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Properties of Life

Every living organism possesses certain characteristics in common; every organism has a certain chemical makeup, must obtain and use energy, responds to environmental stimuli, is able to grow and reproduce, must regulate internal functions, and is capable of adapting to certain environmental changes. Different texts list these characteristics in different ways and order. Life is extremely diverse, with an estimated 20 million or more species (mostly insects), divided into six different kingdoms. Representatives of each are shown below.


1. Hierarchical Organization 

All organisms consist of one or more cells containing highly ordered structures. Complex chemical structure is determined by DNA. This complex molecule contains coded information which directs the manufacture of proteins (which are themselves very complex molecules). This code is determined by sequences of three of the gray portions shown in the center of the illustration below. Just as sequences of three combinations of dots and/or dashes can denote certain letters of the alphabet in Morse Code, each sequence in the DNA code represents a particular building block of a protein. As an example of the amount of DNA, human chromosome 14 (one of the shorter chromosomes) contains 87,410,661 pairs of bases, and consists of 1,050 genes, of which over 60 are known to cause diseases, including one linked to early Alzheimer disease. As of January 2003, this is the longest contiguous piece of human DNA that has been sequenced. 

In addition to DNA and proteins, all organisms contain certain lipids (fats), and carbohydrates (sugars and starches). (See The Science of Biology)

A human karyotype showing the 23 pairs of chromosomes, which are made of long strands of DNA wrapped around proteins. The color patterns on these represent similar genes on each chromosome.

2. Sensitivity (often listed as Irritability or Responsiveness)

All organisms respond to certain stimuli (both external and internal), such as light, the pull of gravity, electrical fields, touch, pain, pressure, sound, certain chemicals 

3. Growth, Development, and Reproduction

All organisms can become larger by growing, and/or producing new cells, and passing hereditary information (DNA) on to their offspring; reproduction of new individuals can occur through asexual or sexual methods; multicellular organisms go through various mechanisms of development from a single cell to many, usually specialized cells.

4. Regulation and Homeostasis

All organisms are capable of regulating their internal environment by supplying cells with nutrients, respiratory gases, eliminating wastes, and transporting many substances throughout the body.

5. Obtaining and using energyMetabolism 

All organisms must be able to obtain and use energy from an outside source. Metabolism is the total of all chemical activities within the cells and/or body, and thousands of different reactions continually occur in different cells. Almost all of these reactions use water in the reactions.

6. Adaptation and Evolution - Adaptation is used as a noun and as a verb. An adaptation is a characteristic that allows an organism to better survive in its environment, and when genetic mutations occur that promote survival (and reproduction), the organism is said to have adapted to its environment. As individuals adapt, survive, and reproduce, their genetic makeup (DNA) spreads throughout the population. Populations evolve, not individuals. Evolution is defined as changes in gene frequencies within populations over a period of time (and/or location). This process is usually extremely slow, and depends upon changes in the environment. However, some organisms can undergo doubling or other changes in chromosome numbers (polyploidy), and a new species can appear in one generation. The example below is one that I am very familiar with, in which chromosome numbers doubled, creating a new species of Gray Treefrog. These two species look identical, but they cannot produce viable offspring, due to failure of hybrid survival beyond early stages. Hyla chrysoscelis, the Southern Gray Treefrog, is diploid (with 24 chromosomes and a high trill-rate for a call), while Hyla versicolor, the Northern Gray Treefrog) is tetraploid (with 48 chromosomes and a slow trill-rate).

Burkett, Ray D. 1989. Status of diploid-tetraploid gray treefrogs ( Hyla chrysoscelis-Hyla versicolor) in the Mid-south. Pp. 51-57 in: Scott, A. F. (ed), Proc. of the contributed papers session of the second annual symposium on the natural history of lower Tennessee and Cumberland River valleys. The Center for Field Biology, Austin Peay State University, Clarksville, TN.    

Although the belief had been questioned for some years, it was generally believed in the early 19th Century that species of organisms were immutable, that is (in spite of observed variations) they couldn't change. In 1838, Charles Darwin began a five-year voyage on H. M. S. Beagle, in which he served as the ship's naturalist. He was hired by Captain Fitzroy to prove the inerrancy of the creation stories in Genesis. What Darwin learned on this voyage led to the theory that forms the core of the science of biology (and that has allowed for the development of modern medicine). At the same time as Darwin was making his discoveries, Alfred Russell Wallace was making the same discoveries in Malaysia (in the area east of the label below for Keeling Islands). However, Darwin published his book in 1859, before Wallace finished his manuscript.  

The evidence that Darwin used to develop his theory was based upon several lines of inquiry. Charles Lyell, in his Principles of Geology (1830), showed that the earth was much older than previously believed. Enormous amounts of time were needed for evolution to have occurred. Geographical distribution of plants and animals indicated more diversity than could be accounted for by climate and environmental differences. Life on each continent was distinctive, and was similar, but different from life on islands near each continent. Oceanic species show more variation that that found on continents, indicating more rapid change. Darwin was able to observe differences from many groups of islands (see map of his voyage). Endemic species unique to each island group. Darwin did not draw his conclusions until a few years after his voyage. Some of his major lines of evidence came from the huge variety of farm plants and animals that had been developed by artificial selection.

Fossil evidence of evolution is illustrated here by the extinct glyptodont and the modern armadillo, both found in South America by Darwin.


Also see understand the environment (which includes more detail about Charles Darwin and the Galapagos Islands.

History of life: The earliest indication of life is indirect - Iron oxides and other oxides, which indicate the presence of free oxygen in the atmosphere, date back about 3.5 billion years. Free oxygen is only produced through the process of photosynthesis, indicating that photosynthetic bacteria were present at that time. At the beginning of the Cambrian Period (about 600 million years ago), almost all phyla of animals appeared. Five great prehistoric extinctions have occurred since then: 1) Ordovician (440 mya) - 25% of families extinct; 2) Devonian (370 mya) - 19% of families lost; 3) Permian (250 mya) - 54% of families lost; 4) Triassic (210 mya) - 23% of families lost; 5) Cretaceous (65 mya) - 17% of families lost, including dinosaurs. We are now at the beginning of a 6th major extinction, this time caused by humans. So far, beetles, amphibians, birds, and large mammals have suffered the most. The extinction of a majority of the earth's life forms over millions of years does not present any evidence of intelligent design; instead, random mutation has abundant evidence to support it.

Evidence for evolutionary change comes from numerous sources, some of which are illustrated below:  Evidence from homologous structures (similar structures having a similar origin), but not necessarily a similar function (analogy) is shown here in vertebrate front limbs.

Biochemical evidence in the form of DNA and serology (comparison of blood chemistry) links modern whales to hoofed mammals. Also, evidence from vestigial organs (such as rudimentary pelvic bones in whales and in boas and pythons), indicates that these structures were once different. In humans, the appendix and the nictitating membrane (in the inner corner of the eye) are vestigial organs.

Fossils occur in layers of rock, with the youngest usually on top, and the oldest in deeper layers. On occasion, buckling of rocks can reverse these layers in relatively small areas, but radioactive dating methods can help to identify the proper sequence of ages.

Evolutionary change in horses over the last 60 million years.

Over a period of 12 million years during the Jurassic Period, the shells of this group of oysters became larger, thinner and flatter.

Embryological evidence of a common ancestor.  All of these also possess a dorsal, hollow nerve cord, a notochord (stiffening rod of cartilage) in the back, similar, membranes in the embryos, yolk sac that produces the first blood cells and germ cells, and similar development of many organs.

Evidence from artificial selection by man. The tassels and seeds of a wild grass, shown on the left, became the male tassels and female ears of modern corn, which has been selected for stalk and seed size, as well as chemical and oil content, and color.

Biochemical evidence: Differences in amino acids from humans in vertebrate hemoglobin polypeptides.

Animals that are "ecological equivalents" from different continents. These animals live in similar habitats, and have similar habits. The ecological "niche" can be thought of as the "job" of an animal.

Documented record of natural selection in the peppered moth (See Raven & Johnson, Biology, 6Ed. p. 446). Research of Bernard Kettlewell. This study illustrates that a mutation must be present before the environment changes (illustrated by melanism preceding pollution by soot from the burning of soft coal, in England). Predators (birds) eliminated most of the conspicuous moths in each location, resulting in adaptive changes to match the environment in each location.

Evidence from geographic variation. These are all considered to be the same species, but different geographic variants (subspecies), a common occurrence in widespread species. A similar situation exists in racers (Coluber constrictor), with the blue racer, black racer, and black-lipped racer all found in Shelby County, TN. In east Texas and western Louisiana, a similar situation exists between the blue racer, the buttermilk snake, and the black racer. Although each geographical variant is phenotypically distinct, intermediates exist between the populations.

Although individuals living close together within each species can interbreed, as distance increases, fertility decreases, indicating increasing genetic divergence as the habitat and climate change. These frogs are now considered to be four different species: 1) Rana pipiens, 2) R. blairi, 3) R. utricularia, and 4) R. berlandieri. The mating calls of each species differ, and defective embryos occur between certain combinations, indicating that both pre-mating and post-mating isolating mechanisms exist.

Notice that isolated populations on small islands (upper left) differ quite substantially in color, pattern, and tail feather structure and length, and bill size, whereas kingfishers on the large island (right) have similar characteristics. This pattern is typical for species of many kinds of organisms that are isolated on islands, mountain tops, etc. Isolation over periods of time provides opportunity for mutations to spread through small populations, thus leading to physical and physiological diversity (microevolution).

Hundreds of species of fruit flies evolved on the Hawaiian Islands as a result of behavioral isolation on the many species of Hawaiian plants. They are extremely variable in appearance, but they are almost identical genetically. 

The homeotic gene clusters of the fruit fly, Drosophila melanogaster, and the house mouse, Mus musculus, show similarity of sequences that control development of front and back parts of the body in both species. In spite of the fact that these genes are located on different parts of chromosomes in each species, and that flies belong to Arthropods, a separate evolutionary line from the mouse, a Chordate, a remarkable similarity exists between each group, indicating inheritance of similar patterns for over 600 million years for this gene cluster. 

Evolution of Cichlid fish in Lake Victoria, in east Africa, is an example of sympatric speciation, where several species diverge within one area. Each species developed unique food habits, which lessened competition between each diverging population. The second set of jaws in the throats of these fish has allowed for evolutionary changes in the modification of the oral jaws and head shape.

The formation of species (speciation) requires reproductive isolation, which can occur as a by-product of evolutionary change. Reproductive isolation can occur by at least nine different mechanisms, some occurring before mating, and some after mating. A species is usually defined as "a group of organisms actually or potentially capable of reproducing with one another, but reproductively isolated from other such groups." An immediate problem with this definition is that some organisms only reproduce asexually (amoebas, for instance), and other species are composed of only females.


Below are shown the methods of both pre-mating (pre-zygotic) and post-mating (post-zygotic) mechanisms of keeping species distinct (preventing hybrid swarms).

Postzygotic isolating mechanisms which keep closely related species apart.

Mules (a hybrid between a horse and donkey) are an example of sterile offspring. The chromosome number is different between the parents, and although much of the DNA is similar, the chromosomes cannot pair up (synapse) properly during meiosis. Thus, a mating between mules will usually result in sperms and eggs having different numbers of chromosomes (with the omission of necessary genetic material for development).

In order to understand fully how populations change, you will need to review details of the structure and functions of DNA and RNA, mutations, cell division (especially meiosis), Mendelian genetics, pre- and post-mating isolating mechanisms of reproduction, animal behavior, and population genetics. After Mendel's research was rediscovered in 1903, the answer to the question of how genetic variation remains in a population, was independently published in 1908 by an English mathematician (G. H. Hardy) and a German physician (G. Weinberg). Their findings have come to be known as the Hardy-Weinberg Law or as Hardy-Weinberg equilibrium. This principle essentially describes the conditions under which evolution will NOT occur.

The proportions of genotypes in a population will remain constant, as long as the following conditions are met:

  1) The number of individuals in the population is very large.

  2) Mating is entirely random. There can be no choosing of mates by specific criteria.

  3) No mutations can occur.

  4) No immigration or emigration can occur, i.e., no genetically different individuals can enter the population.

  5) No natural selection can occur, i.e., every individual has the same chance of surviving and mating.

These conditions are NEVER met in a population because: 

1) populations, especially near the edge of the geographic range, tend to be small and more variable, since environmental conditions are different from those in the center of a large geographical area; in small populations, genetic drift often occurs, in which gene frequencies change dramatically from those near the center of the species' geographical range.

2) mating usually follows species-specific, very elaborate mating rituals, and mates are not chosen at random. 

3) mutations occur at random, due to exposure to harmful chemicals, cosmic radiation, etc. Furthermore, randomness occurs in the order in which the chromosomes line up in meiosis, the cell divisions in which sperms and eggs are formed, and randomness occurs in crossing-over, the process in which DNA breaks and pieces of chromosomes from paired strands trade places.

4) organisms move (or are carried) from one area to another. 

5) natural selection almost always favors the survival (and subsequent reproduction) of certain individuals over others. The individuals of the prey species that are more camouflaged or are faster than others have an advantage, just as the predators that can see better or hunt better, or are faster, catch more prey, eat and survive longer than those who are not as skilled.

It is not surprising that organisms evolve, so much as it is sometimes surprising that it takes so long for some changes to take place. Hardy-Weinberg equilibrium is written as an algebraic equation:

(p + q)2  = p2  + 2pq + q2

Suppose we have a population of 100 individuals in which 9% of the individuals are albinos (genotype aa), and 91% have normal skin pigmentation. This is a recessive characteristic, in that one must have inherited the characteristic from both parents in order for the characteristic to be expressed. Since we have two parents and receive a complete set of genetic instructions from both the sperm and egg, we have two alleles (alternative forms of a gene) per genetic characteristic.

Let p + q = 1.  Therefore, q = 1 - p.  If q2 = 0.09, then q = 0.3

p = 1 - 0.3, therefore p = 0.7    Therefore, p2 = 0.49 

and 2pq = 2(0.7 X 0.3), which = 0.42 

 Substituting these numbers for the frequencies, we now have 49% genotype AA (normal individuals who do not carry the allele for albinism), 42% genotype Aa (carriers, who inherited the defective allele from one parent, and an allele for normal skin pigmentation from the other), and our original 9% of the population which are albinos. The percentage of albinos in the human population is actually much lower than this.

If this population breeds at random, the gene frequency will remain indefinitely, without change. But if albinos are more easily seen and eliminated more easily than normal colored individuals, the population will change, or evolve. This is how nature works. Another example of the Hardy-Weinberg principle is illustrated below, in which we begin with 84 black cats and 16 white cats, of a total of 100 cats. In this example q2 = 0.16.  If you need help with genetics terms used here, go to Genetics.

In the illustration below, a gene frequency of 1.0 would represent 100% of the population. If we modify the example of albinos above, and let q2 = 0.04, and q = .2, then follow the pink line (aa) upward, we would find that 64 % of the population would have the genotype AA (normal pigmentation), and 32% would be Aa (carriers of the recessive gene for albinism).