Chapter 13 Outline
Chapter 13 Evolution: Evidence of Change
I. Evolution and Life’s Diversity
A. Darwin’s Dilemma
1. The process by which modern organisms have descended from ancient organisms is called evolution.
2. Charles Robert Darwin set sail on HMS Beagle for a cruise around the world to several continents and many remote islands.
3. Darwin examined his specimens and filled notebooks with his thoughts and observations, and spent many hours reading the most current scientific books.
4. Darwin witnessed countless wonders of nature for which his bright young mind demanded an explanation.
B. The Diversity of Life
1. The diversity of life is the variety of living things of every imaginable shape, size, and habit.
2. Darwin’s observations helped him to realize that an enormous number of species inhabit the Earth.
3. It is estimated that 3 million to more than 20 million different living species exist on Earth.
4. Darwin found evidence that even more organisms had vanished from the Earth, and it is estimated than more than 99.9% of all species on Earth are extinct.
C. Fitness: To Survive and Reproduce
1. The physical traits and behaviors that enable organisms to survive and reproduce in their environment give them what Darwin called fitness.
2. In 1859, Darwin provided evidence to support his theory of evolution in The Origin of Species by Means of Natural Selection.
3. Common descent is the fact that all species have shared, or common, ancestors.
4. Adaptations are traits that arise from the need for an organism to be better suited to its environment.
II. The Age of the Earth
A. Evidence in Stone
1. Darwin and other scientists have accumulated a vast amount of evidence that proves that evolution has occurred, and some of this evidence certifies that planet Earth is more than 4 billion years old.
2. In the past, many people believed the Earth was relatively young-only a few thousand years old, and that it had remained unchanged over time.
3. In the eighteenth and nineteenth centuries, scientists began to examine the Earth in great detail and offered the hypothesis that the Earth was indeed very old and had changed slowly over a long period of time by natural forces like weather.
4. Fossils are the preserved remains of ancient organisms.
B. The Geologic Time Scale: A Clock in the Rocks
1. The geologic time scale is a record of the age of earth from the dating of rocks.
2. Relative dating is a technique used by scientists to determine the age of fossils relative to other fossils in different layers of rock.
3. Because geologists did not know how long it took for different rock layers to form, they could not determine the actual age of the fossils.
C. Radioactive Dating
1. Near the middle of the century, our growing understanding of radioactivity provided scientists with a tool that could determine the actual age of rocks.
2. Radioactive elements decay, or break down, into nonradioactive elements at a very steady rate.
3. The unit used to measure this rate of radioactive decay in a unit is called a half-life.
4. Elements with different half0lifes provide natural “clocks” that “tick” at different rates, and when properly interpreted, these clocks help scientists date rocks and specimens of different ages.
5. Absolute dating has enabled geologists to compile a remarkably accurate history of our planet.
6. Using these data, scientists have determined that the Earth is about 4.5 billion years old.
7. Eras are broken down sections of the Earth’s 4.5 billion year history, and are farther divided into periods and epochs.
III. The Fossil Record
A. How Fossils Form
1. A fossil can be as large and complete as an entire perfectly preserved animal or plant, or it might be as small and incomplete as a tiny fragment of a jawbone or leaf.
2. Fossils have been formed in a variety of ways, all of which depend a great deal on chance, such as animals in cold places falling into crevasses in ice or in snow fields.
3. Most fossils are found in sedimentary rock.
4. Sedimentary rocks are formed when exposure to rain, heat, and cold breaks down existing rocks into small particles of sand, silt, and clay.
B. Fossil Evidence: Problems in Assembling the Puzzle
1. The chancy process by which organisms are fossilized means that the fossil record is not as complete as we would like it to be. For every organism that leaves a proper fossil, many die and vanish without leaving a trace.
2. The Grand Canyon is an example of a place where many layers of rock have been exposed-in this case, by the moving water of the Colorado River.
3. When a fossil is exposed, a fortunate scientist may happen along at the right time and remove it for study.
4. The quality of fossil preservation also varies. Some fossils are preserved so perfectly that we can see the microscopic structure of tiny bones and feathers. Other fossils are not preserved as well and so raise fascinating questions about their meaning and importance.
C. What the Fossil Record Tells Us
1. A paleontologist is a scientist who studies fossils.
2. Over the years, paleontologists have collected millions of fossils to make up the fossil record.
3. Paleontologists have assembled good evolutionary histories for many animal groups.
4. The fossil record also tells of major changes that occurred in Earth’s climate and geography.
5. The fossil record shows that change followed change on Earth; species changed as the Earth changed.
IV. Evidence from Living Organisms
A. Similarities in Early Development
1. All living organisms carry within their bodies traces of the history that links them to their ancestors.
2. Embryos are organisms at early stages of development that are similar in related animals.
3. All genes in an organism are not active at the same time, but those that are active during the early development of fish, birds, humans, and related animals are the shared heritage from a common ancestor.
B. Similarities in Body Structure
1. In the embryos of many animals, the clumps of cells that develop into limbs look quite similar, but as the embryos mature, the limbs grow into arms, wings, legs, and flippers that differ greatly in form and function.
2. Homologous structures are different structures that develop from the same body parts.
3. Vestigal organs are organs that serve little or no purpose.
C. Similarities in Chemical Compounds
1. All organisms, from bacteria to humans, share many bio-chemical details.
2. All organisms use DNA and/or RNA to carry information from one generation to the next and to control growth and development.
3. The more two closely related two species are, the more closely their important chemical compounds resemble each other.
D. What Homologies Tell Us
1. Similarities in structure and biochemistry provide powerful evidence that all living things evolved from common ancestors.
2. The structural and biochemical similarities among living organisms are best explained by Darwin’s conclusion: Living organisms evolved through gradual modification of earlier forms-descent from a common ancestor.
3. If organisms had arisen independently of one another, there would be very little chance that they would have similar structures and biochemistries.
4. The very complexity of life and its processes supports Darwin’s conclusion.