Evolutionary theory gives unity and explanation to the things you can observe in the fossil record. As T. Dobzhansky wrote (1973):

"Seen in the light of evolution, biology is, perhaps, intellectually the most satisfying and inspiring science. Without that light it becomes a pile of sundry facts - some of them curious but making no meaningful picture as a whole."

Evolution asks two large questions:
1. What causes evolution?
2. What has been the history of life on Earth?

Early Greeks - The Materialists

Western science traces its origins to Greece for it was the Greeks who provided a new way of looking at nature. Their theories about biological diversity were hopelessly wrong in most details, but it was their approach that was so important. Gone are supernatural causes of natural phenomena. We find instead attempts to explain natural phenomena in terms of rules and regularities that derive from nature itself.

Later Greeks - The Classical Tradition

The classical tradition began with Socrates, his student Plato, and Plato's student Aristotle. Their ideas were incorporated into Western theology and philosophy and became part of the common way of looking at the world and man's place in it until Darwin proposed his evolutionary theory in the nineteenth century.

Plato established the philosophy of essentialism. According to essentialists, objects observed in the real world are reflections of a limited number of essences or eide. Variation is merely the manifestation of imperfect reflections of constant essences. Plato called this the theory of forms. For example, in the world, statements I make today may be just today but tomorrow the same statements may be unjust. This does not mean that there is no such thing as justice. Justice exists, unchanging and perfect, in the world of forms. The same concept can be applied to objects: the student desk you sit in during class and a lawn chair are different from each other in many respects. But you still recognize that they are both chairs. How? Because both conform to the basic essence of a chair. The student desk and the lawn chair are in the real world. The essence of chair is in the world of forms.

Aristotle provided a method for learning about nature that consists of asking so that data can be sought for the answer. Aristotle also applied Plato's theory of forms to the natural world. According to Aristotle, the many different individuals of a species were unimportant variants of the important unchanging form in the world of forms. He thus established an important idea that remained unchallenged until Darwin's time - basic forms of life were fixed or unchangeable.

Aristotle made many other contributions to man's view of nature that have become so much a part of our thought patterns that they are taken for granted. For example, he argued that nature is arranged from simple, imperfect forms of life to the more complex and perfect forms. He called this progression from the lowest forms (inanimate matter) to intermediate forms (such as jellyfish) to highest (such as man) the scala naturae, or scale of life. This great chain of being establishes man as the dominate and perfect form of life. This position sets man above and apart from nature.

Aristotle's scientific approach was not used by succeeding generations. Instead, ideas of the Greek philosophers became academic dogma during the middle ages. Rather than original research, scholars produced elaborate commentaries on these ancient works. During this time, Aristotle's ideas on the fixity of species and the scala naturae were incorporated into the Judea-Christian belief that the earth and its creatures are the result of special creation, that they have not changed since they were created, and that man is above and apart from nature.

The 1600's and 1700's were times of worldwide explorations. In their quests for new trade routes, precious minerals and products, governments financed explorations to all parts of the world. Travelers to Africa, the Orient, and New World brought back new plants and animals. At the same time in Europe, the expanding industry and trade prompted growth in mining and construction. While digging into the earth, workers began to uncover fossils. At first, fossils were accounted for in various ways (magical items of great occult value ["dragon bones" of china]; rocks molded into the shape of animals by chance; they were bones of currently living animals that existed in remote corners of the world) and presence of fossils did not disturb the classical view of nature. But as more fossils were uncovered, and more strange and new forms of life were discovered by explorers, an explanation was sought for the question - Why are there so many forms of life and why have some of them apparently died out?

Georges Cuvier (1769-1832), known as the father of paleontology, established that fossils were remains of animals that had existed at one time but had become extinct. Extinction, according to Cuvier, was caused by periodic waves of destruction, such as fires and floods (Noah's flood being one of them). Each one of these catastrophes was followed by a special creation of new forms of life. This theory is called catastrophism (the present state of the earth is the consequence of violent catastrophes of short duration). Although catastrophism led to the recognition that species could go extinct it did not lead to a theory of evolution.

A related idea, progressionism, took the view that, after each cataclysmic episode, life was created again but in a more advanced form. Progressionism ties together catastrophism with the older ideas about the scale of nature.

The Breakdown of the Classical Tradition

By the beginning of the 1800's, exploration had greatly increased man's knowledge of the diversity of life and many naturalists began to toy with the idea that one species could change into another. The earliest theory of evolution to be logically developed was that of Jean Baptiste de Lamark (1744-1829). Lamark believed that all living things are endowed with a vital force that controls their development and functioning and enables them to overcome handicaps in the environment. In 1809, he published his four main principles:

1. The first organisms arose by abiogenesis. The belief that animals could arise from inorganic matter (abiogenesis) was widespread during the middle ages. Before the 1700's, most people believed decaying meat turned into flies and trash into rats.

2. Organisms have an innate power to progress toward more complex and perfect forms.

3. Organisms have an inner disposition to adapt their characteristics in response to changes in the environment. In other words, an organism's need for a structure encouraged its growth. Lamark suggested that giraffes had evolved their long necks because they stretched up to reach leaves of trees, thereby stretching their necks in their lifetimes.

4. Characters acquired in response to changes in the environment were passed on to offspring (the inheritance of acquired traits). Giraffes, for example, after having stretched their necks reaching for leaves, passed this characteristic to their offspring.

Although Lamark's theory is usually rejected by biologists today because of his belief in the inheritance of acquired traits, his theory has had an important impact on society (it was the basis for Lysenkoism in the Soviet Union) and is undoubtedly widely believed today.

At roughly the same time, geologists began to challenge the idea that the earth had not changed since its creation and to challenge the theory of catastrophism. James Hutton (1726-1797), a Scottish geologist, believed that the present state of the earth could be accounted for by the slow, uniform action of the same geological forces that exist today but have acted over vast amounts of time. This principle, called uniformitarianism, states that the earth is extremely ancient and that its surface is constantly changing under the effects of erosion, volcanism, and other geologic processes. Today uniformitarianism seems obvious but, at the time it was proposed, there was stern opposition from those who believed the earth's surface remained unchanged from the time it was created and from catastrophists.

In 1831, Charles Lyell published his revolutionary book The Principles of Geology and established the modern science of geology. Lyell, who was a uniformitarian, collected overwhelming evidence that the earth's surface is constantly changing and did much to advance the idea that the earth changes through time (inorganic evolution).

Charles Darwin

In 1831, a young Charles Darwin was taken aboard the HMS Beagle as an unpaid naturalist and companion for her moody captain, Robert Fitz-Roy. Darwin, the son of a wealthy doctor and recent graduate of divinity school, had obtained the job through the influence of professor John S. Henslow. Henslow gave Darwin a most influential parting gift - a copy of the first volume of Lyell's Principles of Geology. Darwin read the Principles as the Beagle crisscrossed the oceans. The book had an enormous impact on Darwin because he witnessed uniformitarianism in action. In Chile alone, he saw a spectacular array of geologic events -pyrotechnics of the volcano Osorno, earthquake in Valdivia, and the ocean bottom lifted 10 feet above the high water mark in the Bay of Conception. Darwin was convinced that Lyell and Hutton were correct - the earth's surface is constantly changing. Importantly, Darwin realized, if the earth's surface is constantly changing, the life on it must be forced to change in concert.

Darwin also contemplated the apparent irrationality of special creation for every species. He noted that identical climates in different parts of the world were populated by different types of plants and animals, but there were remarkable similarities among the organisms within each continent despite wide ranges in climatic differences. The evidence most compelling to Darwin was provided by the Galapagos islands, which have since come to symbolize Darwin's work aboard the Beagle. It struck Darwin that, although the animals on the islands are clearly similar to each other, each island had its own distinct tortoises and finches. Darwin could not understand why a Creator would make different animals for each island. He concluded that special creation and fixity of species was unbelievable and the similarities among the animals on the Galapagos were due to common ancestry while their differences due to different environments on separate islands.

When Darwin returned to England, he continued to collect evidence for evolution from domestic animal breeding, but he was also searching for a natural mechanism that would cause for change in species over time.

Darwin writes in his biography:

"... fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which goes on everywhere from long-continued observation of habits of animals and plants, it struck me that under these circumstances favorable variations would tend to be preserved, and unfavorable ones destroyed. The result would be the formation of new species. Here, then, I had at last got a theory by which to work."

He called this mechanism natural selection.

(C. Darwin in later years)

For two decades, Darwin delayed publishing his ideas on evolution, although he discussed the idea with fellow scientists. In 1958, Darwin received a letter from a naturalist working on the flora and fauna of Malaysia and the East Indies, Alfred Wallace. Wallace had also read Malthus's work and independently derived the theory of natural selection. Darwin knew he now had to publish his theory of evolution by natural selection. So Darwin read both Wallace's paper and then one of his own to a meeting of the Linnean Society of London in 1958. In 1859, twenty three years after the return of the Beagle, On the Origin of Species was finally published. With its publication, conceptual biology shifted gears. A vast amount of data about living organisms began to make sense to a degree never achieved before. It changed man's view of nature and his own place in it.

Thus Charles Darwin made two great contributions to science:

1. He presented a wealth of detailed evidence and cogent arguments to show that organisms change through time.

2. He conceived the theory of natural selection to explain how this change took place.

1. Members of a species show variation in morphology, physiology, etc.

2. Offspring have the same variations as their parents (offspring inherit features from their parents).

3. Organisms have the physical capability to produce more offspring than they actually do. Something prevents them from having as many

offspring as they are capable of.

4. The environment changes through time.

1. Resources are limited so that individuals must compete and struggle for their own existence and that of their offspring.

2. Therefore, only some survive and leave offspring. These are the ones

that have some favorable characteristic that enables them to "win" resources.

3. In the next generation, the species is represented by the offspring

that have inherited the favorable characteristic.

4. After several generations, all members of the species have the favorable feature.

5. Since the environment constantly changes, what is favorable constantly changes, and species keep changing accordingly or goes extinct.

To answer these questions, we must ask a more fundamental one:

The Scientific Method

There are many different views of scientific reasoning but we will restrict our discussion to two broad approaches:

The inductive method of reasoning was described by Sir Francis Bacon (1561 1626) who rejected the classical and medieval theological method of accepting a theory (usually a theological belief) and deducing the consequences. Bacon argued that the problem with interpreting observations in terms of what is already assumed to be true is that we see what we believe rather than believe what we see. Bacon believed every scientific investigation should begin by assembling all the data from observation and experiment that related to some natural phenomena. From these facts, general statements, or hypotheses, to explain the phenomena can be made. Bacon's method can be summarized:

1. Scientific knowledge grows by the accumulation of independent facts. Scientific knowledge begins with close and intelligent observation of natural events or phenomena.

2. General laws are inferred from particular facts. A guess or tentative explanation of natural phenomena is made based on our observations. The guess is our hypothesis. The guessing is not haphazard: the process, in which we combine the bits of information and logic to produce a hypothesis is known as induction.

3. The truth content of a hypothesis is judged by the number of additional observations that support the hypothesis.

The fundamental difference between Bacon's approach and earlier ones was that scientific statements were based on data derived from observations and experiments of natural phenomena and not on preconceived principles, imagination, or superstition.

Although it is vastly better than the reasoning methods used in medieval times, there are problems with a strict inductivist approach:

1. Despite, best intentions, particular facts can be perceived with bias that reflects the scientist's nationality, gender, race, or time of reference.

2. Supporting observations for a hypothesis can usually be found.

Example: Carniometry, or the measurement of the volume of the brain case was once thought to be an index of intelligence. The hypothesis was, the greater the size of the cranium, the greater the intelligence. Samuel George Morton (18xx-1851) undertook to rigorously test this hypothesis. let me assure you that Morton was not considered a crackpot. He was a Philadelphia patrician with two medical degrees. Morton collected human skulls and had over 1000 (mostly of native americans) when he died. Because he actually tabulated the volume of these skulls, he was considered, by his contemporaries, to be a great objective data-gather. For over 100 years he was regarded as the one who elevated a fanciful speculation (brain size = intelligence) into established fact.

Morton made his measurements by filling the skull's brain case with shot, pouring the shot into a graduated cylinder, and reading the volume. His results match every good Yankee's prejudice - whites on top, North American Indians in the middle, African Blacks, then South American Indians, and finally a tie for last place between the Australian Aborigines and the Hottentotts. Among the Caucasians - Germans and Anglo-Saxons on top, Jews next, Irish and Mediterranean peoples next, and east Indians on the bottom.

He tested his theory inductively by examining skulls from Egyptian tombs - once again, Egyptian nobles had larger brain sizes than the black slaves buried with them. He also found other evidence - ancient texts from Greece depicted blacks only as servants and slaves - an indication of their innately menial status.

But let's examine Morton's work more carefully. first, Morton was a product of the time (early nineteenth century) and place (United States) he lived. He believed races of humans were, in fact, separate species. He believed that they could be ranked according to their intelligence.

Second, Morton never tried to disprove his hypothesis or challenge is own prejudices. He only sought evidence that confirmed his views. Take, for example, the historical texts which only depict blacks as servants - what about other texts and art from other parts of the world that depict blacks as rulers and slave owners? S.J. Gould reexamined Morton's data and found many discrepancies:

1. Morton often chose to include or delete subsamples (he gave arguments for why he deleted small caucasians, but included small indians). He excluded a large Eskimo and Chinese from the sample.

2. Morton did some early measurements with seed instead of shot. When he discovered that this method gave inconsistent results, he re did the caucasian values with shot, but not the blacks.

3. Morton was convinced that brain volume indicated intelligence and was never able to see another hypothesis - but his own observations cried out for it:

a. No sexual bias accounted for (the hottentotts measured were all females; the Englishmen were all mature men)

b. No age

c. No body size

A more defensible scientific method is the hypothetico-deductive method used by logical empiricists. In many evolution articles, the philosopher Karl Popper is often associated with popularizing the method:

1. A hypothesis is formed by the first two steps of inductive reasoning. But there is a difference - There is an absolute requirement for a scientific hypothesis: it must be testable.

2. We test a hypothesis by validating or falsifying a prediction made from the hypothesis. These predictions can be formed by saying "If the hypothesis is true, XX must follow." These predictions are called deductions.

3. If the hypothesis is true, the prediction must be true. Then we set about testing the deduction by making observations or conducting experiments to see if the deduction is true.

4. If we show that the deductions are incorrect, then we must reject the hypothesis.

5. If the deduction proves to be true, the hypothesis is not proven to be true. At this point other deductions are made from the hypothesis and tested (the process is repeated). The more deductions tested and found true, the more likely it is that the hypothesis is true. If the deductions continue to be verified, we reach the stage when we can say the hypothesis is true beyond reasonable doubt. The hypothesis then becomes a statement that is part of the conceptual framework of the field.

There are two points to keep in mind about formulating and testing hypotheses:

(1) the data collected in testing the deduction must be obtainable by other scientists. If an important discovery is made, it is never fully accepted by the scientific community until it has been verified by other scientists. If the original discoverer has made an error, the error would likely be corrected when others attempt to verify the original report. Science is a self-correcting method (e.g., cold fusion; dino DNA).

(2) We must avoid circular reasoning by keeping our observations that test our deduction logically free from our initial hypothesis.

As Popper has pointed out, not all hypotheses are scientific hypotheses. Only statements from which a testable prediction can be deduced are scientific. Evolution is a scientific field because it can be studied by this hypothetico-deductive method.

The opposite of scientific hypotheses are matters of taste or aesthetic judgments, moral or ethical statements, or metaphysical beliefs. Occasionally, a special interest group will claim that their viewpoint is "scientific." For example, scientific creationists ("God created the earth and all its creatures") and social Darwinists ("this race is morally superior to that one") often invoke supernatural forces or aesthetic judgments. While their proponents may use scientific terminology to present their views, their statements are not scientific hypotheses because testable consequences cannot be deduced from them.

Terms Defined

When discussing science, especially evolution, it is important to say how we intend to use terms such as "theory" and "hypothesis." For non scientists a theory can be a pejorative term: "evolution is just a theory," meaning that it is a dubious notion. A theory for a scientist is a synthesis of a large and important body of information about natural phenomena. Thus the theory of evolution would be all the many sorts of observations relating to explaining why there are so many different kinds of organisms.

Hypothesis is the term for the tentative explanation of observed phenomena. In the formative years of a science, a hypothesis may grow into a theory (theory and hypothesis should not be used interchangeably as synonyms). Some early speculators about the underlying causes of organic diversity and of adaptation adopted a hypothesis of evolutionary change to account for these phenomena. As data increased and the hypothesis was not falsified, the large body of information and verified hypotheses was recognized as the "theory of evolution."

1. If the hypothesis of evolution is true, the species that lived in the remote past must be different species from the ones alive today.

It seems obvious to us today that dinosaurs and other ancient forms of life are different from the species living today. But there was reasonable doubt in the nineteenth century. It was suspected that members of the species might still be living but not yet discovered. Darwin tested this hypothesis with had data from Cuvier's research. To test the hypothesis that some fossils were the remains of extinct taxa, Cuvier used fossils most likely to give an unambiguous answer: large terrestrial mammals and reptiles. Cuvier could show that many fossil mammals (such as elephants) and reptiles (such as dinosaurs) were similar to, but not identical with, living species. Although Africa and South America had not been fully explored, he realized that it was unlikely the dozens of fossil species of large animals remained hidden in unknown lands. Since the time of Cuvier, more fossil species have been described that have no living representative. Thus it is true beyond a reasonable doubt that species living today are different from species living in the past. The deduction is shown to be true, so the hypothesis of evolution is made more probable.

2. If the hypothesis of evolution is true, the further one goes back in time by looking at fossils in older sedimentary strata, the less chance of finding fossils of contemporary species.

The hypothesis that species in progressively older rock layers would be ever less like living species was tested by Charles Lyell. Although he did not know the absolute age of the fossils, Lyell did know which layers were older using the relative time scale of the geological column.

Perhaps Darwin's best argument that life evolved was the observed fact that all organisms are united in a pattern of increasing similarity. This is simply a common sense observation such as mice are more like rats than like squirrels. Rats, mice, squirrels and other rodents share many unique anatomical and behavioral characters not found in other creatures, but we also observe that they are similar to other mammals in having hair, mammary glands and a four-chambered heart. Darwin saw these ever widening circles of group membership as clearly supporting the hypothesis that species were related to varying degrees.

It was not until the twentieth century (especially since 1940) that reliable methods for detecting the age of rocks have been perfected. These methods, which use the rate of radioactive decay of materials in the rocks, have shown that the earth is approximately 4.5 billion years old.

5. If the hypothesis that evolution proceeds by natural selection is true, there must be variation among organisms.

Darwin described ample and convincing evidence of variation among individuals in the Origin of Species. Darwin had been impressed by the variation in natural populations while on the Beagle expedition. Later, in publications of other scientists, he found additional examples of variation not only in external features but in internal ones as well. Darwin also provided many examples of variation among members of domesticated species (e.g., dogs).

6. If the hypothesis that evolution proceeds by natural selection is true, there must be fewer offspring surviving to reproduce than organisms are capable of producing.

Darwin observed that the population size of species seems to remain about the same year after year. A single oyster produces millions of eggs each year yet the ocean does not fill up with oysters. A single oak tree can produce hundreds of acorns each year, yet the number of oak trees remains about the same.

The struggle for existence is a fact of nature. Thus the deduction that more offspring are produced than can survive can be accepted beyond reasonable doubt.

7. If the hypothesis that evolution proceeds by natural selection is true, there must be differences between generations.

To test whether natural selection could result in change in species over time, Darwin compared it with selective breeding practiced by plant and animal breeders. For many centuries, man has effectively chosen which plants or animals to breed to improve agricultural varieties and to breed various kinds of dogs, cats, pigeons, horses, and other domestic animals. In this practice of artificial selection, the human breeder selects the parents deemed desirable for each generation and eliminates the undesirable types. Since the selected parents produce a variety of offspring, the breeder can usually continue to select in a particular direction until he consistently gets the results he wants. The result often produces varieties significantly different from the original breeding stock. Artificial selection demonstrated to Darwin and his contemporaries that continued selection was powerful enough to cause large-scale changes within a species. To suppose that natural selection could produce similar changes in natural environments seemed, therefore, a reasonable idea. One of the most extreme examples of artificial selection can be seen in dog breeds, all of which are derived from the same genetic stock - the wolf. In a few thousand years, artificial selection has produced such opposites as the chihuahua and the Great Dane, the Pekinese and the Saint Bernard.

The deduction that natural selection could result in change over generations was eventually tested by other means and shown to be correct. The most dramatic evidence comes from situations in which a population is presented with an environmental challenge never before encountered and therefore never selected for. When the population encounters this new environment, most individuals are killed, but any that have genes that confer adaptation to the new conditions survive and reproduce.

A well-documented example of this is the change in coloration of peppered moths (Biston betularia) that occurred during the industrial revolution in England. The moths exist in three forms: dark gray, pale gray and intermediate gray (there are photographs of this moth on page 358 in Futuyama).

Examination of insect collections in museums show that prior to the 1850's, most members of the species living in English industrial areas were light to intermediate colored and delicately camouflaged to match the lichens on trees and rocks. The moth rested on these lichens and so was hidden from predators. In the later half of the 19th century, booming industrial cities released tons of black soot from coal-burning factories into the air, blackening nearby tree trunks and rocks, and killing off the lichens. The light color of the pepper moth was no longer protective and they became vulnerable to birds. The dark variety became better adapted and, by 1898, the percentage of dark moths had jumped to 98% of the population near the industrial city, Manchester.

The effects of industrial soot on the frequency of the dark peppered moths were verified experimentally in a study completed in the 1950's. An equal number of speckled and pigmented moths were marked with a spot of paint under their wings and then released into both polluted and pollution-free areas. When survivors were recaptured, the results confirmed that speckled moths were favored in pollution-free areas and dark moths were favored in polluted areas. Such mutations (known as industrial melanism, after the dark brown pigment melanin) are common in many species that live in industrial areas. There are now several species of melanic moths, and also melanic spiders and ladybugs.

The evidence seems irrefutable that the rise in frequency of the melanic form of B. betularia in the last century was the result of selection caused by industrial pollution. Consequently it seems reasonable to suppose that if pollution decreased, as has happened since clean air legislation was enforced, the light colored moth should start to increase again. This has clearly happened in the West Kirby area where studies have shown that the pale form has increased from six to 30% between 1959 and 1984.

In Darwin's time, it was obvious that many features were passed from parent to offspring but neither he nor most of his contemporaries could explain the basic principles of inheritance. In the 20th century, the principles of genetics were discovered and this deduction was shown to be true. Biologists also discovered genetic mechanisms that cause organisms to change from generation to generation and they were able to explain how natural selection and these other mechanisms can lead to a change in the hereditary material found in a population. The synthesis of Darwin's original ideas and modern genetic theory is called either neoDarwinism or the modern synthesis.