Transcript Descent with Modification: A Darwinian View of Life

Descent with Modification: A
Darwinian View of Life
Chapter 22
Overview: Darwin Introduces a
Revolutionary Theory
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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
This manuscript:
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Focused biologists’ attention on the great
diversity of organisms
Darwins Two Major Points from Origin
of Species
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Presented evidence that many species of organisms
presently inhabiting the Earth are descendents of
ancestral species (common descent)
Proposed a mechanism for the evolutionary process
(natural selection)
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a population’s allele frequency can change over generations
if individuals that possess certain heritable traits leave more
offspring than others
Results in evolutionary adaptation – accumulation of
inherited characteristics that enhance organisms’ ability to
survive and reproduce in specific environments
evolution – change over time in genetic composition of a
population and could eventually lead to new species
Darwin’s Ideas Challenged Traditional
Views
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The Darwinian revolution challenged
traditional views of a young Earth inhabited
by unchanging species
In order to understand why Darwin’s ideas
were revolutionary, it is important to examine
his view in context of other Western ideas
about Earth and its life
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There was MUCH resistance to the idea of
evolution
On the Origin of Species
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Shook the deepest roots
of Western culture
Challenged a worldview
that had been prevalent
for centuries
The Historical Context of Darwin’s Life
and Ideas – Figure 22.1
The Scale of Nature and Classification of
Species
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Aristotle (Greek Philosopher)
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Old Testament of the Bible
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viewed species as fixed and unchanging
Scale of Nature – all living things are arranged on a scale of
increasing complexity
holds that species were individually designed by God and
therefore are perfect
interpreted organismal adaptations as evidence that each
species was designed to serve a specific purpose
Carolus Linnaeus (Swedish Botanist)
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developed two-part (binomial) system of naming organisms
– still used today
taxonomy – branch of biology concerned with naming and
classifying organisms
Fossils, Cuvier, and Catastrophism
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Study of fossils (paleontology) helped lay
groundwork for Darwin’s ideas
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Fossils - remains or traces of organisms from the
past
Usually found in sedimentary rock, which
appears in layers/strata
Fossils in each layer provide a glimpse of
some of the organisms that populated Earth at
the time that layer formed
Formation of Sedimentary Rock – Figure
22.3
Fossils, Cuvier, and Catastrophism
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Georges Cuvier - noted that sedimentary
strata contain unique groups of organisms
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Deeper strata contain species that are often very
different from those present today
Rather than follow evolution, Cuvier believed
in Catastrophism - the idea that each strata
boundary corresponds to some catastrophic
event, such as flood
Theories of Gradualism
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James Hutton introduced the theory of gradualism
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Charles Lyell introduced the theory of
uniformitarianism
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Gradualism - holds that geological changes are the
cumulative product of slow, continuous processes
Uniformitarianism - incorporates gradualism, and refers
to the idea that geologic processes have not changed
throughout earth’s history
So, there have been very few catastrophes and the earth
must be older than 4-6000 years old
Exerted a strong influence on Darwin’s thinking—
applied the principle of gradualism to biological
evolution
Lamarck’s Theory of Evolution
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Jean Baptiste Lamarck placed fossils in an
evolutionary context and was the first person to
present a functional mechanism describing evolution
He used two common ideas of his time:
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Use and disuse - organs that are used become stronger
and more functional; those that are not used deteriorate
Inheritance of acquired characteristics - after acquiring
a new trait, an organism is able to pass it on to offspring
Lamarck’s theory was based on observations; they
were unsupported by evidence
Lamarck v/s Darwin
Darwin’s Proposals
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In The Origin of Species, Darwin proposed that
species change through natural selection
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19th century—generally believed species remained
unchanged since their creation, but a major change would
challenge this thinking
Darwin’s Research :
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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
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During his travels on the Beagle, Darwin:
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Observed and collected many specimens of South American plants
and animals
Observed various adaptations of plants and animals that inhabited
many diverse environments
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
Galapagos Islands
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Learned that most species there occur nowhere else in the world, but
many resemble species in South America
Species on the islands are related but, like finches, were adapted to
different lifestyles
Voyage of the Beagle
Galapagos Finches
Darwin’s Focus on Adaptation
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Darwin reassessed all observations made during the voyage
He began to perceive adaptation to the environment and the
origin of new species as closely related processes
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Species adapt to succeed in changing/different environments
Confirmed years later—Galapagos finches’ beaks and behaviors are
adapted to the specific foods available on their home islands
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
In 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 next year
The Origin of Species
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Evolution explains life’s unity and diversity
Natural selection is a cause of adaptive evolution
Descent with Modification
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Think of descent in terms of a branching tree
Over time, organisms enter/experience new environments
which lead to changes in the organisms
These changes cause divergence, the branches of the tree
Closely related species are closest together in the tree
The trunk represents a common ancestor
The Linnaean classification scheme fits the concept of an
evolutionary tree; genus and species may differ, but still
belong in the same family
Descent with Modification – Figure 22.7
Natural Selection and Adaptation
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Darwin’s Observations:
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All species have the potential to overproduce
offspring. If they continued to do so, they would
use up all of the world’s resources
Populations are stable, except for seasonal
fluctuations, or unless changed by man altering
the environment
Environmental resources are limiting
Individuals within a population have varying
characteristics
Much of the variation is heritable (genetic)
Summary of Darwin’s Ideas
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Natural selection is differential success in
reproduction (unequal ability of individuals
to survive and reproduce).
Natural selection occurs through an
interaction between the environment and the
variability inherent among the individual
organisms making up populations.
The product of natural selection is the
adaptation of populations of organisms to
their environment.
Natural Selection & Adaptation
Mayr’s Inferences Regarding Natural
Selection
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Conditions lead to a struggle for existence
among individuals of a population
Individuals with traits that allow them to
outcompete others will survive and reproduce
Favorable traits are maintained, leading to
gradual changes in the population
Thomas Malthus
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His ideas had a profound influence on
Darwin’s thinking:
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“man will overpopulate the planet unless there is
disease, war, or famine” - eventually things will
start dying
Populations in nature DO NOT GROW
EXPONENTIALLY – there are always limits to
population growth!
Artificial Selection
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Humans have modified other species over
many generations by selecting and breeding
individuals that possess desired traits
Results in plants and animals bred as
livestock/pets that often bear little
resemblance to their wild ancestors (ex. wild
mustard)
Summary of Natural Selection
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Overpopulation - more organisms are born than
can survive
Variation within a population - there will be many
variation for different traits among individuals
Competition within the population - individuals
will compete for survival: food, mates, shelter, etc.
Survival of the fittest - those with traits best suited
to the environment will be more likely to survive
Reproduction - individuals that survive will pass
their traits on to the next generation
Natural selection is
differential success in
reproduction
–That results from the
interaction between
individuals that vary in
heritable traits and
their environment
Key Points to Remember
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Population - group of interbreeding individuals
belonging to a particular species and sharing a
common geographic area
A population is the smallest unit that can evolve
Natural selection occurs because of interaction
between organisms and their environments
Natural selection works to increase or decrease the
occurrence of heritable traits
Individuals are selected, but populations evolve
Evolution
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Theory - an accepted hypothesis that has been tested over
and over again without yet being disproved
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Definition - Evolution is the change in the overall genetic
makeup (allele frequency) of a population over time
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Three Basic Components
a. Individuals cannot evolve. Populations evolve.
b. Natural selection is the mechanism of evolution.
c. Evolution occurs by chance (NOT GOAL ORIENTED).
Darwin’s Theory Explains a Wide Range
of Observations
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Natural Selection in Action:
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The evolution of insecticide resistance in insect
populations
The evolution of drug resistant HIV
Note: an environment does not create resistance, it selects for
resistant individuals that were already present in the
population.
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And, natural selection depends on time and place. It favors those
characteristics in a genetically variable environment that increase
fitness in the current, local environment.
Evolution of Insecticide Resistance
1. By spraying crops with poisons to kill
insects, humans have unwittingly favored
the reproductive success of insects with
inherent resistance to poisons.
2. Resistant individuals survive and
reproduce, passing the gene for resistance
to offspring.
3. Additional applications of the same
insecticide will be less effective, and the
frequency of resistant insects in the
population will grow.
The Evolution of Drug Resistant HIV
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Researchers developed numerous drugs to
combat HIV, but using these medications
selects for viruses resistant to the drugs
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A few drug-resistant viruses may be present by
chance at the beginning of treatment
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
As a result, the frequency of drug resistance in
the viral population rapidly increases
Antiviral Resistance
Darwin’s Theory Explains a Wide Range
of Observations
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The example of the insect resistance/HIV highlight
2 important points about natural selection:
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It is an editing mechanism, not a creative force; it only act
on existing variation in population; cannot create
favorable traits
It favors traits that increase fitness in current, local
environment; what is adaptive in one situation may not be
adaptive in another
In the cases described, natural selection brought
about change rapidly enough that it could be
observed directly
Evidence of Evolution
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Evidence that the diversity of life is a product of
evolution pervades every research field of biology.
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Comparative Anatomy
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Biogeography
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Anatomical Homologies
Embryological Homologies
Molecular Homologies
Continental Drift
Geographic Distribution of Species
Fossil Record Evidence
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Succession of Fossil Forms
Comparative Anatomy
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The study of different structures contributes to
scientists’ understanding of the evolution of
anatomical structures and of evolutionary
relationships.
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Homologous structures
Analogous structures
Vestigial Structures
Homologies
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Homologous structures are
similar structures occurring in
different species that are
believed to be derived from a
common ancestor.
Analogous structures are
similar structures occurring in
different species that are
believed to be the result of
convergent evolution (similar
environmental pressures).
Homology
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Species with common ancestry should display
underlying similarities, even in features that
no longer match in function.
Homology refers to similarities in
characteristics resulting from common
ancestry
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3 types: anatomical, embryological & molecular
Anatomical Homologies
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Descent with modification is indeed evident in
anatomical similarities between species grouped in
the same taxonomic category.
The forelimbs of human, cats, whales, and bats share
the same skeletal elements, even though the
appendages have very different functions
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These forelimbs are homologous structures that represent
variation on the ancestral tetrapod forelimb
The most likely explanation is that the basic
similarity of these forelimbs is the consequence of
the descent of mammals from a common ancestor –
rather than all being uniquely engineered and
unrelated.
Homologous Structures – Figure 22.14
Anatomical Homologies
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Many homologies that are not obvious in
adult organisms become evident when we
look at embryonic development:
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All vertebrate embryos have tails posterior to the
anus, as well as structures called pharyngeal
(throat) pouches
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Develop into gills in fishes and parts of ears/throat in
humans
Embryological Homologies
Closely related organisms go through similar stages in their embryonic development.
For example, all vertebrate embryos go through a stage in which they have gill
pouches on the sides of their throats.
Early Embryonic Development
Vestigial Homologies
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Vestigial organs - structures that have
marginal, if any, importance to living
organism, but had important functions in
organism’s ancestors
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Comparative anatomy confirms evolution is a
remodeling process, an alteration of existing
structures (not perfect = back/knee problems)
Vestigial Structures
The skeletons of some
snakes retain vestiges of
the pelvis and leg bones
of walking ancestors.
We would not expect to
see these structures if
snakes had an origin
separate from other
vertebrate animals.
Molecular Homologies
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All life forms use the same genetic machinery of
DNA/RNA and the genetic code is essentially
universal
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Due to this, all species may have descended from a
common ancestor
Humans and bacteria share genes inherited from a
distant ancestor
Homologies and the Tree of Life
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Molecular homologies basically follow the tree of life more closely related organisms have more similar
homologies.
Molecular Homologies – Table 22.1
The data show the same pattern of
evolutionary relationships that
researchers find when they
compare other proteins or assess
relationships based on nonmolecular methods, such as
skeletal anatomy.
Homologies mirror the taxonomic
hierarchy of the tree of life – the
Darwinian view of life predicts that
different kinds of homologies in a
group of organisms will all tend to
show the same branching pattern
through their evolutionary history.
Biogeography
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Darwin’s observations of the geographic distribution
of species (biogeography) formed an important part
of his theory of evolution.
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Species tend to be more closely related to other species
from the same area than to other species with the same
way of life but living in different areas.
Austraila is the home of marsupials (develop in pouch) –
distinct from eutherians (live elsewhere and develop in
uterus)
Some similar mammals that have adapted to similar
environments have evolved independently from different
ancestors (ex. Squirrels)
Biogeography
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Darwin’s experience in the Galapagos were
pivotal to his thinking:
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Islands generally have species found no where else in the
world, yet it was observed by Darwin that most island
species are closely related to species from the nearest
mainland or neighboring island.
This explains why 2 islands with similar environments in
different parts of the world are populated not by closely
related species, but rather by species that resemble those
of the nearest mainland.
Biogeography
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Some similar mammals that have adapted to similar
environments
 Have evolved independently from different ancestors
 This is convergent evolution
Sugar
glider
NORTH
AMERICA
AUSTRALIA
Flying
squirrel
Fossil Record
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We can see the development of present day species
from ancestral species
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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Prokaryote fossils precede all eukaryotic fossils
Darwinian view predicts evolutionary transitions
leave signs in fossil record
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Paleontologists have discovered fossils of many
transitional forms
Endurance of the Darwinian View
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The Darwinian view of life endures in biology
because it is supported by independent types
of evidence:
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Anatomical and molecular homologies that match
patterns in space (biogeography) and time (the
fossil record).
Natural selection can also explain how similar
adaptations can evolve independently among
distantly related species, such as sugar gliders and
flying squirrels.
Summary of Darwin’s Theory
http://bcs.whfreeman.com/thelifewire/content/chp23/2302003.html
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Individual organisms in nature differ from one another and
some of this variation is inherited
Organisms in nature produce more offspring than can survive
– and many that survive do not reproduce
Members of each species must compete for resources
Individuals best suited to their environment survive and
reproduce most successfully – they pass their traits onto their
offspring
Species change over time – this is caused by natural selection
– new species arise and other species disappear
Species alive today have descended with modifications from
species that lived in the past
All organisms on Earth are united into a single tree of life by
common descent