Evolution and Darwin's Theory

Lima Bean Variation Experiment

• Objective: To observe and measure variation within a sample of lima beans.
• Procedure:
  1. Count out 10 lima beans.
  2. Measure the length of each bean in millimeters and record the data.
  3. Combine data with two classmates.
  4. Create a graph with length on the x-axis and the number of beans of each length on the y-axis.
• Think About It:
  1. Calculate the average length of the beans. Determine if most beans are close to the average length.
  2. Predict how a graph of the entire class's data would differ from a smaller group's graph.

15-1 The Puzzle of Life's Diversity

• Biological Diversity: The variety of living things on Earth.
• Evolutionary Theory: A collection of scientific facts, observations, and hypotheses that explain the diversity of life; also known as evolution, or change over time.
• Evolution: The process by which modern organisms have descended from ancient organisms.
• Scientific Theory: A well-supported, testable explanation of phenomena that have occurred in the natural world.
• Charles Darwin's Contribution: Darwin contributed more to the understanding of evolution than anyone else.
• Voyage of the Beagle:
  • Darwin joined the crew of the H.M.S. Beagle in 1831 for a voyage around the world.
  • He made observations and collected evidence that led him to propose a revolutionary hypothesis about how life changes over time.

Darwin's Observations

• Patterns of Diversity: Darwin was intrigued by how well-suited plants and animals were to their environments.
• Geographical Distribution: Darwin was puzzled by the distribution of species; for example, similar grassland ecosystems in Argentina and Australia were inhabited by very different animals.
• Fossils: Darwin collected preserved remains of ancient organisms (fossils).
  • Some fossils resembled living organisms, while others looked completely different.

The Galapagos Islands

• Unique Environments: The Galapagos Islands, located 1000 km west of South America, have diverse climates among the islands.
• Tortoise Variation: Giant tortoises varied in predictable ways from one island to another.
  • The shape of a tortoise's shell could be used to identify its island of origin.
• Finches: Darwin collected several types of small, ordinary-looking brown birds that had differently shaped beaks.
  • He initially thought that some of the birds were wrens, some were warblers, and some were blackbirds.
• Mockingbirds: Individual birds collected from different islands (e.g., Floreana, James Island) looked different from one another.
• Darwin's Hypothesis: Animals living on different islands may have evolved from a common South American ancestor species after becoming isolated from one another.

15-2 Ideas That Shaped Darwin's Thinking

• Prevailing Beliefs in Darwin's Time:
  • Most Europeans believed that the Earth and all its forms of life had been created only a few thousand years ago.
  • Neither the planet nor its living species had changed since that original creation.
  • Geological features were thought to have been produced suddenly by catastrophic events.
• Fossil Record: Discoveries of a rich fossil record challenged the traditional view of life.
• Scientists' Adjustments: Some scientists adjusted their beliefs to include multiple periods of creation, each preceded by a catastrophic event.

Ancient, Changing Earth

• James Hutton:
  • In 1795, Hutton proposed that Earth is shaped by geological forces that operate over extremely long periods.
  • He suggested that layers of rock form slowly, some rocks are moved up by forces beneath Earth's surface, and others are shaped by natural forces.
  • Hutton proposed that Earth had to be much more than a few thousand years old.
• Charles Lyell:
  • Lyell stressed that scientists must explain past events in terms of processes that they can actually observe.
  • His work explained how geological features could be built up or torn down over long periods of time.
  • Lyell's work helped Darwin appreciate the significance of geological phenomena and realize the Earth's age.
• Impact on Darwin: If Earth could change over time, might life change as well? It would have taken many years for life to change in the way he suggested, possible only if Earth was extremely old.

Lamarck's Evolution Hypotheses

• Jean-Baptiste Lamarck:
  • Lamarck was among the first scientists to recognize that living things have changed over time and that all species were descended from other species.
  • He also realized that organisms were somehow adapted to their environments.
• Lamarck's Hypotheses:
  • By selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime, and these traits could be passed on to their offspring.
  • All organisms have an innate tendency toward complexity and perfection.
  • Organisms could alter the size or shape of particular organs by using their bodies in new ways.
  • Acquired characteristics could be inherited.
• Evaluating Lamarck's Hypotheses:
  • Lamarck's hypotheses of evolution are incorrect in several ways.
  • However, Lamarck was one of the first to develop a scientific hypothesis of evolution and to realize that organisms are adapted to their environments.

Population Growth

• Thomas Malthus:
  • In 1798, Malthus published a book in which he noted that babies were being born faster than people were dying.
  • He reasoned that if the human population continued to grow unchecked, there would be insufficient living space and food for everyone.
  • The only forces he observed that worked against this growth were war, famine, and disease.
• Impact on Darwin:
  • Darwin realized that Malthus's reasoning applied even more strongly to plants and animals than it did to humans.
  • A mature maple tree can produce thousands of seeds in a single summer, and one oyster can produce millions of eggs each year.
  • The overwhelming majority of a species' offspring die, and only a few of those offspring that survive succeed in reproducing.
  • What causes the death of so many individuals? What factor or factors determine which ones survive and reproduce, and which do not?

15-3 Darwin Presents His Case

• Darwin's Findings:
  • Darwin learned that his Galapagos mockingbirds actually belonged to three separate species found nowhere else in the world.
  • The brown birds that Darwin had thought to be wrens, warblers, and blackbirds were all finches, also found nowhere else.
  • The island species looked a great deal like a similar species on the South American mainland, yet they were clearly different.
• Publication of On the Origin of Species:
  • In 1858, Darwin received a short essay from Alfred Russel Wallace, who had been doing field work in Malaysia, which summarized Darwin's thoughts on evolutionary change.
  • In 1859, Darwin published the results of his work, On the Origin of Species.
  • In his book, he proposed a mechanism for evolution that he called natural selection.

Inherited Variation and Artificial Selection

• Inherited Variation: Members of each species vary from one another in important ways.
  • Heritable variation: differences that are passed from parents to offspring.
  • Genetic variation is found in wild species as well as in domesticated plants and animals.
• Artificial Selection: Nature provided the variation, and humans selected those variations that they found useful.
  • Artificial selection has produced many diverse domestic animals and crop plants by selectively breeding for different traits.

Evolution by Natural Selection

• The Struggle for Existence: Members of each species compete regularly to obtain food, living space, and other necessities of life.
• Survival of the Fittest:
  • Fitness: The ability of an individual to survive and reproduce in its specific environment.
  • Adaptation: Any inherited characteristic that increases an organism's chance of survival.
  • Individuals with characteristics that are not well suited to their environment either die or leave few offspring.
  • Individuals that are better suited to their environment survive and reproduce most successfully.
• Natural Selection:
  • In natural selection, the traits being selected contribute to an organism's fitness in its environment.
  • Natural selection takes place without human control or direction.
  • Over time, natural selection results in changes in the inherited characteristics of a population.

Descent With Modification

• Descent with Modification: Over long periods, 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.
• Common Descent: All species, living and extinct, were derived from common ancestors.
  • A single