Evolution
The modern view of the living world came into focus on November 24, 1859, the day Charles Darwin published On the origin of Species by Means of Natural Selection. In the work Darwin argued from evidence that species were not specially created in their present forms but had evolved from ancestral species, and that the mechanism for this evolution was Natural selection. The theory of evolution is one of the most pervasive and explanatory themes in modern biology. The idea that organisms can change from one generation to the next, that life changes as time progresses was an idea contrary to the teachings of many of the old respected philosophers.
Plato:
Plato (427-347 B.C.) believed in two worlds, a real world, which was ideal and eternal, and an illusory world of imperfection. It was this imperfect world that we perceive through our senses. The variations we see in plants and animals were to Plato merely imperfect representatives of the ideal forms.
Aristotle:
Aristotle (384-322 B.C.) who was a student of Plato, was a great naturalist and an excellent observer. He was one of the first philosophers to see "continuous variation" in nature. However, he held a belief that constancy and perpetuity were the main themes of nature. He saw everywhere well defined species, fixed and unchanging despite of all the stress on them. Aristotle stated that species and their forms had to be eternal. Aristotle recognized that organisms ranged from relatively simple forms to very complex. He believed that all living forms could be arranged on a scale of increasing complexity. This "scale" became known as the scala naturae ("scale of nature"). Each form along this ladder of life had its allotted rung and every rung was taken. In this view of life, which was held for over 2000 years, species were fixed, or permanent, and did not evolve.
Natural Theology
This prejudice against the idea of evolution was maintained by the Judeo-Christian culture and by the old testament account of special creation. Even in Darwin's time, biology in Europe and America was dominated by natural theology a philosophy dedicated to discovering the Creator's plan by studying nature. The major objective of natural theology was to classify species in order to reveal the steps of the scale of life that God had created. These natural theologians cataloged and described much of the plant and animal life on earth. As a result biology during this time was primarily a descriptive science. These natural theologians saw adaptations of organisms to their environments as evidence that the Creator had designed each and every species for a particular purpose.
Carolus Linnaeus:
Carlous Linnaeus (1708-1778), was a Swedish physician and Botanist, who sought order in the diversity of life, has been called the "Father of Taxonomy". Taxonomy is the branch of biology concerned with naming and classifying the diverse forms of life. Linnaeus developed the Binomial system of naming organisms, which is still used today. In this system each species is given a unique two-part name based on the Genus and the species. As well Linnaeus adopted a classification scheme for grouping species into a hierarchy of increasing general categories. For example, similar species are grouped into the same genus. Similar Genera (pl. of genus) are grouped into the same family etc. Today modern Taxonomy uses 7 principle categories or "Taxons". From the largest and most general to the most specific these taxons are Kingdom, Phylum (or in Botany "Division") Class, Order, Family, Genus, Species. For Linnaeus, grouping organisms into these taxons implied no evolutionary relationship between them. He was simply trying to reveal God's plan. Ironically, Linnaeus's scheme does indeed reflect the evolutionary kinships of a great many plants and animals.
Fossils:
Fossils are relics or impressions of organisms from the past. Fossils form in several ways but often what is left after the fossilization process is a reproduction, were the organic components (most often the hard body parts) of the organism are replaced with minerals of rock. Fossils are found in Sedimentary Rocks, which are rocks that formed from the sand and mud sediments that settle to the bottom of oceans, lakes and marshes. New layers of mud cover older layers and as more and more layers are deposited the older ones are compressed into rock. The body parts of any dead plant or animal that are trapped in these layers could become fossils if the conditions are right.
Georges Cuvier (1769-1832)
The science of the study of fossils is called Paleontology. The great French anatomist Georges Cuvier (1769-1832) developed this science. Cuvier realized the history of life on earth was recorded in these strata of rocks. As he documented the succession of fossil species in the Paris Basin, he noted that each stratum was characterized by a unique assemblage of fossil species. The oldest (deepest) strata contained species that were very different from those living today. Cuvier even recognized that extinction had been a common occurrence in the past. From stratum to stratum new species would appear and others would disappear. Yet Cuvier was staunch opponent of evolution. But how did he explain the dynamic story being told in the fossil record with a concept that species are immutable? Cuvier speculated that the boundaries between the fossil strata corresponded to catastrophic events, such as floods or drought, that had destroyed many of the local species that had lived in that area at the time. The appearance of new species in upper strata was explained by the migration and repopulating by new species from outside that area after the catastrophe. This view of earth's history is known as catastrophism.
Gradualism and Uniformatarianism
In 1795, the Scottish geologist James Hutton proposed a principle that became known as Gradualism. Hutton proposed that it was possible to explain the variation in landforms by looking at mechanisms currently operating in the world. For example, canyons were cut by rivers running down their lengths. Sedimentary rock was created by sediments eroded from the land and deposited in at the bottoms of oceans and lakes. Such profound changes in the earth's crust were the cumulative product of slow but continuous processes. Charles Lyell (1797-1875) incorporated Hutton's ideas into a theory known as Uniformitarianism. Lyell felt that geological processes are so uniform that their rates and effects must balance out through time. Processes that build mountains must eventually be balanced by the erosion of those mountains. Darwin was influenced by two conclusions of Lyell's theory, first geological change results from slow continuous processes not sudden events, and the earth must be very, very old!
James Hutton
Charles Lyell (1797-1875)
Charles Lyell (1797-1875) incorporated Hutton's ideas into a theory known as Uniformitarianism.
The Coming of Evolution
Evolution in way contradicts common sense. The offspring of any organism always develop again into the parental type. A cat always produces only cats. It required a veritable intellectual revolution before one could even conceive of evolution. The greatest obstacle to the establishment of the theory of evolution was the fact that it can not be observed directly, like many of the phenomena of physics etc. Evolution can only be inferred. The century from about 1740 to 1840 is crucial to the story of evolution because this was the period when the concept of evolution made a breakthrough in the minds if most thinkers. Progress in natural sciences, economics and social and political sciences become the theme. Therefore "evolution in the natural world "and progress in the social world".
Evolution before Darwin
As people began to view their world as more dynamic new theory was needed to explain the two well-known phenomena in nature.
1) Plants and animals show a graded series of "perfection" (i.e. continuous variation).
2) The amazing diversity of organisms (i.e. that anything that is possible to imagine has effectively taken place.)
There were several naturalists before Darwin who had suggested that life had evolved along with evolution of the earth. However only one, Jean Baptiste Lamark (1744 - 1829) had developed a comprehensive model to explain how life evolves.
Lamarck
Lamark was in charge of the invertebrate collection at the Natural History Museum in Paris. He compared the species in the fossil collection with those species living today. He could see what appeared to be several lines of decent, each a chronological series of older to younger fossils leading to a modern species. Lamarck refereed to this as the "Transformation of a species in a phyletic line".
"After a long succession of generations.... individuals, originally belonging to one species become at length, transformed into a new species distinct from the first."
Lamark believed evolution was driven by an innate tendency towards greater and greater complexity, which he equated with perfection. Lamark also felt that such change would be a slow gradual process. Lamarck then had essentially a correct description of natural events. His notion of evolution explained the two phenomena seen in nature.
HOWEVER, Lamark failed to come up with a workable "mechanism' of how evolution could have come about. One of Lamarck's explanations of evolutionary change was a capacity (of an organism) to react to special conditions in his environment. Unfortunately Lamark is remembered mostly for the mechanism he proposed for this evolutionary change. Lamark's mechanism involved two ideas. First was the idea of "use and disuse". This was the idea that those parts of the body that are used extensively to cope with the environment become larger or stronger, while those body parts that are not used deteriorate. The second component of Lamark's mechanism was called the inheritance of acquired characteristics. In this concept of heredity, the modifications an organism acquires during its lifetime can be passed along to its offspring. Lamark deserves much credit for his theory, which was visionary in many respects, in its claim that evolution is the best explanation for both the fossil record and the current diversity of life, in its emphasis on the great age of the Earth, and in its stress on adaptation to the environment as a primary product of evolution.
By way of example let us look at how Lamark's mechanism would have answered a question that had perplexed biologist for a very long time: Why does the giraffe have such a long neck?
The Lamarckian view was that somatic characteristics acquired by an individual during its lifetime could be passed to its offspring. Thus, the characteristics of each generation would be determined in part, by all that had happened to members of the proceeding generations (by all the modifications that had occurred in them, including those caused by experience, use or disuse of body parts and accidents). Evolutionary change would be a gradual accumulation of such acquired modifications over many generations. Therefore according to the Lamarckian view, ancestral giraffes with short neck tended to stretch their necks as much as they could to reach the tree foliage (the major part of their food). This frequent neck stretching caused their offspring to have slightly longer necks. Therefore, as a result of neck stretching to reach higher and higher leaves each successive generation would have longer necks.
Darwin and "Evolution through Natural Selection"
Charles Darwin was born in the Shropshire town of Shrewsbury, in western England, in 1809. At the age of 16 Darwin was sent by his father , a prominent physician, to medical school at the University of Edinburgh. Darwin found these studies boring and left without completing his degree. He was then enrolled at Christ College at Cambridge University to study to become a clergyman. At this time in England most of the prominent naturalists and other scientists belonged to the clergy. It was at Cambridge that Darwin met Reverend John Henslow a professor of botany. Soon after Darwin received his degree in 1831, Professor Henslow recommended the young graduate to Captain Robert FitzRoy, who was preparing the survey ship Beagle for a voyage around the world. It was Darwin's observations on this voyage that were to plant the seeds of a theory as to how organisms could evolve, a process he called "natural selection".
The HMS Beagle's mission was to chart the poorly known coast of South America. While the ship surveyed the coast, Darwin spent most of his time ashore collecting specimens of the exotic and diverse flora and fauna. As the ship made its way around the coast, Darwin observed the various adaptations of plants and animals that inhabited the many different habitats of South America, from jungles, to grasslands, and mountains. Darwin noted that the flora and fauna of South America was distinctly different from that seen in similar habitats in Europe. He also noted the fossils he found while different from the living species, were also distinctly South American.
One of the last South American stops the Beagle made on its around the world trip was at a small group of islands about 900 km off the west coast of Ecuador, the Galapagos Islands. Darwin's observations on the Galapagos Islands showed that the animal species were unique to the islands, but they resembled those species found on the South American mainland. From a collection of finches Darwin found that some species were unique to individual islands while some species were found on two or more adjacent islands. These small birds may have given Darwin a clue to how new species originate.
Darwin began to perceive the origin of new species and adaptation as closely related processes. A new species would arise from an ancestral form by gradual accumulation of adaptations to a different environment. For example if a single population of a species was to become fragmented into two smaller populations, and these populations were to become isolated from one another by physical barriers, these populations could diverge from one another as each population adapted to changing local conditions. This hypothesis for the origin of species predicted that over many generations the two populations could become dissimilar enough to be designated separate species.
Darwin and A. R. Wallace both came to the same conclusions about evolution at almost the same time, however Darwin published his book, "The Origin of Species" first. Darwin proposed a workable mechanism for evolution, through a process of natural selection.
A. R. Wallace
Natural Selection and Adaptation
Natural Selection implies that individuals with traits that better adapt them to a specific environment will survive and out number other less well suited individuals. Evolutionary biologist Ernst Mayr has dissected the logic of Darwin's theory of natural selection into three inferences based on five observations
Observation #1. Over production: Organisms commonly produce far greater numbers of offspring than can survive. Most populations have such great fertility that their population sizes would increase exponentially if all individuals that were born lived to reproduce.
Observation #2. Stable populations. Most populations are normally stable in size except for seasonal fluctuations.
Observation #3. Limited Resources. The food supply needed to sustain a population is usually in short supply. That is there is usually not enough of the necessary "resources" to go around.
Inference #1. Struggle for existence. Production of more individuals than the environment can support leads to competition among those individuals for those limited resources.
Observation #4. Variation. Individuals of a population vary extensively in their characteristics; no two individuals are exactly alike.
Observation #5. Inheritance of variation. Much of this morphological (phenotypic) variation is inherited from one generation to the next.
Inference #2. Survival of the best adapted. Only those individuals with those combinations of traits or characteristics which give them an advantage at obtaining those limited resources are going to survive. These individuals will be more likely to leave more offspring than those individuals who are less fit.
Inference #3. Accumulation of beneficial adaptations. This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over the generations.
Therefore Natural selection is this differential success in reproduction, and its product is adaptation of organisms to their environment. In other words, each generation is made up of the offspring of the most successful reproducers of previous generations. Therefore, in a sense, "Nature (the environment) selects" which lines of living things will survive to reproduce and which lines will not survive, therefore, not reproduce therefore, die away.
Back to the Giraffes:
We saw how Lamark's mechanism explained why giraffes have such long necks. Now lest see how Darwin's mechanism of natural selection would explain this phenomena.
Modern evolutionary theory proposes ancestral giraffes probably had short necks but that precise length varied from individual to individual, (phenotypic variation), due to varying genotypes.
When food supply was limited those individuals with longer necks would survive better because they could utilize a food source unavailable to the others (decrease competition) therefore, longer necked giraffes survive to pass on the longer neck trait (allele) therefore progeny are longer necked.
A great deal of evidence has been accumulated since Darwin formulated his mechanism of evolution through natural selection. Much of this evidence is far stronger than that available to Darwin and his contemporaries. What follows are only some of the many lines of evidence.
1. Patterns of Distribution (Biogeography)
The study of the distribution of plants and animals is the science of Biogeography. One of its basic tenets is that each species of animal and plant originated only once. The particular place where this occurred is known as the species center of origin. The center of origin is not a single point but the range of the population when the new species was formed. From its center of origin, each species spreads out until halted by a barrier of some kind - physical, such as an ocean or mountain; environmental such as an unfavorable climate; or ecological, such as the presence or organisms that compete with it for food or shelter.
The processes of Continental Drift and Plate Tectonics has given us much insight into the patterns of biogeography.
2. The Fossil Record ("Paleontology")
When fossils are arranged in the order of their age, a progressive series of changes are seen. Fossils are created when organisms become buried in sediment, the calcium in bone or other hard tissue is mineralized and the sediment eventually is converted to rock. The fossils contained in sedimentary rock layers reveal a history of life on earth.
Trilobites
Transitional fossils
3. Comparative Anatomy
Many organisms exhibit structures that appear to have been derived from a common ancestral form. The forelimbs of all mammals for example, contain the same pattern of bones, although they now carry out a variety of different functions. All vertebrates have the same pattern of bones, muscles, nerves, blood circulation and organs. The pattern becomes gradually more complex as one moves from fishes to amphibians to reptiles to mammals. Organs of different organisms that have a similarity of form due to a common evolutionary origin are termed homologous.
Examples:
A bird's wing, a dolphin's flipper, a bat's wing and a human arm and hand, although superficially dissimilar are composed of a very similar arrangement of bones, muscles and nerves. Therefore, they are homologous. Both the bird's wing and the bat's wing evolved from the forelimb of a common vertebrate ancestor. However, the flying surfaces of their wings are quite different. Feathers grow out of the posterior margin of the wings on the bird, while the flight surface of the bat's wing is essentially a webbed hand. Flight evolved independently in the two groups. Therefore, although the forelimbs are utilized as wings in both birds and bats, they are modified in quite different ways.
Organs that are not homologous but simply have similar functions in different organisms are termed analogous organs. The wings of various unrelated flying animals, such as insects and vertebrates resemble one another superficially, BUT the structures are very different. Vertebrate wings are modified forelimbs supported by bones while insect wings are outgrowths of the upper wall of the thorax and are supported by chitinous veins.
4. Development (Embryology)
In many cases the evolutionary history of an organism can b e seen to unfold during its development, with the embryo exhibiting characteristics of the embryos of its ancestors. For example, early in their development embryos possess gill slits like a fish and later exhibit a tail, the vestige of which we carry to adulthood as the coccyx (tail bone) at the end of our spine. Human fetuses even possess a fine fur (called lanugo) during the fifth month of development.
The relict developmental forms suggest strongly that our development has evolved with new instruction being laid on top of old ones and the overall developmental program getting progressively longer.
5. The Molecular Record (Biochemistry and Molecular Biology)
The fact that organisms have evolved progressively from relatively simple ancestors implies that a record of the evolutionary change is present in the cells of the organisms, in their DNA. According to evolutionary theory, every evolutionary change involves the substitution of new versions of genes for old ones, the new arising from the old by mutation. Then coming to predominance because of evolutionary changes involves a progressive accumulation of genetic change in the DNA. Organisms that are more distantly related will accumulate a greater number of evolutionary differences. This is indeed what is seen when DNA sequences are compared between various organisms. For example, the longer the time since the organisms diverged, the greater the number of differences in the nucleotide sequence of genes for cytochrome-c, a protein that you will recall, plays a key role in oxidative metabolism. The same regular pattern of change is seen in hemoglobins and many other proteins. Again we see that evolutionary history involves a pattern of progressive change.
