Development of the Theory of Evolution | The Modern View of the Age of the Earth | Development of the Modern View of Evolution
Darwinian Evolution | The Diversity of Life | Characteristics of Living things | Levels of Organization | Learning Objectives
Terms | Review Questions | Links | References
Biology literally means "the study of life". Biology is such a broad field, covering the minute workings of chemical machines inside our cells, to broad scale concepts of ecosystems and global climate change. Biologists study intimate details of the human brain, the composition of our genes, and even the functioning of our reproductive system. Biologists recently all but completed the deciphering of the human genome, the sequence of deoxyribonucleic acid (DNA) bases that may determine much of our innate capabilities and predispositions to certain forms of behavior and illnesses. DNA sequences have played major roles in criminal cases (O.J. Simpson, as well as the reversal of death penalties for many wrongfully convicted individuals), as well as the impeachment of President Clinton (the stain at least did not lie). We are bombarded with headlines about possible health risks from favorite foods (Chinese, Mexican, hamburgers, etc.) as well as the potential benefits of eating other foods such as cooked tomatoes. Informercials tout the benefits of metabolism-adjusting drugs for weight loss. Many Americans are turning to herbal remedies to ease arthritis pain, improve memory, as well as improve our moods.
Can a biology book give you the answers to these questions? No, but it will enable you learn how to sift through the biases of investigators, the press, and others in a quest to critically evaluate the question. To be honest, five years after you are through with this class it is doubtful you would remember all the details of meatbolism. However, you will know where to look and maybe a little about the process of science that will allow you to make an informed decision. Will you be a scientist? Yes, in a way. You may not be formally trained as a science major, but you can think critically, solve problems, and have some idea about what science can and cannoit do. I hope you will be able to tell the shoe from the shinola.
Science is an objective, logical, and repeatable attempt to understand the principles and forces operating in the natural universe. Science is from the Latin word, scientia, to know. Good science is not dogmatic, but should be viewed as an ongoing process of testing and evaluation. One of the hoped-for benefits of students taking a biology course is that they will become more familiar with the process of science.
Humans seem innately interested in the world we live in. Young children drive their parents batty with constant "why" questions. Science is a means to get some of those whys answered. When we shop for groceries, we are conducting a kind of scientific experiment. If you like Brand X of soup, and Brand Y is on sale, perhaps you try Brand Y. If you like it you may buy it again, even when it is not on sale. If you did not like Brand Y, then no sale will get you to try it again.
In order to conduct science, one must know the rules of the game (imagine playing Monopoly and having to discover the rules as you play! Which is precisely what one does with some computer or videogames (before buying the cheatbook). The scientific method is to be used as a guide that can be modified. In some sciences, such as taxonomy and certain types of geology, laboratory experiments are not necessarily performed. Instead, after formulating a hypothesis, additional observations and/or collections are made from different localities.
Steps in the scientific method commonly include:
After a hypothesis has been repeatedly tested, a hierarchy of scientific thought develops. Hypothesis is the most common, with the lowest level of certainty. A theory is a hypothesis that has been repeatedly tested with little modification, e.g. The Theory of Evolution. A Law is one of the fundamental underlying principles of how the Universe is organized, e.g. The Laws of Thermodynamics, Newton's Law of Gravity. Science uses the word theory differently than it is used in the general population. Theory to most people, in general nonscientific use, is an untested idea. Scientists call this a hypothesis.
Scientific experiments are also concerned with isolating the variables. A good science experiment does not simultaneously test several variables, but rather a single variable that can be measured against a control. Scientific controlled experiments are situations where all factors are the same between two test subjects, except for the single experimental variable.
Consider a commonly conducted science fair experiment. Sandy wants to test the effect of gangsta rap music on pea plant growth. She plays loud rap music 24 hours a day to a series of pea plants grown under light, and watered every day. At the end of her experiment she concludes gangsta rap is conducive to plant growth. Her teacher grades her project very low, citing the lack of a control group for the experiment. Sandy returns to her experiment, but this time she has a separate group of plants under the same conditions as the rapping plants, but with soothing Led Zeppelin songs playing. She comes to the same conclusion as before, but now has a basis for comparison. Her teacher gives her project a better grade.
Modern biology is based on several great ideas, or theories:
Robert Hooke (1635-1703), one of the first scientists to use a microscope to examine pond water, cork and other things, referred to the cavities he saw in cork as "cells", Latin for chambers. Mattias Schleiden (in 1838) concluded all plant tissues consisted of cells. In 1839, Theodore Schwann came to a similar conclusion for animal tissues. Rudolf Virchow, in 1858, combined the two ideas and added that all cells come from pre-existing cells, formulating the Cell Theory. Thus there is a chain-of-existence extending from your cells back to the earliest cells, over 3.5 billion years ago. The cell theory states that all organisms are composed of one or more cells, and that those cells have arisen from pre-existing cells.
Figure 1. James Watson (L) and Francis Crick (R), and the model they built of the structure of deoxyribonucleic acid, DNA. While a model may seem a small thing, their development of the DNA model fostered increased understanding of how genes work. Image from the Internet.
In 1953, American scientist James Watson and British scientist Francis Crick developed the model for deoxyribonucleic acid (DNA), a chemical that had (then) recently been deduced to be the physical carrier of inheritance. Crick hypothesized the mechanism for DNA replication and further linked DNA to proteins, an idea since referred to as the central dogma. Information from DNA "language" is converted into RNA (ribonucleic acid) "language" and then to the "language" of proteins. The central dogma explains the influence of heredity (DNA) on the organism (proteins).
Homeostasis is the maintainence of a dynamic range of conditions within which the organism can function. Temperature, pH, and energy are major components of this concept. Theromodynamics is a field of study that covers the laws governing energy transfers, and thus the basis for life on earth. Two major laws are known: the conservation of matter and energy, and entropy. These will be discussed in more detail in a later chapter. The universe is composed of two things: matter (atoms, etc.) and energy.
These first three theories are very accepted by scientists and the general public. The theory of evolution is well accepted by scientists and most of the general public. However, it remains a lightening rod for school boards, politicians, and television preachers. Much of this confusion results from what the theory says and what it does not say.
Modern biology is based on several unifying themes, such as the cell theory, genetics and inheritance, Francis Crick's central dogma of information flow, and Darwin and Wallace's theory of evolution by natural selection. In this first unit we will examine these themes and the nature of science.
The Ancient Greek philosopher Anaxiamander (611-547 B.C.) and the Roman philosopher Lucretius (99-55 B.C.) coined the concept that all living things were related and that they had changed over time. The classical science of their time was observational rather than experimental. Another ancient Greek philosopher, Aristotle developed his Scala Naturae, or Ladder of Life, to explain his concept of the advancement of living things from inanimate matter to plants, then animals and finally man. This concept of man as the "crown of creation" still plagues modern evolutionary biologists (See Gould, 1989, for a more detailed discussion).
Post-Aristotlean "scientists" were constrained by the prevailing thought patterns of the Middle Ages -- the inerrancy of the biblical book of Genesis and the special creation of the world in a literal six days of the 24-hour variety. Archbishop James Ussher of Ireland, in the late 1600's calculated the age of the earth based on the geneologies from Adam and Eve listed in the biblical book of Genesis. According to Ussher's calculations, the earth was formed on October 22, 4004 B.C. These calculations were part of Ussher's book, History of the World. The chronology he developed was taken as factual, and was even printed in the front pages of bibles. Ussher's ideas were readily accepted, in part because they posed no threat to the social order of the times; comfortable ideas that would not upset the linked applecarts of church and state.
Figure 2. Archbishop James Ussher. Image from the Internet.
Often new ideas must "come out of left field", appearing as wild notions, but in many cases prompting investigation which may later reveal the "truth". Ussher's ideas were comfortable, the Bible was viewed as correct, therefore the earth must be only 5000 years old.
Geologists had for some time doubted the "truth" of a 5,000 year old earth. Leonardo da Vinci (painter of the Last Supper, and the Mona Lisa, architect and engineer) calculated the sedimentation rates in the Po River of Italy. Da Vinci concluded it took 200,000 years to form some nearby rock deposits. Galileo, convicted heretic for his contention that the Earth was not the center of the Universe, studied fossils (evidence of past life) and concluded that they were real and not inanimate artifacts. James Hutton, regarded as the Father of modern geology, developed the Theory of Uniformitarianism, the basis of modern geology and paleontology. According to Hutton's work, certain geological processes operated in the past in much the same fashion as they do today, with minor exceptions of rates, etc. Thus many geological structures and processes cannot be explained if the earth was only a mere 5000 years old.
Radiometric age assignments based on the rates of decay of radioactive isotopes, not discovered until the late 19th century, suggest the earth is over 4.5 billion years old. The Earth is thought older than 4.5 billion years, with the oldest known rocks being 3.96 billion years old. Geologic time divides into eons, eroas, and smaller units. An overview of geologic time may be obtained at http://www.ucmp.berkeley.edu/help/timeform.html.
Figure 3. The geologic time scale,
hilighting some of the firsts in the evolution of life. One
way to represent geological time. Note the break during the
precambrian. If the vertical scale was truly to scale the
precambrian would account for 7/8 of the graphic.
This image is from http://www.clearlight.com/~mhieb/WVFossils/GeolTimeScale.html.
Figure 3. The geologic time scale, hilighting some of the firsts in the evolution of life. One way to represent geological time. Note the break during the precambrian. If the vertical scale was truly to scale the precambrian would account for 7/8 of the graphic. This image is from http://www.clearlight.com/~mhieb/WVFossils/GeolTimeScale.html.
Erasmus Darwin (1731-1802; grandfather of Charles Darwin) a British physician and poet in the late 1700's, proposed that life had changed over time, although he did not present a mechanism. Georges-Louis Leclerc, Comte de Buffon (pronounced Bu-fone; 1707-1788) in the middle to late 1700's proposed that species could change. This was a major break from earlier concepts that species were created by a perfect creator and therefore could not change because they were perfect, etc.
Swedish botanist Carl Linne (more popularly known as Linneus, after the common practice of the day which was to latinize names of learned men), attempted to pigeon-hole all known species of his time (1753) into immutable categories. Many of these categories are still used in biology, although the underlying thought concept is now evolution and not immutability of species. Linnean hierarchical classification was based on the premise that the species was the smallest unit, and that each species (or taxon) belonged to a higher category.
Linneus also developed the concept of binomial nomenclature, whereby scientists speaking and writing different languages could communicate clearly. For example Man in English is Hombre in Spanish, Mensch in German, and Homo in Latin. Linneus settled on Latin, which was the language of learned men at that time. If a scientist refers to Homo, all scientists know what he or she means.
William "Strata" Smith (1769-1839), employed by the English coal mining industry, developed the first accurate geologic map of England. He also, from his extensive travels, developed the Principle of Biological Succession. This idea states that each period of Earth history has its own unique assemblages of fossils. In essence Smith fathered the science of stratigraphy, the correlation of rock layers based on (among other things) their fossil contents. He also developed an idea that life had changed over time, but did not overtly state that.
Abraham Gottlob Werner and Baron Georges Cuvier (1769-1832) were among the foremost proponents of catastrophism, the theory that the earth and geological events had formed suddenly, as a result of some great catastrophe (such as Noah's flood). This view was a comfortable one for the times and thus was widely accepted. Cuvier eventually proposed that there had been several creations that occurred after catastrophies. Louis Agassiz (1807-1873) proposed 50-80 catastrophies and creations.
Jean Baptiste de Lamarck (1744-1829) developed one of the first theories on how species changed. He proposed the inheritance of acquired characteristics to explain, among other things, the length of the giraffe neck. The Lamarckian view is that modern giraffe's have long necks because their ancestors progressively gained longer necks due to stretching to reach food higher and higher in trees. According to the 19th century concept of use and disuse the stretching of necks resulted in their development, which was somehow passed on to their progeny. Today we realize that only bacteria are able to incorporate non-genetic (nonheritable) traits. Lamarck's work was a theory that plainly stated that life had changed over time and provided (albeit an erroneous) mechanism of change.
Additional information about the biological thoughts of Lamarck is available by clicking here.
Charles Darwin, former divinity student and former medical student, secured (through the intercession of his geology professor) an unpaid position as ship's naturalist on the British exploratory vessel H.M.S. Beagle. The voyage would provide Darwin a unique opportunity to study adaptation and gather a great deal of proof he would later incorporate into his theory of evolution. On his return to England in 1836, Darwin began (with the assistance of numerous specialists) to catalog his collections and ponder the seeming "fit" of organisms to their mode of existence. He eventually settled on four main points of a radical new hypothesis:
Figure 4. Charles Darwin (right) and Alfred Wallace (left), the co-developers of the theory of evolution by means of natural selection. Image of Charles Darwin from http://zebu.uoregon.edu/~js/glossary/darwinism.html.Image of A.R. Wallace (right) is modified from http://www.prs.k12.nj.us/schools/phs/science_Dept/APBio/Natural_Selection.html.
Unlike the upper-class Darwin, Alfred Russel Wallace (1823-1913) came from a different social class. Wallace spent many years in South America, publishing salvaged notes in Travels on the Amazon and Rio Negro in 1853. In 1854, Wallace left England to study the natural history of Indonesia, where he contracted malaria. During a fever Wallace managed to write down his ideas on natural selection.
In 1858, Darwin received a letter from Wallace, in which Darwin's as-yet-unpublished theory of evolution and adaptation was precisely detailed. Darwin arranged for Wallace's letter to be read at a scientific meeting, along with a synopsis of his own ideas. To be correct, we need to mention that both Darwin and Wallace developed the theory, although Darwin's major work was not published until 1859 (the book On the Origin of Species by Means of Natural Selection, considered by many as one of the most influential books written [follow the hyperlink to view an online version]). While there have been some changes to the theory since 1859, most notably the incorporation of genetics and DNA into what is termed the "Modern Synthesis" during the 1940's, most scientists today acknowledge evolution as the guiding theory for modern biology.
Recent revisions of biology curricula stressed the need for underlying themes. Evolution serves as such a universal theme. An excellent site devoted to Darwin's thoughts and work is available by clicking here. At that same site is a timeline showing many of the events mentioned above in their historical contexts.
Evolutionary theory and the cell theory provide us with a basis for the interrelation of all living things. We also utilize Linneus' hierarchical classification system, adopting (generally) five kingdoms of living organisms. Viruses, as discussed later, are not considered living. Click here for a table summarizing the five kingdoms. Recent studies suggest that there might be a sixth Kingdom, the Archaea.
Figure 5. A simple phylogenetic representation of three domains of life" Archaea, Bacteria (Eubacteria), and Eukaryota (all eukaryotic groups: Protista, Plantae, Fungi, and Animalia). Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Table 1. The Five Kingdoms. Monera (in the broadest sense,
including organisms usually placed in the Domain
Archaea). Photosynthesis, chemosynthesis,
decomposer, parasitic. Single-celled, filament, or
colony of cells; all prokaryotic. Monerans play various roles in
almost all food chains, including
producer,consumer, and decomposer. Cyanobacteria are important
oxygen producers. Many Monerans also produce
nitrogen, vitamins, antibiotics, and are important
compoents in human and animal
intestines. Bacteria (E. coli),
cyanobacteria (Oscillatoria), methanogens,
and thermacidophiles. Protista Photosynthesis, absorb food from
environment, or trap/engulf smaller
organisms. Single-celled, filamentous,
colonial, and multicelled; all
eukaryotic. Important producers in
ocean/pond food chain. Source of food in some human
cultures. Phytoplankton component that is
one of the major producers of oxygen Plankton (both phytoplankton and
zooplankton), algae (kelp, diatoms,
dinoflagellates),and Protozoa (Amoeba,
Paramecium). Fungi Absorb food from a host or from
their environment. All heterotrophic. Single-celled, filamentous, to
multicelled; all eukaryotic. Decomposer, parasite, and
consumer. Produce antibiotics,help make
bread and alcohol. Crop parasites (Dutch Elm
Disease, Karnal Bunt, Corn Smut, etc.). Mushrooms (Agaricus
campestris, the commercial mushroom), molds,
mildews, rusts and smuts (plant parasites), yeasts
(Saccharomyces cerevisae, the brewer's
yeast). Plantae Almost all photosynthetic,
although a few parasitic plants are known.
All multicelled, photosynthetic,
autotrophs.. Food source, medicines and
drugs, dyes, building material, fuel. Producer in most food
chains. Angiosperms (oaks, tulips,
cacti),gymnosperms (pines, spuce, fir), mosses,
ferns,liverworts, horsetails (Equisetum, the
scouring rush) Animalia All heterotrophic. Multicelled heterotrophs capable
of movement at some stage during their life history
(even couch potatoes). Consumer level in most food
(herbivores,carnivores,omnivores). Food source, beasts of burden
and transportation, recreation, and
companionship. Sponges, worms,molluscs,
insects, starfish,mammals, amphibians,fish, birds,
reptiles, and dinosaurs, and people.
Table 1. The Five Kingdoms.
(in the broadest sense, including organisms usually placed in the Domain Archaea).
Photosynthesis, chemosynthesis, decomposer, parasitic.
Single-celled, filament, or colony of cells; all prokaryotic.
Monerans play various roles in almost all food chains, including producer,consumer, and decomposer.
Cyanobacteria are important oxygen producers.
Many Monerans also produce nitrogen, vitamins, antibiotics, and are important compoents in human and animal intestines.
Bacteria (E. coli), cyanobacteria (Oscillatoria), methanogens, and thermacidophiles.
Photosynthesis, absorb food from environment, or trap/engulf smaller organisms.
Single-celled, filamentous, colonial, and multicelled; all eukaryotic.
Important producers in ocean/pond food chain.
Source of food in some human cultures.
Phytoplankton component that is one of the major producers of oxygen
Plankton (both phytoplankton and zooplankton), algae (kelp, diatoms, dinoflagellates),and Protozoa (Amoeba, Paramecium).
Absorb food from a host or from their environment.
Single-celled, filamentous, to multicelled; all eukaryotic.
Decomposer, parasite, and consumer.
Produce antibiotics,help make bread and alcohol.
Crop parasites (Dutch Elm Disease, Karnal Bunt, Corn Smut, etc.).
Mushrooms (Agaricus campestris, the commercial mushroom), molds, mildews, rusts and smuts (plant parasites), yeasts (Saccharomyces cerevisae, the brewer's yeast).
Almost all photosynthetic, although a few parasitic plants are known.
All multicelled, photosynthetic, autotrophs..
Food source, medicines and drugs, dyes, building material, fuel.
Producer in most food chains.
Angiosperms (oaks, tulips, cacti),gymnosperms (pines, spuce, fir), mosses, ferns,liverworts, horsetails (Equisetum, the scouring rush)
Multicelled heterotrophs capable of movement at some stage during their life history (even couch potatoes).
Consumer level in most food chains (herbivores,carnivores,omnivores).
Food source, beasts of burden and transportation, recreation, and companionship.
Sponges, worms,molluscs, insects, starfish,mammals, amphibians,fish, birds, reptiles, and dinosaurs, and people.
Monera, the most primitive kingdom, contain living organisms remarkably similar to ancient fossils. Organisms in this group lack membrane-bound organelles associated with higher forms of life. Such organisms are known as prokaryotes. Bacteria (technically the Eubacteria) and blue-green bacteria (sometimes called blue-green algae, or cyanobacteria) are the major forms of life in this kingdom. The most primitive group, the archaebacteria, are today restricted to marginal habitats such as hot springs or areas of low oxygen concentration.
Figure 6. Representative photosynthetic cyanobacteria: Oscillatoria (left) and Nostoc (right). The left image is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cyanobacteria/Oscillatoria_130. The right image is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cyanobacteria/Nostoc_130.
Protista were the first of the eukaryotic kingdoms, these organisms and all others have membrane-bound organelles, which allow for compartmentalization and dedication of specific areas for specific functions. The chief importance of Protista is their role as a stem group for the remaining Kingdoms: Plants, Animals, and Fungi. Major groups within the Protista include the algae, euglenoids, ciliates, protozoa, and flagellates.
Figure 7. Scanning electron micrographs of diatoms (Protista).There are two basic types of diatoms: bilaterally symmetrical (left) and radially symmetrical (right). Images are from http://WWW.bgsu.edu/departments/biology/algae/index.html.
Figure 8. Light micrographs of some
protistans. The images are Copyright 1994 by Charles
J. O'Kelly and Tim Littlejohn, used by permission from:
Figure 8. Light micrographs of some protistans. The images are Copyright 1994 by Charles J. O'Kelly and Tim Littlejohn, used by permission from: http://megasun.bch.umontreal.ca/protists/gallery.html.
Fungi are almost entirely multicellular (with yeast, Saccharomyces cerviseae, being a prominent unicellular fungus), heterotrophic (deriving their energy from another organism, whether alive or dead), and usually having some cells with two nuclei (multinucleate, as opposed to the more common one, or uninucleate) per cell. Ecologically this kingdom is important (along with certain bacteria) as decomposers and recyclers of nutrients. Economically, the Fungi provide us with food (mushrooms; Bleu cheese/Roquefort cheese; baking and brewing), antibiotics (the first of the wonder drugs, penicillin, was isolated from a fungus Penicillium), and crop parasites (doing several billion dollars per year of damage).
Figure 9. Examples of fungi. The images are from http://www.cinenet.net/users/velosa/thumbnails.html.
Plantae (click here for more information about the Plantae) include multicelled organisms that are all autotrophic (capable of making their own food by the process of photosynthesis, the conversion of sunlight energy into chemical energy). Ecologically, this kingdom is generally (along with photosynthetic organisms in Monera and Protista) termed the producers, and rest at the base of all food webs. A food web is an ecological concept to trace energy flow through an ecosystem. Economically, this kingdom is unparalleled, with agriculture providing billions of dollars to the economy (as well as the foundation of "civilization"). Food, building materials, paper, drugs (both legal and illegal), and roses, are plants or plant-derived products.
Figure 10. Examples of plants. The left image of species of Equisetum is cropped and reduced from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Fern_Allies/Sphenophyta/Equisetum/E._arvense_and_E._laevigatum_KS. The center image of Iris, is reduced and cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.401/Flowering_Plants/Monocots/Iridaceae/Iris/Iris_pumula_habit. The right image of Pereskia (Cactaceae) is reduced from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.401/Flowering_Plants/Dicots/Cactaceae/Pereskia/Pereskia_leafy_stem_RK.
Animalia consists entirely of multicelluar heterotrophs that are all capable (at some point during their life history) of mobility. Ecologically, this kingdom occupies the level of consumers, which can be subdivided into herbivore (eaters of plants) and carnivores (eaters of other animals). Humans, along with some other organisms, are omnivores (capable of functioning as herbivores or carnivores). Economically, animals provide meat, hides, beasts of burden, pleasure (pets), transportation, and scents (as used in some perfumes).
Figure 11. Examples of animals. The left image of a jellyfish is from http://www.smoky.org/~mtyler/bio/coelenterata.html. The center image of a tree frog is from http://frog.simplenet.com/froggy/images/wild28.gif. The right image of the chimpanzee is from http://www.selu.com/~bio/PrimateGallery/art/Copyright_Free02.html.
Living things have a variety of common characteristics.
Biosphere: The sum of all living things taken in conjunction with their environment. In essence, where life occurs, from the upper reaches of the atmosphere to the top few meters of soil, to the bottoms of the oceans. We divide the earth into atmosphere (air), lithosphere (earth), hydrosphere (water), and biosphere (life).
Ecosystem: The relationships of a smaller groups of organisms with each other and their environment. Scientists often speak of the interrelatedness of living things. Since, according to Darwin's theory, organisms adapt to their environment, they must also adapt to other organisms in that environment. We can discuss the flow of energy through an ecosystem from photosynthetic autotrophs to herbivores to carnivores.
Community: The relationships between groups of different species. For example, the desert communities consist of rabbits, coyotes, snakes, birds, mice and such plants as sahuaro cactus (Carnegia gigantea), Ocotillo, creosote bush, etc. Community structure can be disturbed by such things as fire, human activity, and over-population.
Species: Groups of similar individuals who tend to mate and produce viable, fertile offspring. We often find species described not by their reproduction (a biological species) but rather by their form (anatomical or form species).
Populations: Groups of similar individuals who tend to mate with each other in a limited geographic area. This can be as simple as a field of flowers, which is separated from another field by a hill or other area where none of these flowers occur.
Individuals: One or more cells characterized by a unique arrangement of DNA "information". These can be unicellular or multicellular. The multicellular individual exhibits specialization of cell types and division of labor into tissues, organs, and organ systems.
Organ System: (in multicellular organisms). A group of cells, tissues, and organs that perform a specific major function. For example: the cardiovascular system functions in circulation of blood.
Organ: (in multicellular organisms). A group of cells or tissues performing an overall function. For example: the heart is an organ that pumps blood within the cardiovascular system.
Tissue: (in multicellular organisms). A group of cells performing a specific function. For example heart muscle tissue is found in the heart and its unique contraction properties aid the heart's functioning as a pump. .
Cell: The fundamental unit of living things. Each cell has some sort of hereditary material (either DNA or more rarely RNA), energy acquiring chemicals, structures, etc. Living things, by definition, must have the metabolic chemicals plus a nucleic acid hereditary information molecule.
Organelle: A subunit of a cell, an organelle is involved in a specific subcellular function, for example the ribosome (the site of protein synthesis) or mitochondrion (the site of ATP generation in eukaryotes).
Molecules, atoms, and subatomic particles: The fundamental functional levels of biochemistry.
Figure 12. Organization levels of life,
in a graphic format. Images from Purves et al.,
Life: The Science of Biology, 4th Edition, by Sinauer
and WH Freeman (www.whfreeman.com),
used with permission.
Figure 12. Organization levels of life, in a graphic format. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
It is thus possible to study biology at many levels, from collections of organisms (communities), to the inner workings of a cell (organelle).
Darwin, Charles R. 1859 On the Origin of Species by Means of Natural Selection. Available at http://www.literature.org/Works/Charles-Darwin/origin/
Gould, Stephen J. 1989 Wonderful Life.
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