Evolution

Principles of Evolution

1. To understand the origins of existing life forms it is necessary to understand some of the basic principles of evolution. Charles Darwin proposed the idea of evolution in his book 'On The Origin Of Species' in 1859. He called evolution 'descent with modification'. It is the process by which all life on earth has diversified from bacterial mats that existed over 3.6 billion years (3,600,000,000,000 years) ago. The process of evolution has taken a long time. Throughout the time line the process has not proceeded at a constant rate. Rather it is a process puncuated by rapid bursts of change interspaced between realtively long periods during which little or no change was taking place. The exact causes and nature of these puncuated bursts are not well understood.
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2. For a long time it was mistakenly thought that evolution was a simple linear progression, with humankind at the top of the ladder. This old view was replaced long ago as new evidence came to light. We now understand that evolution proceeds in a kind of branching pattern, with species on one branch giving rise to other branches and so on.
3. Evolution proceeds as changes (mutations) occur in the genetic code of an organism. These mutations are 'tested' by the environment in which the organism exists. Many (most) of the changes will be harmful to the organism, and it will die before the harmful genes are passed on to its offspring. Occasionally, however, a gene combination will arise the actually improves the adaptation of the organism to its' particular environment and these genes are more likely to be passed on. By this process, called 'natural selection', all life has branched. The process of natural selection is the means by which the great diversity of dinosaurs was able to evolve.
4. To reconstruct the evolutionary history of a related group of animals, such as the dinosaurs, it is necessary to look at the detailed shape of the bones. Not all dinosaurs are completely different. Some share certain similarities in their bone structure. By finding out what dinosaurs share these similarities, or 'characters', it is possible to produce an evolutionary tree, or phylogeny, showing the relationships of the animals to their ancestors.
5. The modern theory of evolution explains the diversity of life from the perspective of science. It provides a scientific explanation of how the multitude of species was created. Advances in science, especially genetics, have strengthened this view of life in part by explaining the basis of variation and inheritance. As you will see, variation and inheritance are both very important to the theory. All aspects of modern biology are affected by the theory of evolution by means of natural selection. It has been said that 'nothing makes sense in biology except in light of the theory of evolution.' Note the often used phrase, "The Darwinian Revolution" a scientific revolution that has changed the focus of biology.
6. Major Basic Theories of Evolution
   1. The Theory of Inheritance of Acquired Characteristics. (1809) by Jean-Baptiste Lamarck. Modifications acquired during one's lifetime are inherited by the next generation, ex. giraffes acquired a long neck slowly over time as each generation of giraffe stretched its neck slightly longer in trying to reach leaves high in trees. At fist glance this theory is deceptively close to Darwin's theory (both include the concept that evolution produces life forms adapted to their environments) but the inheritance of acquired characteristics implies that the organism itself can control the direction of change. Unfortunately, there have been no discoveries of any such mechanism of change.
   2. The Theory of Evolution by means of Natural Selection - The founders: Charles Darwin (1809-1882) an English Scientist, At the age of 22 he sailed on the HMS as ship's naturalist for a 5 year voyage 1831-1836. Alfred Wallace (1823-1913) an English Naturalist who spent the years 1848-1852 on the Amazon River and the years 1854-1861 in Indonesia collecting and studying insects. Both men were excellent observers and prolific collectors and both worked as taxonomists. In1858 - Wallace sends Darwin a letter, an essay in which he (Wallace) clearly communicates evolution by means of natural selection. In1859 Darwin publishes On the Origin of Species by Means of Natural Selection.
   3. Seven Influential Factors for both Wallace and Darwin:
   1. Geology the study of the earth's structure, origin, and history. The book, Principles of Geology by Charles Lyell (1797-1875), includes the "theory of uniformity" or "uniformitarianism" - Geological processes have been uniform through time. Geological changes of the past were caused by same, observable processes of today. The process of change has been uniform through time. Canyons and thick layers of sedimentary rock are the profound results of accumulated gradual change over vast stretches of time (gradualism). The significance to biology is this, slight changes over a long time have large-scale impact and one can look at the present to see the processes of the past.
      Three concepts from geology are important in shaping the theory of evolution by means of natural selection:
      1. the earth must be very old [4.6 billion years]
      2. one can look at the present to see processes of the past
      3. slight changes over a long time have large-scale impact
   2. Fossils - extinct forms similar yet distinct from extant forms. Perhaps a history of change (evolution) connects the living with the extinct.
   3. Island Life. Isolated island populations differ slightly from nearby mainland populations. e.g. Galapagos Islands (600 mi. off W coast of S.A) the birds, lizards, huge tortoises, similar to 'nearby' mainland species yet slightly different. Therefore there must have been Descent with modification (the island life evolved from mainland ancestors).
   4. Overproduction of individuals - Thomas Malthus (British Economist) in his "Essay on the Principle of Populations" (1798) attempts to explain that much of human suffering, hunger, sickness, homelessness, and war are due to competition between an ever increasing number of individuals or groups of individuals for limited resources. The number of individuals (population size) tends to increase exponentially. This exponential capacity can't be sustained (if it were, earth would be covered over many times with individual life forms). Eventually, more individuals are born than can live to reproduce, leading to a struggle for existence. Darwin and Wallace saw the concept relative to all species not just humans. Only some individuals live to reproduce, many (in fact, most) will die before reaching reproductive age. What determines, in nature [not human society] who lives and who dies? An answer lies in part in variations among individuals. Social Darwinism is a distortion of the Theory of Evolution in the Darwinian sense, Social Darwinism was used by Hitler to justify the extirpation of Jews and his quest to destroy other countries; Antievolutionists use Social Darwinism as an attack on the Theory of Evolution. Social Darwinism is the erroneous application of "survival of the fittest" to human societies.
   5. Individuals within a species vary extensively. How does one know variations exist? Taxonomy! A taxonomist observes and describes the variation within a species. You don't have to be a taxonomist to know that individual people vary. People vary not only in morphological traits such as hair texture and body shape but also in physiological traits as seen in the susceptibility of American Indians to European diseases [measles, mumps]. A knowledge of the variation in species of wild animal be they bird, beetle, or barracuda, requires the careful and detailed observation of many specimens. Only a scrutinizing taxonomist, whether professional or amateur, sees the variation between individuals of wild species.
   6. Many of the variations are inheritable and some variations may impart greater reproductive success, as in artificial selection below.
   7. Artificial Selection "selective breeding by humans of another species". Variation among crops & livestock may be favored (selected) or disfavored (selected against), or neutral, i.e., neither favored nor disfavored, by man's control over which individuals are allowed to reproduce. Man selects the best (according to his taste) and breeds these for future generations

CHARLES DARWIN AND THE PRINCIPLE OF EVOLUTION

Darwin showed little interest in his early education at Shrewsbury School and in medical studies at Edinburgh University (1825-27). He turned away from becoming a physician after witnessing several operations performed without anesthesia. At the same time he began to be interested in geology and natural history. He was finally sent to study for the holy orders for the Church of England at Christ Church College, Cambridge University (1828), but had no inclination for the ministry. Instead, he became more and more interested in natural history. After he received his degree in 1831, a Cambridge professor and friend, John Stevens Henslow, recommended him for an unpaid position as naturalist on a scientific expedition. Darwin embarked on a 5-year voyage on the H.M.S. Beagle, a turning point in his life. The Beagle set sail on Dec. 27, 1831, to study the Pacific coast of South American and some Pacific islands as well as to set up navigational stations in the area. Darwin's duties were to study the geology and biology of these areas.

Darwin's biological findings gave him a lasting place in history. The Galapagos Islands were probably the scene of Darwin's most important and best-known research. On these islands, Darwin found an array of animal life and found that related but different species lived on different islands even though all the islands were very similar in geological, climatic, and other physical conditions. He found an array of ground finches with beaks ranging from large and powerful to small or fine. He correlated these differences not with physical conditions on the islands but with the birds' feeding habits. Those with powerful beaks ate large seeds; those with smaller beaks ate small seeds; those with fine beaks primarily fed on insects. He stated that each finch was particularly suited to the food that was available in its environment--an example of what later became known as adaptation.

Along the Pacific coast of South America, Darwin observed that closely related species often lived in adjacent geographic areas and that species found on isolated oceanic islands often resembled species found on the nearest continent. Established biological thinking held that all species were unchanging and specially created; however, Darwin found that similar species were always present in adjacent geographic areas. When he found, at a given location, fossils of extinct species that were similar to living species, he began to question why similar species existed in successive geologic time periods.

Darwin collected many geologic and biologic specimens, studied many fossils, and made observations of the form, numbers, diversity, and living habits of different forms of life. From his meticulous research he arrived at the idea that species descend, with modifications, from other species, or that species evolve from other species.

Darwin waited for years after his return to England (1836) to present his conclusions. He reported on some of the data he had accumulated during his trip in the Journal of Researches (1839). He also wrote a brief summary of his ideas on evolution that became known to a few scientists, but for the most part Darwin turned to other work, including detailed study of barnacles.

In 1856, Darwin began to write his theory of evolution by natural selection, but before he had finished (1858), he received a paper from naturalist Alfred Russel Wallace outlining a theory similar to his own. Friends arranged for the two men to present a joint paper before the Linnaean Society of London in 1858. On Nov. 24, 1859, an abstract of Darwin's theory was published, The Origin of Species.

In this book, Darwin presented his idea that species evolve from more primitive species through the process of natural selection, which works in nature. In his account of how natural selection occurs, he pointed out that not all individuals of a species are exactly the same but, that individuals have variations and that some of these variations make their bearers better adapted to particular ecological conditions. He pointed out that most species produce more eggs and offspring than ever reach maturity. He theorized that well-adapted individuals of a species have more chance of surviving and producing young than the less adapted, and that over the passage of time the the ones that are less adapted are weeded out. The accumulation of adaptations to a particular ecological way of life leads--if there is a geographic split of the population--into the development of separate species, each adapted to its own particular ecological living space.

The effect of on the Origin of Species was immediate and widespread. The book upset many established patterns of thought, contradicted firmly held religious beliefs, and brought into focus the concept that humans are one species among many that had evolved from a more primitive one. Controversies and debates on the theory raged all over England, Europe, and the United States. Though the evidence Darwin presented was strong, some scientists aligned themselves with orthodox churchmen and others who opposed the theory. Other scientists enthusiastically embraced it.

Darwin continued to write and do research, expanding on ideas he had presented in On the Origin of Species. In The Descent of Man and Selection in Relation to Sex (1871), Darwin provided evidence for human evolution from more primitive species and discussed the role of sexual selection in evolution.

GLOSSARY TERMS

1. Natural Selection - is the outcome of differences in survival and reproduction among individuals that vary in heritable traits.
2. Directional Selection - allele frequencies underlying a range of variation tend to shift in a consistent direction in response to directional change in the environment.
3. Stabilizing Selection - intermediate forms of a trait are favored, and alleles that specify extreme forms are eliminated from a population.
4. Disruptive Selection - forms at both ends of the range of variation are favored and intermediate forms are selected against.
5. Gene Flow is the physical movement of alleles into and out of a population, through immigration and emigration.
6. Genetic drift is a random change in allele frequencies over the generations, brought about by chance alone.
7. Bottleneck is a severe reduction in population size. As a result of this reduction allele frequencies will have been altered at random. An example of this would be the cheetah. There was a huge reduction in the cheetah population. Parents mated with their own offspring when no other options were available. As a result of this, they have the same alleles and are almost like clones.
8. Founder effect is when a population is isolated. They take with them the alleles that were in that group. This is based on chance, not natural section. An example of this would be the Amish. They maintain isolation by pre-mating behavior (lifestyle, behavior, religious beliefs).