CPB722 LEC

Chapter 22: 

Chapter 22 Descent with Modification: A Darwinian View of Life

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Overview: Darwin Introduces a Revolutionary Theory A new era of biology began on November 24, 1859 The day Charles Darwin published On the Origin of Species by Means of Natural Selection

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The Origin of Species Focused biologists’ attention on the great diversity of organisms

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Darwin made two major points in his book He presented evidence that the many species of organisms presently inhabiting the Earth are descendants of ancestral species He proposed a mechanism for the evolutionary process, natural selection

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Concept 22.1: The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species In order to understand why Darwin’s ideas were revolutionary We need to examine his views in the context of other Western ideas about Earth and its life

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The historical context of Darwin’s life and ideas

Resistance to the Idea of Evolution: 

Resistance to the Idea of Evolution The Origin of Species Shook the deepest roots of Western culture Challenged a worldview that had been prevalent for centuries

The Scale of Nature and Classification of Species: 

The Scale of Nature and Classification of Species The Greek philosopher Aristotle Viewed species as fixed and unchanging The Old Testament of the Bible Holds that species were individually designed by God and therefore perfect

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Carolus Linnaeus Interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose Was a founder of taxonomy, classifying life’s diversity “for the greater glory of God”

Fossils, Cuvier, and Catastrophism: 

Fossils, Cuvier, and Catastrophism The study of fossils Helped to lay the groundwork for Darwin’s ideas Fossils are remains or traces of organisms from the past Usually found in sedimentary rock, which appears in layers or strata

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Paleontology, the study of fossils Was largely developed by French scientist Georges Cuvier Cuvier opposed the idea of gradual evolutionary change And instead advocated catastrophism, speculating that each boundary between strata represents a catastrophe

Theories of Gradualism: 

Theories of Gradualism Gradualism Is the idea that profound change can take place through the cumulative effect of slow but continuous processes

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Geologists Hutton and Lyell Perceived that changes in Earth’s surface can result from slow continuous actions still operating today Exerted a strong influence on Darwin’s thinking

Lamarck’s Theory of Evolution: 

Lamarck’s Theory of Evolution Lamarck hypothesized that species evolve Through use and disuse and the inheritance of acquired traits But the mechanisms he proposed are unsupported by evidence

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Concept 22.2: In The Origin of Species, Darwin proposed that species change through natural selection As the 19th century dawned It was generally believed that species had remained unchanged since their creation, but a major change would challenge this thinking

Darwin’s Research: 

Darwin’s Research As a boy and into adulthood, Charles Darwin Had a consuming interest in nature Soon after Darwin received his B.A. degree He was accepted on board the HMS Beagle, which was about to embark on a voyage around the world

The Voyage of the Beagle: 

The Voyage of the Beagle During his travels on the Beagle Darwin observed and collected many specimens of South American plants and animals Darwin observed various adaptations of plants and animals That inhabited many diverse environments

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Darwin’s interest in the geographic distribution of species Was kindled by the Beagle’s stop at the Galápagos Islands near the equator west of South America

Darwin’s Focus on Adaptation: 

Darwin’s Focus on Adaptation As Darwin reassessed all that he had observed during the voyage of the Beagle He began to perceive adaptation to the environment and the origin of new species as closely related processes

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From studies made years after Darwin’s voyage Biologists have concluded that this is indeed what happened to the Galápagos finches

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In 1844, Darwin wrote a long essay on the origin of species and natural selection But he was reluctant to introduce his theory publicly, anticipating the uproar it would cause In June 1858 Darwin received a manuscript from Alfred Russell Wallace Who had developed a theory of natural selection similar to Darwin’s Darwin quickly finished The Origin of Species And published it the next year

The Origin of Species: 

The Origin of Species Darwin developed two main ideas Evolution explains life’s unity and diversity Natural selection is a cause of adaptive evolution

Descent with Modification: 

Descent with Modification The phrase descent with modification Summarized Darwin’s perception of the unity of life States that all organisms are related through descent from an ancestor that lived in the remote past

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In the Darwinian view, the history of life is like a tree With multiple branchings from a common trunk to the tips of the youngest twigs that represent the diversity of living organisms

Natural Selection and Adaptation: 

Natural Selection and Adaptation Evolutionary biologist Ernst Mayr Has dissected the logic of Darwin’s theory into three inferences based on five observations

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Observation #1: For any species, population sizes would increase exponentially If all individuals that are born reproduced successfully

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Observation #2: Nonetheless, populations tend to be stable in size Except for seasonal fluctuations Observation #3: Resources are limited Inference #1: Production of more individuals than the environment can support Leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

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Observation #4: Members of a population vary extensively in their characteristics No two individuals are exactly alike

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Observation #5: Much of this variation is heritable Inference #2: Survival depends in part on inherited traits Individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals

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Inference #3: This unequal ability of individuals to survive and reproduce Will lead to a gradual change in a population, with favorable characteristics accumulating over generations

Artificial Selection: 

Artificial Selection In the process of artificial selection Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits

Summary of Natural Selection: 

Summary of Natural Selection Natural selection is differential success in reproduction That results from the interaction between individuals that vary in heritable traits and their environment

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Natural selection can produce an increase over time In the adaptation of organisms to their environment

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If an environment changes over time Natural selection may result in adaptation to these new conditions

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Concept 22.3: Darwin’s theory explains a wide range of observations Darwin’s theory of evolution Continues to be tested by how effectively it can account for additional observations and experimental outcomes

Natural Selection in Action: 

Natural Selection in Action Two examples Provide evidence for natural selection

Differential Predation in Guppy Populations: 

Differential Predation in Guppy Populations Researchers have observed natural selection Leading to adaptive evolution in guppy populations

The Evolution of Drug-Resistant HIV: 

The Evolution of Drug-Resistant HIV In humans, the use of drugs Selects for pathogens that through chance mutations are resistant to the drugs’ effects Natural selection is a cause of adaptive evolution

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Researchers have developed numerous drugs to combat HIV But using these medications selects for viruses resistant to the drugs Figure 22.13 Patient No. 1 Patient No. 2 Patient No. 3 Percent of HIV resistant to 3TC Weeks

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The ability of bacteria and viruses to evolve rapidly Poses a challenge to our society

Homology, Biogeography, and the Fossil Record: 

Homology, Biogeography, and the Fossil Record Evolutionary theory Provides a cohesive explanation for many kinds of observations

Homology: 

Homology Homology Is similarity resulting from common ancestry

Anatomical Homologies: 

Anatomical Homologies Homologous structures between organisms Are anatomical resemblances that represent variations on a structural theme that was present in a common ancestor

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Comparative embryology Reveals additional anatomical homologies not visible in adult organisms

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Vestigial organs Are some of the most intriguing homologous structures Are remnants of structures that served important functions in the organism’s ancestors

Molecular Homologies: 

Molecular Homologies Biologists also observe homologies among organisms at the molecular level Such as genes that are shared among organisms inherited from a common ancestor

Homologies and the Tree of Life: 

Homologies and the Tree of Life The Darwinian concept of an evolutionary tree of life Can explain the homologies that researchers have observed

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Anatomical resemblances among species Are generally reflected in their molecules, their genes, and their gene products

Biogeography: 

Biogeography Darwin’s observations of the geographic distribution of species, biogeography Formed an important part of his theory of evolution

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Some similar mammals that have adapted to similar environments Have evolved independently from different ancestors

The Fossil Record: 

The Fossil Record The succession of forms observed in the fossil record Is consistent with other inferences about the major branches of descent in the tree of life

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The Darwinian view of life Predicts that evolutionary transitions should leave signs in the fossil record Paleontologists Have discovered fossils of many such transitional forms

What Is Theoretical about the Darwinian View of Life?: 

What Is Theoretical about the Darwinian View of Life? In science, a theory Accounts for many observations and data and attempts to explain and integrate a great variety of phenomena

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Darwin’s theory of evolution by natural selection Integrates diverse areas of biological study and stimulates many new research questions