Macroevolution
 
History of Evolutionary Thinking:
 
Thinking about evolution can be traced as far back as Ancient Greece. Plato ( 427-347 B.C.) and his student Aristotle ( 384-322 B.C.) held opinions contrary to any form of evolution. Plato believed in a philosophy called idealism. He conceived that there were two coexisting worlds: an ideal real world and an imaginary imperfect one, humans perceive with their senses. Plants and animals were mearly imperfect representatives of the ideal forms. Only the ideal forms were real. Evolution cannot occur since the real organisms are already perfect. Aristotle questioned Plato's thinking and believed organisms were placed on a ladder of increasing complexity. Each organism had an immovable place on this ladder and each rung was occupied. His view of life was fixed and incapable of evolving. This view of life prevailed for over 2,000 years.
 
The creationist-essentialist dogma, that species were individually created and fixed, became embedded in Western thought while Judeo-Christian culture fortified prejudice against evolution.
 
Carolus Linnaeus: (1707-1787) the father of Taxonomy; developed the system of naming organisms with two names ( binomial nomenclature). He grouped species into a hierarchical categories based on their similarities. Linnaeus believed that creatures were permanent organisms incapable of change. Linnaeus only grouped organisms to display God's plan and was not an evolutionist.
 
Cuvier the Father of Paleontology: The study of fossils (paleontology) was developed by the French anatomist Georges Cuvier (1769-1832). Cuvier realized life's history was recorded in fossil-containing strata, and documented the succession of life of fossil species in the Paris Basin. He noted that there was a different set of species at each level of study. He realized that extinction was a common occurrence in the history of life. Cuvier was noted as an antievolutionist. He believed that each successive change in species was caused by a major catastrophe such as a major flood or drought. This kind of thinking was called Catastrophism. Cuvier explained the appearance of new species in strata that did not contain them before by immigration of the new species from other areas after the catastrophic event was over.
 
James Hutton and the Theory of Gradualism: In 1775 Hutton proposed that it was possible to explain the various land forms by looking at the geologic mechanisms currently operating in the world. Charles Lyell , a leading geologist of Darwin's time, expanded on Hutton's theory into the theory of uniformitarianism. In his theory , Lyell, stated that all geologic processes are uniform and have operated from the beginning of time. These processes are so uniform that they balance themselves out through time. The mountain building process will eventually be balanced out by the process of erosion. Darwin rejected this theory but was influenced by the ideas of an ancient earth and the lengthy process that could cause subtle changes.
 
Jean B.Lamarck (1744-1829): Lamarck developed a theory of evolution based on the idea of an organism's movement toward increasing complexity. As these organisms attained perfection, they became better adapted to their environments. These organisms responded to their "felt needs". Organisms that used a certain part of their body to cope with the environment developed these organs, while the organs that were not used deteriorated. This is commonly known as the use and disuse idea. These newly acquired characteristics where then passes on to its offspring. These ideas were in error since acquired characteristics cannot be passed on to an organism's offspring.
 
Charles Darwin : The Voyage of the Beagle, beginning in December of 1831, opened a new chapter in the study of evolution. The voyage was to chart the coast of the poorly known South American coastline. This voyage enabled Darwin to observe the various adaptations of the animals and plants that inhabited the diverse environments of South America. On this voyage Darwin came up with the conclusion that the earth was ancient and not static as the theories of the time presented. By 1840, Darwin had formulated his theory of Natural Selection. In June of 1858, Darwin received a letter from Alfred Wallace who was working on collecting specimens in the East Indies. Accompanying the letter was a manuscript detailing Wallace's own work on natural selection. It was almost identical to Darwin's work. Darwin's was given credit since his work was much more extensive than Wallace's. His The Origin Of Species was published in 1859.
 
Descent with Modification: Darwin did not the term evolution in his book, but used the phrase descent with modification. He perceived that life developed from an unknown common ancestor that lived in the remote past. As these organisms moved into different environments, modifications accumulated over millions of years. Darwin's idea of life was that of a tree with one trunk and many branches. Darwin's book centered on the role of natural selection in adaptation. Natural selection is a differential success in reproduction, and its product is adaptation of organisms to their environment.
 
Evidence that backs up Current Evolutionary Theory.
 
Fossils: any preserved remnant or impression left by organisms that lived in the past.
These remnants of the past are only meaningful in the context of macroevolution only if their vintages relative to other fossils can be determined by methods we shall now discuss.
 
Geological Time Scale:
 
Relative dating: Superimposition of sedimentary rocks tells the relative age of fossils. The fossils in each layer are a local sampling of the organisms that existed when that sediment was deposited. Younger strata are superimposed on the top of older ones , and the succession of fossil species is a story of macroevolution that can be read by paleobiologists.
 
Absolute dating: This does not mean errorless dating. The age is given in years and not in relative terms such as early , before , and after. Radioactive dating is the method often used to determine the age of the fossils and rocks . Fossils contain isotopes of elements accumulated in the organism when they were alive. Because each radioactive isotope has a fixed rate of decay, half-life, it can be used to date the species. Half-life is unaffected by pressure, temperature, or other environmental variables.
 

Examples of half-life:

  • Potassium 40 -- 1.3 x 109
  • Rubidium 87 -- 4.9 x 1010
  • Uranium 238 -- 4.5 x 109
  • Carbon 14 -- 5.7 x 103
 
Another form of dating is called the "clock". Amino acids have right and left handed symmetry (optical isomers). The L- form is synthesized by living organisms. When an organism dies the L form is transformed into the D form . This type of dating is temperature sensitive. Climates that did not change too much over a period of time have fossils that relate well with radioactive dating.
 
Geologic time: There are 4 eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
These eras are divides into periods, and the periods into epochs. Geologic Time Chart.
 
Taxonomy: Linnaeus developed the system used today called Binomial Nomenclature. Each level is called a taxon. The most specific taxon is the species and the most general is the kingdom.
Homology and Analogy: Homology is likeness due to shared ancestry. Sometimes organisms resemble each other but are not related. This is caused by a process called convergent evolution. Organisms living in the same or similar environments develop similar structures based on natural selection. They evolve to fill a niche and help the organism survive. This is called analogy.
 
Protein comparison: Similarity in protein structure indicates a common ancestry.
The amino acid sequence of cytochrome c, an ancient protein common to all aerobic organisms, has been determine for a wide range of organisms from bacteria to complex animals and plants. The sequence found in humans matches the chimpanzees exactly, and differs from the rhesus monkey by just one amino acid. It differs from the dog by 13 the rattlesnake by 20 and the tuna by 31 amino acids.
 
DNA Comparison: Comparison can be made by 3 methods: a). DNA-DNA hybridization, 2 pieces of DNA, one from one species and the other from another closely related species, are mixed together. The more extensive the paring the more homologous the species. b). restriction mapping, use of restriction enzymes to cut the DNA into pieces, and the pieces are compared to a similar species. c). determine the entire DNA sequence of the organism.
 
 
Schools of Taxonomy:
 
1. Cladistic: clades are evolutionary branches. They classify organisms according to the order in time that branches arise along a phylogenic tree. These clades all descended from one ancestral species. This gives the diagrams the appearance of a tree.
 
2. Phenetics: no phylogenetic assumptions are used only measurements in similarities and differences in structure. Analogy and homology are not separated.
 
Continental Drift and Macroevolution:
The continents are not fixed, but drift on the surface of the earth on great plates of crust on the mantle. North America and Europe are presently drifting apart at the rate of 2 cm. per year. About 250 million years ago, near the end of the Paleozoic era, all the land masses formed one huge continent called Pangea. This had a major effects on the isolated populations . 180 million years ago the continents began to split apart. This occurred during the early Mesozoic era. This caused the continents to become major isolated areas for evolution to occur.
 
Mass Extinctions: These allowed for fertile grounds for major adaptive radiations to occur.
 
1). The Ordovician Extinction: Sea creatures killed off by the beginning of the ice age.
 
2). The Devonian Extinction: affected the tropical shallow-water habitats. No major climate changes occurred. "The Oxygen Crisis", The oceans at that time had weak currents due to the lack of the ice caps at the poles. The water is not well oxygenated.
 
3). The Permian Extinction: claimed over 90% of the species of marine animals. This was the time Pangea was formed.
 
4). Cretaceous Extinction: occurred 65 million years ago. This caused the extinction of the dinosaurs, and other major forms of marine and terrestrial life.
 
Click here for an essay on evolution by Stephen J. Gould.