Download Sect 15.1

Section 15.1
Darwin’s Theory of Natural Selection
Objectives
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Discuss the evidence that convinced Darwin that species could change over time.
List the four principles of natural selection.
Show how natural selection could change a population.
Main Idea
• Charles Darwin developed a theory of evolution based on natural selection.
Developing the Theory
• In 1831
◦ most people thought
 the Earth was about 6,000 years old
 that organisms were unchanging
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Charles Darwin
◦ British theologist
◦ also studied
 medicine
 the sciences
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Darwin
◦ was hired on the HMS Beagle
 as a naturalist
 for its 5-year round-the-world voyage
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Darwin
◦ was hired to collect specimens
 biological
 geological
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his vast collection included
 rocks
 fossils
 plants
 animals
During the voyage, Darwin
◦ read Lyell’s Principles of Geology
 which proposed that the Earth was millions of years old
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was greatly influenced by Lyell’s thinking
as he
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saw marine fossils high in the Andes
unearthed giant fossils of smaller living mammals
saw how earthquakes could lift rocks great distances very quickly
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The Galápagos Islands
◦ on the 4 islands, Darwin began to collect
 mockingbirds
 finches
 and other animals
◦ he noticed that the different islands seemed to have their own
 or slightly different varieties of animals
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On the Galápagos Islands
◦ Darwin was disappointed by
 the stark barrenness of the land
◦ almost every specimen that Darwin had collected
 was new to European scientists
 this included:
• Galápagos Finch
• Galápagos tortoise
• Galápagos land iguana
• Galápagos tree cactus
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Darwin
◦ didn’t begin to reconsider his observations
 until several years after his return
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he took note of John Gould’s work
 with the birds specimens that
 Darwin had brought back
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Gould determined that the birds
 did not live anywhere else in South America
Darwin
◦ began to suspect
 populations from the main land
 changed after reaching the Galápagos
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hypothesized that new species could appear gradually through
 small changes in ancestral species
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inferred that if humans could change species by selective breeding
 then perhaps the same process could work in nature
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Darwin
◦ noted that plant and animal breeders would breed only
 the largest hogs
 the fastest horses
 the cows that produced the most milk
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Darwin termed this process artificial selection
 = the selection by humans for breeding of useful traits from the natural
variation among different organisms
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Darwin
◦ read an essay by Thomas Malthus
◦ this essay suggested that
 if humans were unchecked
□ would out grow its food supply
□ leading to competitive struggles
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Darwin realized that
◦ this reasoning applied to
 plants
 animals
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Darwin thought
◦ if all the offspring of almost any species survived for several generations
 they would overrun the world
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Survival of the Fittest
◦ fitness
 = the ability of an individual to survive and reproduce in its specific
environment
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this information was central to Darwin’s explanation of evolutionary change
Darwin proposed that fitness is the result of adaptations
successful adaptations enable organisms to become better
 suited to their environment
 able to survive and reproduce
Darwin’s theory of natural selection
◦ has four basic principles
 individuals in a population show variation
 variations can be inherited
 organisms have more offspring than can survive on available resources
 variations that increase reproductive success will have a greater chance of
being passed on
The Origin of the Species
• Darwin had likely developed his theory of natural selection by 1840
◦ for years, he continued to develop
 evidence for evolution
 explaining how natural selection might provide a mechanism for the
origin of species
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in 1858
 Alfred Russell Wallace
 proposed a theory
 that almost identical to Darwin’s
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Darwin in 1859 published
◦ On the Origin of Species by Means of Natural Selection
 a condensed version of the book he started many years before
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In his book, Darwin
◦ used the word evolution only once
 on the last page of the book
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evolution
 = cumulative changes on groups of organisms through time
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the theory of natural selection
 is not synonymous with evolution
 it is a means of explaining how evolution works