetic medicines
- Physiology — the study of the functioning of living organisms and the organs and parts of living organisms
- Phytopathology — the study of plant diseases (also called Plant Pathology)
- Psychobiology — the study of the biological bases of psychology
- Sociobiology — the study of the biological bases of sociology
- Structural biology — a branch of molecular biology, biochemistry, and biophysics concerned with the molecular structure of biological macromolecules
- Virology — the study of viruses and some other virus-like agents
- Zoology — the study of animals, including classification, physiology, development, and behavior (See also Entomology, Ethology, Herpetology, Ichthyology, Mammalogy, and Ornithology)
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