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Classification
Finding Order in Diversity
Biologists have…
• identified and
named 1.5 million
different species
• estimate 2-100
million additional
species have yet to
be discovered
Why Classify?
• To study the diversity of life, scientists must name
each organism
• Scientists use a classification system to name
organisms and group them in a logical manner
• Taxonomy is the study of naming organisms
– Names are universally accepted
– Names have biological significance
– Categories go from larger groups to smaller groups.
• Ex) teacher= larger, biology teacher = smaller
Assigning Scientific Names
• Calling things by their
common name can be
confusing
– Names change by region
• Ex) puma, mountain lion,
panther & cougar are all
regional names for Felis
concolor
• In the 18th century
scientists used Latin &
Greek to assign scientific
names, because these
languages were common
to all.
Early Naming
• In the beginning, scientific names were
often detailed physical descriptions
– Often names were 20+ words long
• There was difficulty in standardizing names
because different scientists described
organisms differently
– Ex) the blue-winged/red breasted/
white stomached bird
Binomial Nomenclature
• Carolus Linnaeus developed a
two word naming system
called binomial nomenclature
• Each species is give a two part
scientific name:
– The name is written in italics
– The first word is capitalized and
the second is written in lowercase
– The first word is the genus
(closely related species)
– The second word is the species
(name usually describes an important
trait)
Linnaeus’s System of Classification
• Hierarchal system, with
seven levels
• Each level is referred to as a
taxon
• From largest to smallest:
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–
–
–
–
–
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Kingdom
Phylum
Class
Order
Family
Genus
Species
“King Philip Cried Out For Good Soup”
Which similarities are most important?
• Organisms that look very similar may not share a
common ancestor.
• Their similar appearance may be the result of
convergent evolution.
• Darwin’s idea of descent with modification gave rise
to the study of phylogeny, or evolutionary
relationships among organisms.
Evolutionary Classification
• Biologists now group
organisms into categories
that represent lines of
descent, or phylogeny,
not just physical
similarities.
• Species within a genus
are more closely related
to each other than to
those in another genus
because they share a
recent common
ancestor.
• The higher the level of
the first taxon in common
between two organisms,
the farther back in time
the common ancestor.
Classification Using Cladograms
• Cladistic analysis: identification and consideration of only
those characteristics of organisms that are evolutionary
innovations (new characteristics that arise as lineages evolve
over time)
– Characteristics that appear in recent parts of lineage but not older
members are called derived characteristics
– These characteristics can be used to construct a cladogram to show the
evolutionary relationships between organisms
– Cladograms are useful tools for scientists to understand how one lineage
branched from another in the course of evolution.
Similarities in DNA & RNA
• The genes of many
organisms show important
similarities at the
molecular level.
• Similarities in DNA can be
used to help determine
classification and
evolutionary
relationships
• The more similar the DNA
sequence of two species, the
more recently they
shared a common
ancestor and the more
closely related they are in
evolutionary terms.
Molecular Clocks
• Comparisons of DNA can
also be used to mark the
passage of evolutionary
time.
• A molecular clock uses
DNA comparisons to
estimate the length of time
that two species have been
evolving independently.
• A molecular clock relies on
mutations to mark time.
• The degree of
dissimilarity in the DNA
of two species indicates how
long ago the two species
shared a common
ancestor.