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Evolution by Natural Selection
Darwin compared processes in nature to artificial selection.
By doing so, he developed a scientific hypothesis to explain how evolution occurs.
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The Struggle for Existence 
Darwin realized that high birth rates and a shortage of life's basic needs would force organisms to compete for resources.
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How is natural selection related to a species' fitness?
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Survival of the Fittest 
The ability of an individual to survive and reproduce in its specific environment is fitness.
Darwin proposed that fitness is the result of adaptations.
An adaptation is any inherited characteristic that increases an organism's chance of survival.
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Because of its similarities to artificial selection, Darwin referred to the survival of the fittest as natural selection.
In natural selection, the traits being selected contribute to an organism's fitness in its environment.
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6. Evolution by Natural Selection
Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species' fitness in its environment.
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Descent With Modification 
Natural selection produces organisms that have different structures, establish different niches, or occupy different habitats.
Each living species has descended, with changes, from other species over time.
Darwin referred to this principle as descent with modification.
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Evidence of Evolution
What evidence of evolution did Darwin present?
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Evidence of Evolution
Evidence of Evolution
Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process include:
the fossil record
geographical distribution
homologous structures
similarities in early development, or embryology.
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10. EVIDENCE FOR THE THEORY OF EVOLUTION
1. Fossils
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Evidence of Evolution
The Fossil Record 
Darwin saw fossils as a record of the history of life on Earth.
By comparing fossils from older rock layers with fossils from younger layers, scientists could document that life on Earth has changed over time.
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12. Fossils and Ancient Life
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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How Fossils Form
How Fossils Form
Most fossils form in sedimentary rock.
Sedimentary rock forms when exposure to the elements breaks down existing rock into small particles of sand, silt, and clay.
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How Fossils Form
Fossil Formation
The fossil record provides evidence about the history of life on Earth. Most fossils are formed in sedimentary rock.
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16. Interpreting Fossil Evidence
Paleontologists determine the age of fossils using relative dating or radioactive dating.
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17. Interpreting Fossil Evidence
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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18. Interpreting Fossil Evidence
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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21. Mass Extinction Events Percentage of species extinct
Geologic Time Scale
Mass Extinction Events
Percentage of species extinct
76% species
80% species
96% species
The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.
75% species