The Origin of Species

Chapter 22
Descent with Modification: A Darwinian View of Life
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
The Origin of Species
Focused biologists’ attention on the great diversity of organisms
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
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
The historical context of Darwin’s life and ideas
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 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
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
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
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
Gradualism
Is the idea that profound change can take place through the cumulative effect of slow but continuous processes
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 hypothesized that species evolve
Through use and disuse and the inheritance of acquired traits
But the mechanisms he proposed are unsupported by evidence
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
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
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
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
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
From studies made years after Darwin’s voyage
Biologists have concluded that this is indeed what happened to the Galápagos finches
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
Darwin developed two main ideas
Evolution explains life’s unity and diversity
Natural selection is a cause of adaptive evolution
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
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
Evolutionary biologist Ernst Mayr
Has dissected the logic of Darwin’s theory into three inferences based on five observations
Observation #1: For any species, population sizes would increase exponentially
If all individuals that are born reproduced successfully
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
Observation #4: Members of a population vary extensively in their characteristics
No two individuals are exactly alike
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
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
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
Natural selection is differential success in reproduction
That results from the interaction between individuals that vary in heritable traits and their environment
Natural selection can produce an increase over time
In the adaptation of organisms to their environment

If an environment changes over time
Natural selection may result in adaptation to these new conditions
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
Two examples
Provide evidence for natural selection
Differential Predation in Guppy Populations
Researchers have observed natural selection
Leading to adaptive evolution in guppy populations
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
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
The ability of bacteria and viruses to evolve rapidly
Poses a challenge to our society
Homology, Biogeography, and the Fossil Record
Evolutionary theory
Provides a cohesive explanation for many kinds of observations
Homology
Homology
Is similarity resulting from common ancestry
Anatomical Homologies
Homologous structures between organisms
Are anatomical resemblances that represent variations on a structural theme that was present in a common ancestor
Comparative embryology
Reveals additional anatomical homologies not visible in adult organisms
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
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
The Darwinian concept of an evolutionary tree of life
Can explain the homologies that researchers have observed
Anatomical resemblances among species
Are generally reflected in their molecules, their genes, and their gene products
Biogeography
Darwin’s observations of the geographic distribution of species, biogeography
Formed an important part of his theory of evolution
Some similar mammals that have adapted to similar environments
Have evolved independently from different ancestors
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
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?
In science, a theory
Accounts for many observations and data and attempts to explain and integrate a great variety of phenomena
Darwin’s theory of evolution by natural selection
Integrates diverse areas of biological study and stimulates many new research questions