Download Chapter 22: Descent with Modification: A Darwinian View of Life

Vandan Desai
Biology 301: Ecology and Evolution
Chapter 22: Descent with Modification: A Darwinian View of Life
(Textbook otes)
-----------------------------------------------------------------------------------------------------------------------------I.
Overview: Darwin Introduces a Revolutionary Theory
A. A new era of biology began on November 24, 1859—the day Charles Darwin published On
the Origin of Species by Means of atural Selection
B. The Origin of Species
i. Focused biologists’ attention on the great diversity of organisms
ii. Darwin made two major points in the book
1. Presented evidence that many species of organisms presently inhabiting the
Earth are descendants of ancestral species (common descent)
2. Proposed a mechanism for the evolutionary process, natural selection
a. Population can change over generations if individuals that possess
certain heritable traits leave more offspring than others
b. Results in evolutionary adaptation—accumulation of inherited
characteristics that enhance organisms’ ability to survive and
reproduce in specific environments
c. Evolution—change over time in genetic composition of a population
and could eventually lead to new species
II.
Concept 22.1: The Darwinian revolution challenged traditional views of a young Earth
inhabited by unchanging species
A. In order to understand why Darwin’s ideas were revolutionary, important to examine his
views in context of other Western ideas about Earth and its life
B. Resistance to the Idea of Evolution
i. The Origin of Species
1. Shook the deepest roots of Western culture
2. Challenged a worldview that had been prevalent for centuries
ii. The Scale of Nature and Classification of Species
1. The Greek philosopher—Aristotle
a. Viewed species as fixed and unchanging
b. Scale of nature—all living things are arranged on a scale of
increasing complexity
2. The Old Testament of the Bible
a. Holds that species were individually designed by God and therefore
perfect
b. Interpreted organismal adaptations as evidence that each species was
designed to serve a specific purpose
3. Carolus Linnaeus
a. Developed a two-part (binomial) system of naming organisms that we
still use today (eg. Homo sapiens)
b. Taxonomy—branch of biology concerned with naming and
classifying organisms
iii. Fossils, Cuvier, and Catastrophism
1. Study of fossils (paleontology) helped lay groundwork for Darwin’s ideas
2. Fossils—remains or traces of organisms from the past
a. Usually found in sedimentary rock, which appears in layers/strata
b. Fossils in each layer provide a glimpse of some of the organisms that
populated Earth at the time that layer formed
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Vandan Desai
Biology 301: Ecology and Evolution
3. Georges Cuvier—noted that sedimentary strata contain unique groups of
organisms
a. Deeper strata contain species that are often very different from those
present today
b. Rather than follow evolution, Cuvier believed in Catastrophism—the
idea that each strata boundary corresponds to some catastrophic
event, such as flood
4. Theories of Gradualism
a. James Hutton introduced the theory of gradualism
i. Gradualism—holds that geological changes are the
cumulative product of slow, continuous processes
b. Charles Lyell introduced the theory of uniformitarianism
i. Uniformitarianism—incorporates gradualism, and refers to
the idea that geologic processes have not changed throughout
earth’s history
ii. So, there have been very few catastrophes and the earth must
be older than 4-6000 years old
c. Exerted a strong influence on Darwin’s thinking—applied the
principle of gradualism to biological evolution
5. Lamarck’s Theory of Evolution
a. Lamarck placed fossils in an evolutionary context and was the first
person to present a functional mechanism describing evolution
b. He used two common ideas of his time:
i. Use and disuse—organs that are used become stronger and
more functional; those that are not used deteriorate
ii. Inheritance of acquired characteristics—after acquiring a new
trait, an organism is able to pass it on to offspring
c. Lamarck’s theory was based on observations; they were unsupported
by evidence
III.
Concept 22.2: In The Origin of Species, Darwin proposed that species change through natural
selection
A. 19th century—generally believed species remained unchanged since their creation, but a
major change would challenge this thinking
B. Darwin’s Research
i. As a boy and into adulthood, Charles Darwin had a consuming interest in nature
ii. 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
iii. The Voyage of the Beagle
1. During his travels on the Beagle, Darwin…
a. Observed and collected many specimens of South American plants
and animals
b. Observed various adaptations of plants and animals that inhabited
many diverse environments
c. Geologic observations—found fossils of organisms high in the Andes
Mountains; inferred that rocks containing fossils must have been
raised there by a long series of earthquakes
d. Galapagos Islands
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Biology 301: Ecology and Evolution
i. Learned that most species there occur nowhere else in the
world, but many resemble species in South America
ii. Species on the islands are related but, like finches, were
adapted to different lifestyles
iv. Darwin’s Focus on Adaptation
1. Darwin reassessed all observations made during the voyage
2. He began to perceive adaptation to the environment and the origin of new
species as closely related processes
a. Species adapt to succeed in changing/different environments
b. Confirmed years later—Galapagos finches’ beaks and behaviors are
adapted to the specific foods available on their home islands
3. In 1844, Darwin wrote a long essay on the origin of species and natural
selection but reluctant to publish, anticipating the uproar it would cause
4. In 1858, Darwin received a manuscript from Alfred Russell Wallace who
had developed a theory of natural selection similar to Darwin’s
5. Darwin quickly finished The Origin of Species and published it next year
C. The Origin of Species
i. Evolution explains life’s unity and diversity
ii. atural selection is a cause of adaptive evolution
iii. Descent with Modification
1. Think of descent in terms of a branching tree
2. Over time, organisms enter/experience new environments which lead to
changes in the organisms
3. These changes cause divergence, the branches of the tree
4. Closely related species are closest together in the tree
5. The trunk represents a common ancestor
6. The Linnaean classification scheme fits the concept of an evolutionary tree;
genus and species may differ, but still belong in the same family
iv. Natural Selection and Adaptation
1. Darwin’s observations:
a. All species have the potential to overproduce offspring. If they
continued to do so, they would use up all of the world’s resources
b. Populations are stable, except for seasonal fluctuations, or unless
changed by man altering the environment
c. Environmental resources are limiting
d. Individuals within a population have varying characteristics
e. Much of the variation is heritable (genetic)
2. Mayr’s inferences regarding natural selection:
a. Conditions lead to a struggle for existence among individuals of a
population
b. Individuals with traits that allow them to outcompete others will
survive and reproduce
c. Favorable traits are maintained, leading to gradual changes in the
population
3. Malthus’s influence on Darwin—“man will overpopulate the planet unless
there is disease, war, or famine”—eventually things will start dying
4. Artificial Selection
a. Humans have modified other species over many generations by
selecting and breeding individuals that possess desired traits
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Biology 301: Ecology and Evolution
b. Results in plants and animals bred as livestock/pets that often bear
little resemblance to their wild ancestors (ex. wild mustard)
5. Summary of Natural Selection
a. Overpopulation—more organisms are born than can survive
b. Variation within a population—there will be many variation for
different traits among individuals
c. Competition within the population—individuals will compete for
survival: food, mates, shelter, etc.
d. Survival of the fittest—those with traits best suited to the environment
will be more likely to survive
e. Reproduction—individuals that survive will pass their traits on to the
next generation
6. Key points to remember:
a. Population—group of interbreeding individuals belonging to a
particular species and sharing a common geographic area
b. A population is the smallest unit that can evolve
c. Natural selection occurs because of interaction between organisms
and their environments
d. Natural selection works to increase or decrease the occurrence of
heritable traits
e. Individuals are selected, but populations evolve
IV.
Concept 22.3: Darwin’s theory explains a wide range of observations
A. Natural Selection in Action
i. Differential Predation and Guppy Populations
1. John Endler and David Reznick—adaptive evolution in guppy populations
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ii. The Evolution of Drug-Resistant HIV
1. Researchers developed numerous drugs to combat HIV, but using these
medications selects for viruses resistant to the drugs
a. A few drug-resistant viruses may be present by chance at the
beginning of treatment
b. The drug-resistant pathogens are more likely to survive treatment and
pass on the genes that enable them to resist the drug to their offspring
c. As a result, the frequency of drug resistance in the viral population
rapidly increases
iii. The example of the guppies/HIV highlight 2 important points about natural selection:
1. It is an editing mechanism, not a creative force; it only act on existing
variation in population; cannot create favorable traits
2. It favors traits that increase fitness in current, local environment; what is
adaptive in one situation may not be adaptive in another
iv. In the cases described, natural selection brought about change rapidly enough that it
could be observed directly
B. Homology, Biogeography, and the Fossil Record
i. Homology—similarities resulting from common ancestry
1. Anatomical Homologies
a. Forelimbs of human, cats, whales, and bats share the same skeletal
elements, even though the appendages have very different functions
i. These forelimbs are homologous structures that represent
variation on the ancestral tetrapod forelimb
b. All vertebrate embryos have tails posterior to the anus, as well as
structures called pharyngeal (throat) pouches
i. Develop into gills in fishes and parts of ears/throat in humans
c. Vestigial organs—structures that have marginal, if any, importance to
living organism, but had important functions in organism’s ancestors
d. Comparative anatomy confirms evolution is a remodeling process, an
alteration of existing structures (not perfect = back/knee problems)
2. Molecular Homologies
a. All life forms use same genetic machinery of DNA/RNA and the
genetic code is essentially universal
b. Due to this, all species may have descended from a common ancestor
c. Humans and bacteria share genes inherited from a distant ancestor
3. Homologies and the Tree of Life
a. Molecular homologies basically follow the tree of life
b. More closely related organisms have more homologous molecules
ii. Biogeography—the geographical distribution of species
1. Species found in the same area on Earth are typically closely related
2. Some similar mammals that have adapted to similar environments have
evolved independently from different ancestors (ex. Squirrels)
iii. Fossil Record
1. We can see the development of present day species from ancestral species
2. 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
a. Prokaryote fossils precede all eukaryotic fossils
3. Darwinian view predicts evolutionary transitions leave signs in fossil record
a. Paleontologists have discovered fossils of many transitional forms
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