AP Biology Chapter 26


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AP Biology Chapter 26

Phylogeny and the Tree of Life


The Fossil Record and Geologic Time


Distinguish between phylogeny and systematic.

Phylogeny is the evolutionary history of a species or group. Systematics is the

discipline focused on classifying organisms and determining evolutionary



Describe the process of sedimentation and the formation of fossils. Explain what portions
of organisms mostly fossilize and why.

In fossil formation, an an
imal or plant must die in water or near enough to fall

into shortly after death. The water insulates the remains from many of the

elements that contribute to decomposition. The portion that becomes fossilize is

mostly the exoskeleton because ba
cteria would consume the soft body parts. In

sedimentation, sediments bury the exoskeleton as time passes. The faster this

happens, the more likely the fossilization. (land or mud slides help)


Distinguish between relative dating and absolute dating

Relative dating determines the order in which a series of events occurred of an

artifact or paleontological site, whereas absolute dating is the process of

determining a specific date for an archaeological or paleontological site or



Explain how isotopes can be used in absolute dating.

Radioactive isotopes have a fixed rate of decay, which can be used in absolute

dating. For example, carbon
14 has a half life of 5,730 years, so, isotopes that

decay more slowly can be u
sed to infer the age of fossils associated with the

layers of rock. Radioactive isotopes only decay in fixed rate after organism dies.


Explain why the fossil record is incomplete.

Very few organisms were preserved as fossils, organisms tend to d
ecay before

becoming fossilized, only organisms with exoskeletons could be preserved, and

some organisms left over’s may be destroyed in the process.


Describe two dramatic chapters in the history of continental drift. Explain how those
movements affec
ted biological evolution.

One dramatic continental drift was in the southern tip of Labrador, Canada where

it has moved 40 degree north over the last 200million years. Another was in

Wilmington, Delaware, where the shift had caused the weath
er to drop

dramatically. In affect to biological evolution, it has caused temperature shifts,

and geographical shifts. It also causes geographical isolation.


Explain how mass extinctions have occurred and how they affected the evolution of
ng forms.

Mass extinction have occurred due to the organism’s habitat being destroyed,

its environment may have been changed in a manner unfavorable to the species,

and or any other disruptiv
e global environmental changes. When a dominant

specie becomes extinct, its ecological niches passes to another specie.


Describe the evidence related to the impact hypothesis associated with the Cretaceous
extinctions. Describe the hypothesized consequences of such an impact.

The hypothesis was t
hat the cretaceous mass extinction marks the boundary

between the Mesozoic and Cenozoic eras about 65.5 mya, claimed more than

half of the marine species, many families of terrestrial plants and animals,

and most dinosaurs. The
affect is that a

thin layer of clay rich iridium may have

been released from a cloud of dust, which may block sunlight, affecting weather.

: Connecting Classification to Phylogeny


Distinguish between systematics and taxonomy.

Systematics is the di
scipline focused on classifying organisms and determining

evolutionary relationships. Taxonomy is concerned with naming and classifying



Explain how species are named and categorized into a hierarchy of groups.

Charles Linnaeus made an ap
proach for the species’ names. The first part was

called a genus (in Latin, plural, genera), to which the species belong. The second

part is called the specific epithet, which is unique for each species within the

genus. These organisms are grouped

in binomial


List the major taxonomic categories from the most to least inclusive.

The first grouping is built into the binomial: Species that appear to be closely

related are grouped into the same genus. The taxonomic system named after


called the Linnaean system, places related genera into the same family,

families into orders, orders into classes, classes into phyla, phyla into kingdoms,

and recently, kingdoms into domains.


Define the parts and describe the interrelationships within

a cladogram. Explain how a
cladogram is constructed.

In this methodology, biologists place species into groups called clades, each of

which includes an ancestral species and all of its descendents. Clades are nested

within larger clades. Cl
adograms are constructed by comparing organisms to see

if they share a trait or derived character, and are then constructed as a series of

Y’s or branches. At every branch, one of the organisms that do not share a

common character with the rest of
the group is “branched off” into its own clade.


Distinguish between homologous and analogous structures. Explain why the similarity of
complex systems implies a more recent common ancestor.

homologous structures are structures in different species
that are similar because

of common ancestry, whereas analogous structures are structures that have

similar functions but have not evolved from a common ance
stry. The higher the

resemblance of two complex systems, the more that it implies common anc


Distinguish between shared primitive characters and shared derived characters. Compare
the definitions of an ingroup and outgroup.

shared primitive (ancestral) character, is a character shared by members of a

particular clade, that origin
ated in an ancestor that is not a member of that clade.

Shared derived character is an evolutionary novelty that is unique to a particular

clade. An outgroup is a species or group of species from an evolutionary lineage

that is known to have diverge
d before the lineage that includes the species that

we are studying, or an ingroup.


Compare the cladistic and phylocode classification systems.

Cladistics is an approach to systematic in which organisms are placed into groups

called clades
primarily on common descent, and phylocode is a system of

classification of organisms based on evolutionary re
lationships, in which only

groups that include a common ancestor and all of its descendants are named.


Explain how nucleotide sequences and

amino acid sequences can be used to help classify
organisms. Explain the advantage that molecular methods have over other forms of

With molecular comparison, scientists can study the nucleotide sequences of

organisms. They c
reated a program to take into account insertions, deletions, and

substitutions. They found that the many differences between the two in

in comparison revealed a great amount of divergence therefore they’re not

closely related or common in ances


Explain the principle of parsimony. Explain why any phylogenetic diagram is viewed as a

The principle of parsimony favors the hypothesis that requires the fewest or

simplest assumptions to explain an observation.
Any phylog
enetic diagram is

viewed as a hypothesis because there is no way to measure whether a particular

c hypothesis is accurate or not, and the diagram requires a lot of

assumptions and approximations as well.


Explain how molecular clocks are

used to determine the approximate time of key
evolutionary events. Explain how molecular clocks are calibrated in the actual time.

A molecular clock is basically a yardstick for measuring the absolute time of

evolutionary change based on the observat
ion that some genes and other regions

of genomes appear to evolve at a constant rate. We can calibrate the molecular

clock of a gene that has a reliable average by graphing the number of genetic

differences against the dates of evolutionary branch

points that are known from

the fossil record.


Explain how scientists determined the approximate time when HIV first infected humans.

By comparing sequences of HIV viruses from samples taken at various times

during the epidemic, researchers have ob
served a remarkably consistent rate of

evolution. They estimate that the HIV
1 M strain first infected humans in the



Describe an example of a conflict between molecular data and other evidence, such as the
fossil record. Explain how these diff
erences can be addressed.

The problem is that there is no definite and exact approach in solving or exactly

dating the time period. These methods each have a different approach and are

quite accurate, but it is not exact.


The evolutionary history of a species or group of related species.

Fossil record: Fossils that are the preserved remains or traces of animals, plants, and other
organisms from the remote past.

Geologic time scale: The geologic time scale provides a system
of chronologic
measurement relating stratigraphy to time that is used by geologists, paleontologists, and
other earth scientists to describe the timing and relationships between events that have
occurred during the history of the earth.

Radiometric dating:

A method for determining the absolute ages of rocks and fossils,
based on half life of radioactive isotopes.

Half life: The amount of time it takes for 50% of a sample of radioactive isotope to

Pangaea: The supercontinent that formed near the end o
f the Paleozoic era, when plate
movements brought all the landmasses of earth together.

Systematics: A scientific discipline focused on classifying organisms and determining
their evolutionary relationships.

Binomial: The two
part Latinized name of a speci
es, consisting of the genus and specific

Genus: A taxonomic category above the species level, designated by the first word of a
species’ two
part scientific name.

Specific epithet: The second element in the Latin binomial name of a species, which
follows the generic name and distinguishes the species from others in the same genus.

Species: A population or group of populations whose members have the potential to
interbreed in nature and produce viable, fertile offspring, but do not produce viable,
ertile offspring with members of other such groups.

Family: In classification, the taxonomic category above genus.

Order: In classification, the taxonomic category above the level of family.

Class: In classification, the taxonomic category above the level
of order.

Phyla (singular, phylum): In classification, the taxonomic category above class.

Kingdoms: A taxonomic category, the second broadest after domain.

Domain: A taxonomic category above the kingdom level. The three domains are archaea,
bacteria, and

Taxon (plural, taxa):
A named taxonomic unit at any given level of classification.

Phylogenetic tree: A branching diagram that represents a hypothesis about the
evolutionary history of a group of organisms.

Cladogram: A branching diagram showing
the cladistic relationship between a number of

Clade: A group of species that includes an ancestral species and all its descendents.

Monophyletic: Pertaining to a group of taxa that consists of a common ancestor and all its
descendents. A
monophyletic taxon is equivalent to a clade.

Homology: Similarity in characteristics resulting from a shared ancestry.

Convergent evolution: The evolution of similar features in independent evolutionary

Analogy: Similarity between two species tha
t is due to convergent evolution rather than
to descendent from a common ancestor with the same trait.

Shared primitive character: A character, shared by members of a particular clade, that
originated in an ancestor that is not a member of that clade.

ed derived character: An evolutionary novelty that is unique to a particular clade.

Outgroup: A species or group of species from an evolutionary lineage that is known to
have diverged before the lineage that contains the group of species being studied. An
outgroup is selected so that its members are closely related to the group of species being
studied, but not as closely related as any study
group members are to each other.

A species or group of species whose evolutionary relationships we seek to

Phylocode: System of classification or organisms based on evolutionary relationships:
Only groups that include a common ancestor and all of its descendents are named.

A principle that states that when considering multiple explanations

for an
observation, one should first investigate the simplest explanation that is consistent with
the facts.

Molecular clocks: A method for estimating the time required for a given amount of
evolutionary change, based on the observation that some regions
of genomes appear to
evolve at constant rates.

Phylogenetic fuse:
a combination of phylogenics.