I grew up and went to primary, middle, and high school in the United States of America. After graduating from high school I continued my education by taking more advanced college and university courses. In the United States all university courses are numbered, however there is a special number that always stands out from the rest: "101." The number 101 is used when referring to basic ground-level courses such as Mathematics 101, Psychology 101, History 101, etc. In this way, every time the number 101 shows up it points to the difficulty level of a certain subject. By extensión, and as applied to the title of this article, "Evolution 101" simply means a basic and ground-level introductory lesson for novices in the subject.

 

Charles Darwin

 

Charles Darwin (1809-1882) stands out in history with Galileo, Newton, Einstein, and other great scientists who synthesized important theories about nature. In 1831 Charles Darwin, age 22, embarked on a five-year trip around the world as oficial naturalist of the British ship H.M.S. Beagle. His job was to record "anything worthy to be noted in Natural History," which in those days included studies of plant and animal life, rocks and minerals, and astronomy. 

 

During the voyage he began to speculate about the possibility that changes in the earth might affect its living inhabitants. During his exploration he began to find solid evidences to support the concept that the earth and its living organisms have undergone progressive change. Darwin´s observations laid the foundation for a theory of evolution and natural selection that would discredit all former conjectures and speculations.

 

Darwin returned to England at the end of the voyage with the idea of organic evolution implanted in his mind. During the next twenty-five years he organized the things he had seen. Finally in the year 1859 he published his famous book entitled: "On the origin of species by means of natural selection." In this book Darwin showed how evolution could explain the common threads underlying life´s diversity. Darwin concluded that living things were not created as they exist today. Rather, they are modified descendants of earlier ancestors. A process of natural selection is always in operation, choosing those that are to survive and create the next generation. 

 

Theory, Hypothesis, Fact, and Law

 

A theory is an organic body of ideas or a set of hypotheses or propositions that serves to unify a great number of scientific laws. Such ideas are logically or mathematically linked and offered as an explanation for a wide-scoping variety of interactive and/or interconnected phenomena. We also can think of a theory as a determined set of principles which purports to be either a correct description of some aspecto of reality and/or a guideline for successful human action. 

 

On the other hand, a fact is something that is known to have happened or to exist, especially something for which proof exists, or about which there is information. A scientific law is a statement based on repeated experimental observations that describes some aspect of the world. 

 

Striving to understand seems to be a basic human drive. Science has emerged from human curiosity about ourselves, our world, and the universe. Scientists are obsessively curious, they ask questions about nature, pose hypotheses to answer the questions, and test the hypotheses. Scientists are also skeptics, suspicious of poorly documented or contrived answers to their questions. 

 

Therefore, a theory in science is an explanatory idea that is broad in scope and supported by a large body of evidence. A theory differs from a hipótesis necause the latter is an educated guess a scientist proposes as a tentative explanation for a specific phenomenon. The theory of evolution is the central organizing principie of biology. Aspects of the theory may be refined and updated as we learn more, but the basic idea is extremely solid and sound.

 

Nature

 

The term nature serves to define the basic or inherent features, character, or qualities of someone or something. However, the term Nature (with a capital "N") refers to the phenomena of the physical world collectively, including plants, animals, the landscape, and other features and products of the earth, as opposed to humans or human creations. Nature produces large numbers of new plants and animals, and at the same time is highly selective. It weeds out the weak and innefficient and kills off the organisms that cannot adapt to environmental conditions. Only the strongest, most efficient, best adapted organisms are left to survive, prosper, and reproduce. 

 

Overpopulation

 

Charles Darwin and Alfred Russel Wallace (1823-1913) were both familiar with the writings of one of their fellow countryman, Thomas Robert Malthus (1766-1834). In 1798 Malthus had published an essay entitled "Essay on Population," in which he stated that food supplies can never keep pace with the rapid increase in human population. To keep things in balance, there must be a great disaster from time to time that reduces the population. 

 

According to Malthus, disasters such as wars, famines, and epidemics put the population and food supplie back into balance until a new cycle of population growth builds up pressure once again. The essay was originally published anonymously and for good reason. Malthus understood that the implications of his principle of overpopulation did not align well with the religious thoughts of those times.

 

Darwin and Wallace independently came to the realization that Malthus´principle of overpopulation could apply to any other species of living things as well as man. In view of such evidence they thus equated the Malthusian population theory with the theory of natural selection. 

 

The theory of natural selection begins with the idea that nature is extremely prolific. Every organism produces many more offspring than can possibly survive. If all the young grew to maturity and reproduced in turn, it would take only a few generations to fill the world with organisms of that species. Eventually there would be no room for any other kind of living thing. 

 

For example, a female codfish may lay six or seven million eggs. However, we know that the ocean does not become filled with codfish. From year to year, the number of codfish in the ocean remains practically the same. Unless the balance in a given area is upset by a change in conditions, the population of a given species remains relatively constant for long periods of time.

 

Competition for Life

 

From the above example, it is obvious that all offspring do not survive, and not all grow to maturity. There must be a competition for life among organisms, a struggle for existence in which some die while others survive. But what determines which organisms will survive?

 

Even in Darwin´s lifetime it was well known that cattle and crop plants could be improved by the process of selection. A man could pick out his best cows for breeding, those that had the most desireable variations in a particular trait. For example, not all cows produced the same amount of milk: some were good producers, while others were not. Any dairyman would choose for breeding purposes only those cows with the highest milk-producing ability.

 

Could this same principle apply to Nature? Could there be a process of natural selection comparable to the artificial selection practiced by man? Could this process select out the organisms of a species that would survive and produce the next generation? Darwin and Wallace thought that this was the case. They felt that there must be a continuous process of natural selection that weeded out the less fit. In the long run the "favored races," as Darwin called them, were preserved in the struggle for life.

 

Preservation of the Favored Races

 

But what makes one organism more fit than others of its species to survive in the struggle for existence? We know that variations exist in every species and in every trait. Some of these variations enable the organism to fit better into the environment, to compete better for the available food, or to avoid its enemies more readily. Such variations may be said to have "survival value" because they give the organism an advantage in the struggle for life.

 

When an organism dies, whatever traits it has must die with it. They can no longer be passed along to offspring. Obviously the next generation of the especies is produced by those members of the species that succeed in remaining alive long enough to reproduce. Then, it must be these successful organisms, and they alone, that can pass their traits on to future generations.

 

Natural Selection

 

The Darwin-Wallace evolution theory begins with the established facts of overproduction, constancy of population size, variation, and the resemblence of offspring to their parents. From these facts Darwin and Wallace deduced the rest of their theory as follows: 1.- there is a struggle for existence which eliminates organisms having less favorable variations; 2.- the survivors in the struggle are individuals having variations that give the greatest advantage in the environment; 3.- the favorable variations are passed on to future generations; 4.- as a result, the species becomes increasingly modified over many generations; and 5.- at the same time it becomes better adapted to its surrounding environment and thus survives and reproduces accordingly.

 

The environment is the central factor in natural selection because it determines which variations are favorable and which are unfavorable. Any change in the environment can alter the importance of a certain variation. Thus, what is favorable under certain conditions may no longer be an advantage if conditions change. In this way the entire direction of evolutionary development can change.

 

Over long periods of time, the climate and other conditions on earth change in various ways. Under the influence of such changes, natural selection could alter the characteristics of a group of organisms so greatly that they might be considered to be a new kind, or a new species.

 

Replication, Reproduction, Evolution, and Natural Selection

 

Differences must be made between the terms replication and reproduction - they are not the same. The term "replication" applies when a certain DNA molecule produces another DNA molecule exactly like itself. The term "reproduction" applies when living matter as a cell or as a body made of cells, grows another similar being. Furthermore, when living matter continues to reproduce altered forms that, in turn, make altered offspring, we speak of "evolution." Evolution concerns the change in populations of life forms over time. 

 

As Charles Darwin and his legacy makes crystal clear, more reproducing cells and bodies are produced by budding, cell division, spore formation, hatching, birth, and the like than can ever survive on the earth´s multiple and constantly changing environments. Those organisms that adapt or cope long enough to reproduce are "naturally selected" by the process of natural selection. In fact, those that continue and reproduce - the survivors - are not so much selected for their success as those failing to reproduce before dying are selected against. Because of such, and in the continuous mixing and sifting process of natural selection, we must differentiate very clearly between the two key aspects that maintain life and eliminate life: "selecting for" and selecting against."