Evidence of Evolution
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I. Fossil Record
Importance of Fossils - Provides the best proof of the history of life showing how extinct species have
lead to today’s species.
Formation of Fossils –
Transitional Species (Common Ancestors) – The location where the evolutionary paths of two species
connect on the “tree of life”.
Importance of Transitional Species – Scientists search for common ancestors in the fossil record to show
the evolutionary connection between species & how they change over time.
Example(s) of Transitional Species –
Ways to Date Fossils: Relative Age Dating & Absolute Age Dating
Relative Age Dating of Fossils – Approximate dating method that compares rock layers
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Law of Superposition – The rule that older rock is found under younger rock. This rule is used
to estimate the relative age of rocks layers
Unconformity – This is an exception to the law of superposition, when there is a break in the
rock record.
Index Fossil – An organism that lived during a specific period of time and whose fossil
remains are abundant. These common fossils are used to help date the age of unknown fossils.
Example of Relative Age Dating:
Absolute Age Dating of Fossils – Precise dating method that measures radioactive decay in rocks
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Radioactive Elements – unstable atoms giving off radiation (protons & neutrons) to become
stable. Examples include: Uranium-238 & Carbon-14
Radioactive Dating – Radioactive decay (going from unstable to stable) occurs at a constant
rate called a half life. Each radioactive element has its own half life.
Half Life – the amount of time it takes for half the radioactive atoms in a substance to become
stable. Examples include: Uranium-238 with a 4.5 billion year half life (becomes lead) AND
Carbon-14 with a 5730 year half life (becomes Nitrogen)
Example of Absolute Age Dating:
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II. Comparative Anatomy
Homologous Structures:
 Traits that are inherited by TWO different organisms from a common ancestor that
have similar structures, but different functions for these structures.
Homologous Examples –
None Homologous Examples – (such as Analogous Structures of different species with
similar adaptations with no connections to a common ancestor)
Vestigial Structures:
 Structures that are found in an organism but appear to serve NO function (often
reduced in size) they exist as a “left-over” of an organisms evolutionary past.
Animal Examples –
Human Examples –
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III. Comparative Embryology
Importance:
 Similarities in embryonic development of organisms exist because of a common
ancestor.
o Related species are similar in early stages of development
o Differences accumulate as new instructions in the species DNA are added to old
instructions inherited from a common ancestor species.
Examples of similarities between embryos –
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IV. Comparative Genetics
Importance:
 An organism’s evolutionary history is held in their DNA sequence (genetic code).
 If a species changes, their DNA changes.
 The closer two species are related, the more similarities they share in their DNA.
Example –