1. Evolution
Chapter 17
Regents

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 The Fossil Record
Photo credit: Jackie Beckett/American Museum of Natural History
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The fossil record provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time.
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The fossil record provides incomplete information about the history of life.
Over 99% of all species that have lived on Earth have become extinct, which means that the species has died out.
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Interpreting Fossil Evidence
Paleontologists determine the age of fossils using relative dating or radioactive dating.
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Relative Dating
In relative dating, the age of a fossil is determined by comparing its placement with that of fossils in other layers of rock.
Rock layers form in order by age—the oldest on the bottom, with more recent layers on top.
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Index fossils are used to compare the relative ages of fossils.
An index fossil is a species that is recognizable and that existed for a short period but had a wide geographic range.
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Radioactive Dating 
Scientists use radioactive decay to assign an absolute age to rocks.
Radioactive dating is the use of half-lives to determine the age of a sample.
A half-life is the length of time required for half of the radioactive atoms in a sample to decay.
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The First Organic Molecules
Could organic molecules have evolved under conditions on early Earth?
In the 1950s, Stanley Miller and Harold Urey tried to answer that question by simulating conditions on the early Earth in a laboratory setting.
Miller and Urey’s Experiment
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Miller and Urey's experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth.
Although their simulations of early Earth were not accurate, experiments with current knowledge yielded similar results.
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The Puzzle of Life's Origin
Evidence suggests that 200–300 million years after Earth had liquid water, cells similar to modern bacteria were common.
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Free Oxygen
Microscopic fossils, or microfossils, of unicellular prokaryotic organisms resembling modern bacteria have been found in rocks over 3.5 billion years old.
These first life-forms evolved without oxygen.
About 2.2 billion years ago, photosynthetic bacteria began to pump oxygen into the oceans.
Next, oxygen gas accumulated in the atmosphere.
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The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration.
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14. Origin of Eukaryotic Cells
The Endosymbiotic Theory
The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms.
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15. Origin of Eukaryotic Cells
Endosymbiotic Theory
Ancient Prokaryotes
Chloroplast
Plants and plantlike protists
Aerobic
bacteria
Photosynthetic bacteria
Nuclear envelope
evolving
Mitochondrion
Primitive Photosynthetic
Eukaryote
The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms. Ancient prokaryotes may have entered primitive eukaryotic cells and remained there as organelles.
Animals, fungi, and
non-plantlike protists
Ancient Anaerobic
Prokaryote
Primitive Aerobic
Eukaryote
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Patterns of Evolution
Macroevolution refers to large-scale evolutionary patterns and processes that occur over long periods of time.
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Important topics in macroevolution are:
extinction
adaptive radiation
coevolution
punctuated equilibrium
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Extinction
More than 99% of all species that have ever lived are now extinct.
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Extinction
What effects have mass extinctions had on the history of life? Mass extinctions have:
provided ecological opportunities for organisms that survived
resulted in bursts of evolution that produced many new species
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Adaptive Radiation
Adaptive Radiation
Adaptive radiation is the process by which a single species or a small group of species evolves into several different forms that live in different ways.
For example, in the adaptive radiation of Darwin's finches, more than a dozen species evolved from a single species.
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Convergent Evolution
Convergent Evolution
Different organisms undergo adaptive radiation in different places or at different times but in similar environments.
The process by which unrelated organisms come to resemble one another is called convergent evolution. Results in analogous structures.
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Coevolution
Sometimes organisms that are closely connected to one another by ecological interactions evolve together.
The process by which two species evolve in response to changes in each other over time is called coevolution.
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Punctuated Equilibrium
Darwin felt that biological change was slow and steady, an idea known as gradualism.
Biologists have considered two different explanations for the rate of evolution, as illustrated in these diagrams. Gradualism involves a slow, steady change in a particular line of descent. Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent.
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25. Punctuated Equilibrium
Punctuated equilibrium is a pattern of evolution in which long stable periods are interrupted by brief periods of more rapid change.
Biologists have considered two different explanations for the rate of evolution, as illustrated in these diagrams. Gradualism involves a slow, steady change in a particular line of descent. Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent.
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