Chapter 16: Darwin’s Theory of Evolution

16.1 Darwin’s Voyage of Discovery

Darwin’s Big Journey

• @@Evolution@@ @@is the process of change over time; it is the process by which modern organisms have descended from ancient organisms@@
• While still a young man, Darwin was invited to sail around the world on a ship called the Beagle; the captain and his crew would be mapping the coast of South America during the five-year journey
  • Darwin’s observations during the journey would lead to his theory of evolution that explains how modern organisms evolved over long periods of time from common ancestors

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Observations Aboard the Beagle

• As Darwin traveled, he noticed three patterns of biological diversity:
  • Species vary around the world
  • Species vary locally
  • Species vary over time
• Concerning worldwide biodiversity, Darwin noticed that places around the world that had similar habitats often had different animals that were ecologically similar
• Concerning local biodiversity, Darwin noticed that different, related species often lived in different habitats within a local area
  • For example, the tortoises of the Galápagos Islands have shells with different shapes; the shapes are related to the environment on the island where the tortoise lives
• Darwin noticed that some fossils of extinct animals were similar to living species
  • A fossil is the preserved remains or traces of ancient organisms
• Overall, the evidence he gathered on his trip suggested that species change over time by some natural process

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16.2 Ideas that Shaped Darwin’s Thinking

An Ancient, Changing Earth

• In Darwin’s time, many people believed that Earth was only a few thousand years old and that Earth had not changed much in that time
  • However, scientists were beginning to find evidence that supported different ideas about Earth’s history, ideas that were part of the new science of geology
  • Two of the best-known geologists were James Hutton and Charles Lyell, and the ideas of both these geologists influenced Darwin’s thinking
• Hutton described how geological processes shape the land, suggesting that great forces under Earth’s surface push mountains upward
  • As these processes are very slow, Earth would have to be much older than a few thousand years to make the formations he observed
  • Hutton introduced a concept called deep time, the idea that Earth’s history is so long that it is difficult to imagine, to explain his ideas
• Lyell argued that the laws of nature do not change over time and concluded that geological processes in the past worked like the geological processes of today
  • Over time, these processes formed the modern landscape; such changes would take millions of years
• @@Overall, Hutton and Lyell concluded that Earth is extremely old and that the processes that changed Earth in the past are the same processes that operate in the present@@

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Lamarck’s Evolutionary Hypotheses

• Darwin was not the first scientist to say that species evolve over time, though earlier scientists disagreed on how evolution happened
• French scientist Jean-Baptiste Lamarck was one of the first scientists to hypothesize how evolution works, suggesting two ideas
  • Lamarck suggested that individual organisms could change during their lifetimes by using or not using parts of their bodies
  • He also suggested that individuals could pass these changes on to their offspring
• Though Lamarack’s ideas were wrong, they did help Darwin in several ways
  • Lamarck made a connection between an organism’s body and its environment
  • He was one of the first scientists to suggest that organisms can change over time
  • He also tried to explain evolution using natural processes

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Population Growth

• The ideas of Thomas Malthus also helped Darwin
  • Malthus was an English economist who noticed that people were being born faster than people were dying, leading to overcrowding
  • Malthus thought that overcrowding led to conditions such as war, disease, and starvation, that would slow down population growth
  • Malthus reasoned that if the human population were to grow without control, then there would not be enough space and food for everyone to live
• However, most offspring die and only a few survive to reproduce
  • Darwin thought could be a key part of evolution and wondered which individuals survive and why

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Artificial Selection

• Darwin also studied the work of farmers who bred plants and animals
  • Breeders selected only the plants and animals with the best traits for breeding
  • These traits were passed on to their offspring, and over time, selective breeding could form trees with even bigger fruit, or cows that gave even more milk
  • Nature provides the variation, and humans select the traits they find useful, in a process Darwin called artificial selection

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16.3 Darwin Presents His Case

Evolution by Natural Selection

• Darwin did not share his theory with the public for 20 years, spending this time collecting more information to support his theory
• However, in 1858, Darwin was shocked to find that Alfred Russel Wallace’s thoughts on evolution were almost the same as his own and realized that if he did not act quickly, Wallace would be given credit for describing natural selection
  • The next year, Darwin published his first book on evolution, called “On the Origin of Species”
• Combining his thoughts with ideas from Malthus and Lamarck, Darwin saw that organisms produce more offspring than can survive, with members of a population needing to compete for resources in order to live
  • Those that do not get enough will not survive to reproduce, a concept Darwin called “the struggle for existence”
• Darwin also hypothesized that some heritable traits make survival easier
  • Any heritable trait that helps an organism survive in its environment is called an adaptation and can include body parts, such as sharp claws or strong wings, as well as behaviors, such as avoiding predators
• Darwin knew that differences in adaptations affect fitness - how well an organism can survive and reproduce in its environment
  • The difference in success between organisms with high fitness and low fitness is called “survival of the fittest”
• Darwin named his method of evolution natural selection - the process by which organisms that are most suited to their environment survive and reproduce most successfully (also known as survival of the fittest)
  • @@Natural selection occurs when more individuals are born than can survive (the struggle for existence), there is heritable variation (variation and adaptation), and individuals have different fitness rates (survival of the fittest)@@

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Common Descent

• Darwin proposed that species living today are descended from older species that survived and reproduced and that over time, those older species evolved into new species in a process he called “descent with modification”
• Darwin used the idea that species change over time to explain the great variety of life on Earth
  • Darwin drew the first evolutionary tree to show how organisms share ancient ancestors, an idea called “common descent”
  • According to the principle of common descent, all species — living and extinct — are descended from ancient common ancestors

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16.4 Evidence of Evolution

Biogeography

• Darwin used the science of biogeography, the study of where organisms live and where their ancestors lived in the past, to support his theory
  • Darwin noted two important patterns: (1) closely related organisms that live in different environments often show great differences, and (2) distantly related organisms that live in similar environments are often similar

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Comparing Body Structure and Embryos

• Darwin suggested that animals with similar structures are related and that their structures are inherited from a common ancestor

• Structures that are shared by related species and that have been inherited from a common ancestor are called homologous structures

  • Evolutionary theory explains that homologous structures adapted to different purposes are the result of descent with modification from a common ancestor

  

• Analogous structures are structures that are similar in function but not structure, and they do not suggest common ancestry

  • For example, the wings of bees, bats, and birds are similar but do not show common descent

• A vestigial structure is a structure that is reduced in size and has little or no function

  • Vestigial structures are inherited structures that have lost much of their original function
  • Vestigial structures may show evolutionary relationships

• Similar patterns of embryo development provide evidence that organisms have descended from a common ancestor

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The Age of Earth and Fossils

• Today, geologists use radioactivity to find the age of rocks and fossils
  • Radioactive dating suggests that Earth is about 4.5 billion years old
• Today, hundreds of new fossil discoveries show clearly how many modern species have evolved from older species
  • Many recently discovered fossils form series that trace the evolution of modern species from extinct ancestors

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Genetics and Molecular Biology

• The universal genetic code and homologous molecules provide molecular evidence of evolution
  • All living cells use DNA and RNA to make proteins and to pass on genetic information, and the code is nearly the same for all living things—bacteria, yeasts, plants, fungi, and animals
  • Biologists have found many homologous molecules
  • For example, the protein cytochrome c is important in cellular respiration, and versions of this protein are found in almost all living things

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Testing Natural Selection

• The best way to gather evidence about evolution is to observe and test ideas about natural selection in action in the wild

• @@Peter and Rosemary Grant studied finches on the Galápagos Islands@@

  • The Grants showed that natural selection occurs in wild finch populations, sometimes quickly
  • They also showed that variation within a species can help the species adapt to and survive environmental changes

  

  

• Like any scientific theory, evolutionary theory is constantly reviewed with each new bit of information

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