Chapter 22  Descent with Modification: A Darwinian View of Life
Student Guided Notes
Overview: Endless Forms Most Beautiful

On November 24, 1859, Charles Darwin published his hypothesis in ___________________________
_________________________________________________, ushering in the era of evolutionary biology.

Darwin defined evolution as ____________________________________________, proposing that
Earth’s many species are descendants of ancestral species that were very different from those alive today.
○ Evolution can also be defined more narrowly as _________________________________________
________________________________________________________________________________.
Concept 22.1 The Darwinian revolution challenged traditional views of a young Earth inhabited by
unchanging species

Darwin’s hypothesis had its roots in the work of many other individuals.

The Greek philosopher Aristotle (384–322 A.D.) opposed any concept of evolution and viewed species as
________________________________________________________.

____________________________________________________ (1707–1778), a Swedish physician and
botanist, developed a __________________________________ system for naming species according to
genus and species and classifying species into a hierarchy of increasingly complex categories.

Darwin’s views were influenced by fossils, _________________________________________________
____________________________________________________________________________________.
○ Fossils within layers of __________________________________________ rock show that a
succession of organisms have populated Earth throughout time.

Paleontology, the study of _______________________________________, was largely developed by the
French anatomist _____________________________________________ (1769–1832).
○ Instead of evolution, Cuvier advocated _______________________________, speculating that
boundaries between strata were due to local floods or droughts that destroyed the species then present.

In contrast to Cuvier’s catastrophism, Scottish geologist James Hutton (1726–1797) proposed a theory of
gradualism, which held that profound geologic changes took place through the cumulative effect ______
_____________________________________________________________________________________.

Later, geologist Charles Lyell (1797–1875) proposed a theory of ________________________________,
which held that geologic processes had not changed throughout Earth’s history.
22-1

Hutton’s and Lyell’s observations and theories had a strong influence on Darwin.
○ If geologic changes result from slow, continuous processes rather than sudden events, then the Earth
must be __________________________________________________________________________
_________________________________________ estimated by theologians from biblical inference.

In 1809, French biologist ______________________________________________________________
(1744–1829) published a theory of evolution.

Lamarck explained his observations with two principles: ______________________________________
of parts and the ______________________________________________________________________.
○ Use and disuse was the concept that body parts that are used extensively become _______________
_______________________________________________________, while those that are not used
__________________________________________. (he was right on this!!)
○ The inheritance of acquired characteristics stated that modifications acquired during the life of an
organism _____________________________________________________. (he was wrong on this!!)
○ A classic example is the long neck of the giraffe. Lamarck reasoned that the long, muscular neck of
the modern giraffe evolved over many generations as the ancestors of giraffes reached for leaves on
higher branches and passed this characteristic on to their offspring.
Concept 22.2 Descent with modification by natural selection explains the adaptations of organisms and the
unity and diversity of life

When Darwin was 16, his father sent him to the University of ______________________________ to
study _________________________________.

Darwin left Edinburgh without a degree and enrolled at ______________________________ University
with the intent of becoming a __________________________________.
○ At that time, most naturalists and scientists belonged to the clergy.

After graduation, Darwin joined the crew of the survey ship HMS Beagle as ship naturalist and
conversation companion to Captain Robert FitzRoy.
The voyage of the Beagle

Darwin embarked from _____________________________ on the Beagle in December ____________.

Darwin collected thousands of specimens and noted that the plants and animals of South America were
very different from those of Europe.

Darwin also found fossils that were formed by _______________________ similar to one he experienced.
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○ These observations reinforced Darwin’s acceptance of Lyell’s ideas and led him to doubt the
traditional view _________________________________________________________________
__________________________________________ (thought of as 6,000 years old and unchanging.)

Darwin’s interest in the geographic distribution of species was further stimulated by the Beagle’s visit to
the _____________________________________________________, a group of young volcanic islands
900 km west of the ________________________________________________ coast.
Darwin’s focus on adaptation

During his travels, Darwin observed many examples of adaptations, ____________________________
___________________________________________________________________________________
_________________________________ (ex. finches and differences in beaks related to food they ate).
○ Darwin explained that adaptations arise by natural selection_______________________________
________________________________________________________________________________
________________________________________________________________________________.

By the early 1840s, Darwin had developed the major features of his theory of natural selection as the
mechanism for evolution.

In June 1858, ______________________________________- (1823–1913), a young naturalist sent
Darwin a manuscript containing a hypothesis of _____________________________________________
___________________________________________________.

Darwin finished The Origin of Species and published it the next year.

Although both Darwin and Wallace developed similar ideas independently, the theory of evolution by
natural selection is attributed to Darwin because he developed his ideas earlier and supported the theory
much more extensively.
The Origin of Species

Darwin used the phrase _______________________________________________ to describe evolution.
○ All organisms are related through descent from a common ancestor that _______________________
______________________________________. As a result, organisms share many characteristics,
explaining the ___________________________ of life.
○ Over evolutionary time, the descendents of that common ancestor have accumulated _____________
____________________________________________________________, that allow them to survive
and reproduce in specific habitats.
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○ Over long periods of time, descent with modification has led to the rich _______________________
_________________________________________________________________________________.
Artificial selection, natural selection, and adaptation

Darwin proposed a mechanism—__________________________________________________—to
explain the observable patterns of evolution.

Darwin’s views on the role of environmental factors in the screening of heritable variation were heavily
influenced by artificial selection.

Darwin described two observations of nature, from which he drew two inferences.
○ Observation #1: _________________________________________________________________
_______________________________________________________________________________.
○ Observation #2: _________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________.
○ Inference #1: Individuals whose inherited traits give them a ______________________________
_________________________________________________ and reproducing in a given environment
than other individuals tend to _________________________________________________________
_______________________________________________________________.
○ Inference #2: This unequal ability of individuals to survive and reproduce will cause _____________
_________________________________________________________________________________.

A 1798 essay on human population by __________________________________________________
heavily influenced Darwin’s views on “overreproduction.”
○ Malthus contended that much human suffering—_________________________________________—
was the inescapable consequence of _____________________________________________________
__________________________________________________________________________________
_______________________________________ (over-reproduction is characteristic of most species.)

Three important points need to be emphasized about evolution through natural selection.
1. Although natural selection occurs through interactions between individual organisms and their
environment, _________________________________________________________________. A
______________________________________ is the smallest group that can evolve over time.
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2. Natural selection can act only on __________________________________ traits, traits that are
passed from organisms to their offspring. Characteristics acquired by an organism during its lifetime
may enhance its survival and reproductive success, but there is no evidence that such characteristics
can be inherited by offspring.
3. ____________________________________ factors vary from ____________________ to
____________________ and from time to time. A trait that is favorable in one environment may be
_________________________________________________________________________________.
Concept 22.3 Evolution is supported by an overwhelming amount of scientific evidence

Four types of data document the pattern of evolution and tell us about the processes by which it occurs:
direct ____________________________________________________, __________________________,
the _____________________________________________, and ________________________________.
Direct observations of evolutionary change
The evolution of drug resistance is a particular problem in bacteria and viruses, which exhibit rapid rates
of reproduction.

Many people harbor the bacterium Staphylococcus aureus on their _______________________ or in their
__________________________________________ with no _____________________________ effects.
○ Some genetic strains of this species, known as ___________________________________________
___________________________________________________ (MRSA), are formidable pathogens.

In __________, ________________________________________ became the first widely used antibiotic.
○ By 1945, more than _____% of S. aureus in hospitals were already ___________________________
_____________________________________________.
○ New strains of the bacteria were resistant to penicillin within a few years. Doctors tried using another
powerful antibiotic, methicillin, but again, methicillin-resistant strains of bacteria appeared.

The example of drug-resistant S. aureus highlight two important points about natural selection.
1. Natural selection is an ___________________________________ mechanism, not a
_________________________________________ force. It can act only on existing variation in the
population; it cannot create favorable traits, it selects for favorable traits that are already present in the
population.
2. Natural selection favors traits that increase fitness in the ____________________________________
environment. What is adaptive in one situation is not adaptive in another.
22-5
Homology

More evidence for evolution comes from similarities in the characteristics of different organisms.

Similarity in characteristic traits from ___________________________________ is known as homology.
○ For example, the forelimbs of _________________________, ___________________________,
_________________________________, and __________________________ share the same skeletal
elements, even though the appendages have very different _________________________________.
○ These forelimbs are _________________________________________ structures that represent
variations on the ancestral tetrapod forelimb.

Homologies that are not obvious in adult organisms may become evident when we look at embryonic
development.

Some of the most interesting homologous structures are ______________________________ structures,
structures that have marginal, if any, importance to a living organism but that had important functions in
the organism’s _______________________________________.
○ For example, the skeletons of some snakes and of fossil whales retain vestiges of the _____________
and ____________________________ bones of __________________________________ ancestors.
Homologies mirror the taxonomic hierarchy of the tree of life.

Some homologies, such as the ___________________________________, are shared by all living things
because they arose in the _______________________________________________________. Other
homologies that evolved more recently are shared by only smaller branches of the tree of life.

The pattern of descent from common ancestors and the resulting homologies can be shown in an
evolutionary tree, which reflects evolutionary relationships among groups of organisms.

Distantly related organisms may resemble each other because of convergent evolution, ______________
_____________________________________________________________________________________.
o Their resemblance is __________________________________________________, not homologous.
o NOTE: Analogy = common function (eg bat wing, bird wing)
Homology = common ancestry (eg. wing, arm, flipper)
The fossil record

The fossil record documents the pattern of evolution, showing that past organisms differed from presentday organisms and that many species have become ____________________________.
22-6

Over longer time scales, fossils document the origin of major groups of organisms and the evolutionary
changes within these groups.
Biogeography

Evidence for evolution also comes from biogeography, ______________________________________
___________________________________________________________________________________.

The geographic distribution of organisms is influenced by many factors, including _________________
_________________________________.

We can also use our understanding of evolution to explain biogeographic data.
o For example, islands generally have endemic species _____________________________________
___________________________________________________________.
o Most island species are closely related to species from ______________________________________
______________________ or a neighboring island, reflecting the pattern of colonization of the island.
What is theoretical about the Darwinian view of life?

Some people dismiss the Darwinian view as “_______________________________________.”
○ The term theory has a very different meaning in science than in everyday use.

A unifying theory does not become widely accepted unless _____________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________.
○ That has certainly been the case with the theory of evolution by natural selection.

Scientists continue to test Darwin’s theory.
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Chapter 23  The Evolution of Populations
Student Guided Notes
Overview: The Smallest Unit of Evolution

Natural selection does act on individuals. Each individual’s traits affect its survival and its reproductive
success relative to other individuals in the population.

The evolutionary impact of natural selection is apparent only in the changes in a population of organisms
over time and so it is the __________________________, not the ____________________________,
which evolves.

Microevolution is defined as ___________________________________________________________
___________________________________________________________________________________.

Three mechanisms can cause allele frequencies to change: ____________________________________,
__________________________________________ (chance events that alter allele frequencies), and
__________________________________ (the transfer of alleles between populations).
Concept 23.1 Genetic variation makes evolution possible
Genetic variation occurs within a population.

Individual variation occurs in all species and often reflects genetic variation, ____________________
__________________________________________________________________________________
___________________________________________________________________________________.

Both quantitative and discrete characters contribute to variation within a population.

Discrete characters, such as flower color, are usually determined by a single locus with different alleles
that produce distinct phenotypes.

Quantitative characters, such as plant height, vary along a continuum within a population.
Genetic variation occurs between populations.

Species also exhibit geographic variation, differences in the genetic composition of geographically
separate populations.

Geographic variation in the form of graded change in a trait along a geographic axis is called a cline
(example: birds have increasing body fat as latitude increases.)
22-8
New genes and new alleles originate only by mutation.

The genetic variation on which evolution depends originates when _____________________________,
______________________________________, or other processes produce new __________________
and new _______________________________.

New alleles can arise by mutation, ______________________________________________________
___________________________________________________________________________________.

Some genetic variation in populations represents neutral variation that does not __________________
________________________________________________________________ (example: fingerprints.)

The tendency for natural selection to reduce variation is countered by mechanisms that _____________
___________________________________________, including diploidy and balanced polymorphisms.

Diploidy, having two copies of each chromosome, in eukaryotes prevents the elimination of recessive
alleles via selection because recessive alleles do not affect the phenotype in heterozygotes.
o Even recessive alleles that are unfavorable can persist in a population by “hiding” in heterozygous
individuals.

Heterozygote protection maintains a huge pool of alleles that may not be suitable under the present
conditions but may become beneficial when the environment changes. (balanced polymorphism)

Sexual reproduction also increases variety via: _______________________, ____________________
____________________________ and __________________________________.
Concept 23.2 The Hardy-Weinberg equation can be used to test whether a population ________________
 For a population to evolve, individuals must differ genetically and one of the factors that causes evolution
must be at work.
A population’s gene pool is defined by its allele frequencies.
 A population is _____________________________________________________________________
___________________________________________________________________________________.
 The total of all the alleles for all of the loci for all of the individuals in a population is called the
population’s ____________________________.
 Each allele has a frequency or proportion in the population’s gene pool.
 For example, imagine a population of 500 wildflower plants with two alleles (CR and CW) at a locus that
codes for flower pigment.
22-9
o Suppose that in the imaginary population of 500 plants, 20 (4%) are homozygous for the CW allele
(CWCW) and have ______________________ flowers.
o Of the remaining plants, 320 (64%) are homozygous for the CR allele (CRCR) and have __________
flowers.
o These alleles show incomplete dominance, so 160 (32%) of the plants are heterozygous (CRCW) and
produce ___________________ flowers.
 Because these plants are diploid, the population of 500 plants has 1,000 copies of the gene for flower color.
o The dominant allele (CR) accounts for 800 copies (320 × 2 for CRCR + 160 × 1 for CRCW)  640 +
160 = 800.
o The frequency of the CR allele in the gene pool of this population is 800/1,000 = 0.8, or ______%.
o The CW allele must have a frequency of 1.0 − 0.8 = 0.2, or ______%.
 When there are two alleles at a locus, the convention is to use p to represent the frequency of one allele and
q to represent the frequency of the other.
o Thus p, the frequency of the CR allele in this population, is _____.
o The frequency of the CW allele, represented by q, is ______.
 Allele and genotype frequencies can be used to test whether ____________ is occurring in a population.
The Hardy-Weinberg principle describes a non-evolving population.
 The Hardy-Weinberg principle states that the frequencies of alleles and genotypes in a population’s
gene pool will remain constant over generations unless acted upon by agents other than Mendelian
segregation and recombination of alleles.
o The shuffling of alleles by meiosis and random fertilization has no effect on the overall gene pool of a
population.
o Such a gene pool is said to be in _____________________________________________________.

The Hardy-Weinberg principle states that the repeated shuffling of a population’s gene pool over
generations does not increase the frequency of one allele over another.
o Theoretically, the allele frequencies in our flower population should remain 0.8 for CR and 0.2 for CW
forever.

In a population that has two alleles with frequencies p and q, the combined frequencies must add to 1, or
100%.
o Therefore p + q = 1.0
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o ____= frequency of dominant allele (B)
o ____ = frequency of recessive allele (b)
In addition, the genotype frequencies must add to 1.0: _____ + _____ + _____= ______.
o _____ = frequency of homozygous dominant (BB)
o _____ = frequency of heterozygous dominant (Bb)
o _____ = frequency of homozygous recessive (bb)

This general formula is the Hardy-Weinberg equation.

Using this formula, we can calculate the frequencies of alleles in a gene pool if we know the frequencies
of genotypes, or we can calculate the frequencies of genotypes if we know the frequencies of alleles.

TIP: ALWAYS SOLVE FOR q FIRST!!!
Five conditions must be met for a population to remain in Hardy-Weinberg equilibrium.

Populations evolve because five conditions for non-evolving populations are rarely met for long in nature.
A population must satisfy all five conditions to remain in Hardy-Weinberg equilibrium:
1. _______________________________________________________. The gene pool is modified if
mutations alter alleles or if entire genes are deleted or duplicated.
2. _______________________________________________________. If individuals pick mates with
certain genotypes, or if inbreeding is common, the mixing of gametes will not be random and
genotype frequencies will change.
3. _______________________________________________________. Differential survival or
reproductive success among genotypes will alter allele frequencies.
4. _______________________________________________________. In small populations, chance
fluctuations in the gene pool will cause allele frequencies to change over time, a process called genetic
drift.
5. ____________________________________________. Gene flow, the transfer of alleles due to the
migration of individuals or gametes between populations, will change the frequencies of alleles.
NOTE: Hardy-Weinberg explains NON-EVOLVING populations! If one or more of these 5
conditions ARE NOT met, then that means the population is EVOLVING.
We can apply the Hardy-Weinberg principle to a human population.
22-11

We can use the Hardy-Weinberg principle to estimate the percent of the human population that carries the
allele for the inherited disease ___________________________________ (PKU). (refer to Genetic
Disorder Chart CH 14/15)

From the epidemiologic data, we know that the frequency of homozygous recessive individuals (q2 in the
Hardy-Weinberg principle) is one in _____________________, or 0.0001.
o The frequency of the ___________________________ allele (q) is the square root of 0.0001 = 0.01.
o The frequency of the ___________________________ allele (p) is p = 1 − q, or 1 − 0.01 = 0.99.
o The frequency of carriers (heterozygous individuals) is 2pq = 2 × 0.99 × 0.01 = 0.0198, or about __%.

Thus, about 2% of the U.S. population carries the PKU allele.
Concept 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Natural selection is based on differential survival and reproductive success.

Individuals with variations better suited _______________________________________________ tend to
produce more ______________________________ than those with variations that are less well suited.

As a result of selection, alleles are passed on to the next generation in frequencies different from their
relative frequencies in the present population.

For example, the fruit fly Drosophila melanogaster has an allele that confers resistance ______________
____________________________, including ______________.
o In 20 years after the first use of DDT, the allele frequency shifted from 0% to _____%.
o DDT was a strong ______________________ force, favoring alleles that conferred resistance.
o Note: If DDT had eliminated ALL pests in the early application (no survivors) it would have been
effective.

By _________________________________________________________________________ can cause
adaptive evolution (evolution that results in a better match between organisms and their environment).
Genetic drift results from chance fluctuations in allele frequencies in small populations.

__________________________________ occurs when changes in gene frequencies from one generation
to another occur because of ________________ events (sampling errors) that occur in small populations.

Genetic drift in _______________________ populations may occur as a result of two situations: the
bottleneck effect or the founder effect.
22-12

The __________________________ effect occurs when a new population is started by a small
number of individuals who do not represent the gene pool of the larger source population.

The bottleneck effect occurs when the numbers of individuals in a large population are drastically
____________________________ by a disaster.

FOUR key points about Genetic Drift:

_____________________________________________________________________________________
_____________________________________________________________________________________.

_____________________________________________________________________________________
_____________________________________________________________________________________.

_____________________________________________________________________________________
_____________________________________________________________________________________.

_____________________________________________________________________________________
_____________________________________________________________________________________.
A population may lose or gain alleles by gene flow.

___________________________________ is the transfer of alleles among populations due to the
migration of ________________________ individuals or ______________________.

Gene flow tends to reduce _____________________________________________________________.

Gene flow is an increasingly important agent of evolutionary change in _________________ populations.
o
With increased human mobility, mating is more common between previously isolated populations,
leading to an exchange of alleles and reducing _____________________________________ between
human populations.
Concept 23.4 Natural selection is the only mechanism that consistently causes adaptive evolution

Evolution by natural selection is _________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________.
o

Because of this favoring process, the outcome of natural selection is __________________________.
Adaptive advantage can lead to greater relative fitness (aka “Darwinian Fitness”): ________________
_____________________________________________________________________________________
_____________________________________________________________________________________.
22-13
There are three modes of selection: ___________________________, ____________________________, and
________________________________.

Natural selection can alter the frequency distribution of heritable traits in three ways, depending on
_____________________________________________________________________________________.

Directional selection is most common during periods of environmental change or migration and
favors _________________________________________________________________________.

Disruptive (aka “diversifying”) selection occurs when environmental conditions favor
individuals ____________________________________________________________________
____________________________________________ over those with intermediate phenotypes.

Stabilizing selection favors ________________________________________________________
____________________________________ (eg. human _______________________________.)
Sexual selection may lead to pronounced secondary differences between the sexes.

Sexual selection results in sexual dimorphism, _________________________________________________
_____________________________________________________ such as size, coloration, and ornamentation.

_______________________________ selection is direct competition among individuals of one sex for mates
of the opposite sex.

Intersexual selection, or _________________________, occurs when members of one sex (usually females)
________________________________________________________________________________________.
Balancing selection preserve genetic variation.

_______________________________ selection occurs when natural selection maintains stable frequencies of
two or more phenotypes in a population.

One mechanism that produces balanced polymorphism is _________________________________________.
o In some situations, individuals who are heterozygous at a particular locus have greater fitness than
homozygotes. (Ex. Individuals who are heterozygous for ______________________________________
are also resistant to ___________________________)

A second mechanism that promotes balanced polymorphism is ____________________________________.
o Frequency-dependent selection occurs when the fitness of any one morph declines if it becomes too
_______________________________ in the population.
o Frequency-dependent selection has been observed in a number of predator-prey interactions in the wild.
22-14
Natural selection cannot fashion perfect organisms.

There are at least four reasons natural selection cannot produce perfection.
1. ___________________________________________________________________________________.
2. ___________________________________________________________________________________.
3. ___________________________________________________________________________________.
4. ___________________________________________________________________________________.

With these constraints, evolution does not tend to craft perfect organisms.
22-15
Chapter 24  The Origin of Species
Student Guided Notes
Overview: That “Mystery of Mysteries”

Speciation—_______________________________________________________________________
__________________________________________—is at the focal point of evolutionary theory because
the appearance of new species is the source of biological diversity.

Speciation forms a conceptual bridge between ________________________________, changes in allele
frequencies within a population, and _______________________________, the broad pattern of evolution
over time.
Concept 24.1 The biological species concept emphasizes reproductive isolation

Traditionally, morphological differences have been used to distinguish species. Today, differences in
_______________________, _______________________________, behavior, and _____________
sequences are also used to differentiate species.

The biological species concept defines a species as ________________________________________
__________________________________________________________________________________
__________________, but cannot produce ________________________________________________
___________________________________________.
The formation of a new species hinges on the development of reproductive isolation.

Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they
function before or after the formation of zygotes.

Prezygotic barriers block _______________________________ between species in one of three ways:
by impeding members of different species from attempting to mate, by __________________________
______________________________________________________________________________, or by
___________________________________________________________________________________.

Prezygotic barriers include habitat isolation, behavioral isolation, temporal isolation, mechanical
isolation, and gametic isolation.

In habitat isolation, two organisms ________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________.
22-16

In behavioral isolation, species use ______________________________________________
_______________________________________________________________ to attract mates.

In temporal isolation, two species that breed during __________________________________
____________________________________________________________ cannot mix gametes.

In _________________________________ isolation, closely related species may attempt to mate
but fail because they are anatomically incompatible and transfer of sperm is not possible.

In _______________________________ isolation, the gametes of two species do not form a
zygote because of incompatibilities preventing fertilization.

If a sperm from one species does fertilize the _____________________ of another species, postzygotic
barriers may prevent the hybrid _____________________ from developing into a viable, fertile adult.

Postzygotic barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

In reduced hybrid ______________________, genetic incompatibility between the two species
may abort the development of the hybrid at some embryonic stage or produce frail offspring.

In reduced hybrid fertility, even if the hybrid offspring are vigorous, the hybrids may be
_________________________, and the hybrid cannot backbreed with ____________________
___________________________________ (example: mule with horse or donkey.)

In hybrid breakdown, _______________________________________ hybrids are _________
____________________________________. When first-generation hybrids mate with either
parent species or with each other, however, __________________________________________
____________________________________________________.
Evolutionary biologists have proposed many alternative concepts of species.

Although the biological species concept emphasizes the separateness of species due to reproductive barriers,
many alternative species concepts emphasize the unity within a species.

The morphological species concept, the oldest and still most practical, defines a species by _________
___________________________________________________________________________________.

The ecological species concept defines a species in terms of its ecological niche, __________________
___________________________________________________________________________________
___________________________________________________________________________________.
22-17

The phylogenetic species concept defines a species as ______________________________________
______________________________________________________________________________ and
forms one branch on the ___________________________________________.

More than 20 other species concepts have also been proposed. Each species concept may be useful,
depending on the situation and the types of research questions we are asking.
Concept 24.2 Speciation can take place with or without geographic separation

Two general modes of speciation (allopatric and sympatric) are distinguished by the way gene flow
among populations is initially interrupted.
In allopatric speciation, ___________________________________ barriers can lead to the origin of species.

Mountain ranges, glaciers, land bridges, or splintering of lakes may divide one population into isolated
groups.

Alternatively, some individuals may colonize a new, geographically remote area and become isolated
from the parent population.
In sympatric speciation, a new species can originate in the geographic midst of the parent species.

In plants, sympatric speciation can result from accidents during _____________________________ that
result in _____________________________________________________________, a mutant condition
known as _________________________________.

An autopolyploid is an individual that has _______________________________________________
________________________, all derived from _____________________________________.
○ For example, a failure of cell division can double a cell’s chromosome number from diploid (2n) to
__________________________________ (4n).
○ The tetraploid can reproduce with itself (self-pollination) or with other tetraploids.

A more common mechanism of producing polyploid individuals occurs when ____________________
_______________________________ mate to produce allopolyploid offspring.
○ For example, one species has 2n = 4 and the other 2n = 6 and the offspring has 2n = _____.
○ Although the hybrids are usually __________________________, they may be quite vigorous and
propagate asexually and in subsequent generations, various mechanisms may transform a sterile hybrid
into a fertile polyploid.
22-18

In summary, in allopatric speciation, a new species forms while geographically isolated from its parent
population.

Sympatric speciation requires the emergence of a reproductive barrier that isolates a subset of the
population without geographic separation from the parent population.
Concept 24.4 Speciation can occur rapidly or slowly and can result from changes in few or many genes
How long does it take a new species to form?

The fossil record and molecular data provide information about the timing of speciation in specific taxa.
○ Paleontologists Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to
describe _______________________________________________________________________
______________________________________________________________________________.

The total time between speciation events (from the start of genetic divergence to complete speciation)
varies considerably.
22-19
Chapter 26  Phylogeny and the Tree of Life
Student Guided Notes
Overview: Investigating the Tree of Life

Phylogeny is _______________________________________________________________________
__________________________________________________________________________________.

To reconstruct phylogeny, scientists use systematics, an analytical approach to classifying the diversity
and determining the evolutionary relationships of living and extinct organisms.

Evidence used to reconstruct phylogenies can be obtained from the fossil record and from
morphological, biochemical, and genetic similarities between organisms.
Concept 26.1 Phylogenies show evolutionary relationships.

The scientific discipline of taxonomy determines how organisms are ________________________ and
________________________________.
Taxonomy employs a hierarchical system of classification.

The Linnaean system, first formally proposed by ________________________________________ in the
18th century, has two main characteristics.

1) Each species is assigned a two-part Latinized name, a __________________________
reflecting genus and species. (ex. Homo sapiens)

2) A ______________________________________________ groups species into increasingly
inclusive taxonomic categories.

Organisms are grouped into progressively broader categories: species, genus, family, ____________,
_____________________, _________________, ___________________, and ___________________.

The _________________________________________________________________ is called a taxon.
Classification and phylogeny are linked.

The _______________________________________________________________________________
___________________________________ can be represented in a diagram called a phylogenetic tree.

The branching of the tree may match the hierarchical classification of groups nested within more inclusive
groups.
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A phylogenetic tree represents a hypothesis about evolutionary relationships.

Evolutionary relationships are often represented as a series of dichotomies, or ___________________
__________________________________________________.

Each branch point represents ________________________________________________________
_______________________________________________________________________________.
Concept 26.2 Phylogenies are inferred from morphological and molecular data.

___________________________________________________________________________________
_________________________________________________________________ are called homologies.

Organisms that share similar morphologies or DNA sequences are likely to be ____________________
__________________________________________________ than organisms without such similarities.

Similarity due to ______________________________________________________ is called analogy.

Distinguishing homology from analogy is critical in the reconstruction of phylogeny.

In general, the more elements that are similar in two complex structures, the more likely it is that they
evolved from a common ancestor.

Molecular systematics uses DNA and other molecular data to determine evolutionary relationships.
Concept 26.3 Shared characters are used to construct phylogenetic trees.

In an approach to systematics called cladistics, common descent is ____________________________
___________________________________________________________________________________.

Biologists place species into groups called clades, each of which includes _______________________
___________________________________________________________________________________.

A clade is ____________________________, consisting of an ancestral species and all its descendents.

When biologists lack information about some members of a clade, the result may be a
______________________________ grouping that consists of some, but not all, of the descendents.

The result may also be several polyphyletic groupings, which includes distantly related species but
does not include __________________________________________________________________.
Due to descent with modification, organisms share some, but not all, characteristics with their ancestors.

Systematists must sort through homologous features, or characters, to separate shared derived characters
from shared ancestral characters.

A character is any feature that a particular taxon possesses.
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
A shared __________________ character is an evolutionary novelty unique to a particular clade.

A shared _________________________ character originated in an ancestor of the clade.
The principles of maximum parsimony and maximum likelihood help systematists reconstruct phylogeny.

As available data about DNA sequences increase, it becomes more difficult to draw the phylogenetic tree
that best describes __________________________________ history.

According to the principle of ____________________________________, scientists should look for the
________________________________ explanation that is ___________________________________
________________________________. (meaning the fewest evolutionary events or changes in DNA)

Phylogenetic hypotheses (cladograms) are stronger with more than one form of evidence (ex.
fossils and molecular)

If the forms of evidence result in a different conclusion, then the molecular evidence is favored.
Concept 26.5 Molecular clocks help track evolutionary time.

One of the goals of evolutionary biology is to understand the relationships among all organisms, including
those ______________________________________________________________________.

Molecular clocks serve as yardsticks for measuring the absolute time of evolutionary change.

Molecular clocks are based on the observation that _______________________________________
________________________________________________________________________________.

Scientists calibrate the molecular clock of a gene by _________________________________________
____________________________________________________________________________________
(nucleotide, codon, or amino acid differences) against the dates of _____________________________
____________________________________ that are known from the ___________________________.

The average rate of genetic change inferred from such a graph can be used to estimate the absolute
date of evolutionary events that have no fossil record.
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