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Evidence for Evolution

Review (what we’ve learned so far)




Mutations provide variability within
species
Some traits give individuals within a
species an advantage - those
organisms live longer and/or
reproduce more = natural selection
Over time, the population shifts so the
all (or most) individuals have this trait
= adaptation
Evolution of a whole new species
(speciation) takes long periods of time
Main evidence for evolution
Fossils
Geographic distribution of organisms
around the world
Artificial selection
Homologous structures
Vestigial structures
Embryology
Comparative analysis of DNA
Fossils

The bulk of our
understanding about
ancient life
 Bacterial cells to
complete T. rex skeletons
 How are fossils formed?
(Short video)

http://www.teachersdomai
n.org/68/sci/life/div/becfossil/inde
x.html
Fossil Questions
1. How are fossils formed?


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Organism covered by sediments (mud,
sand, silt, clay, ash)
Calcium replaced by minerals in the
sediments over time
Sometimes imprints can also be preserved
(tracks, leaf impressions, etc.)
Fossil Questions, cont.
2. Why do most living things
not leave fossils behind?
 Organism was eaten
 Didn’t die in correct environment to be
preserved
 Body has no ‘hard parts’ (shell, skeleton)

Virtually absent from fossil record: amoebae,
flatworms, jellyfish, sea slugs, etc.)
Fossil Questions, cont.
3. How are fossils uncovered?


Erosion brings fossils back to surface
Humans dig to find fossils
•
Determine specific locations that are likely spots
to hold fossils
Fossil Questions, cont.
4. How do scientists determine
the age of fossils?
Technique known as:
Radiometric Dating
http://www.teachersdomain.org/6-8/sci/life/div/radiodating/index.html
Transitional
Fossils

A.k.a. Intermediate
fossils

Provide a link
between past forms of
an organism
Transitional Fossils: Orca example

1.
2.
3.
4.
We’ve believed for a long time that
whales (and seals, otters, etc) were
mammals because they share COMMON
FEATURES with land mammals
Warm-blooded (endothermic)
Give live birth and provide milk for young
Have hair
Similar respiratory, circulatory, nervous,
and digestive systems
Transitional Fossils - Whales
Modern whale – totally aquatic

Nostrils on top of head = blowhole

Tail evolved into flipper
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Ears modified directional hearing underwater
Basilosaurus – clearly aquatic

Sturdy flippers & long, flexible body

Still has small, weak hind legs
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Nostrils on top of head
Ambulocetus – amphibious (both land and water)
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Forelimbs equipped with fingers/small hooves
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Hind feet and tail adapted for swimming
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Nostrils on top of snout
Mesocynids– terrestrial

Tetrapod – limbs adapted to moving on land

Ears adapted to hearing in air

Nostrils in front of snout
Gaps in Fossil Record
 Fossils
for 350,000 species of ancient
organisms

0nly a tiny fraction of total!
 Fill
in the ‘gaps’ using logical reasoning
and other lines of evidence
Geographic Distribution on
Organisms around the world



Similar environments
around the world contain
organisms that are
DIFFERENT species but
have similar anatomies
and/or behaviors
Similar selective
pressures lead to
common structures and
or behaviors that aid
survival and/or
reproduction
Example: meerkats and
prairie dogs
Beaver
Another
example of
Geographic
Distribution
Muskrat
Beaver
and
Muskrat
Coypu
Capybara
Coypu
and
Capabara
Geographic Distribution, cont.
• Additionally,
organisms from
common ancestor will
change in structure
and function to adapt
to their specific
environment
• Prime example:
islands!
– the more isolated the
island, the more
distinct its organisms
Artificial Selection

When humans breed organisms with desired
trait repeatedly until desired affect is reached
 Plant and animal breeders have long taken
advantage of the heritability of traits to improve
crops and livestock and pets
 Humans use the same mechanisms to change
organisms that nature uses


Artificial – humans ‘select’ the best traits
Natural – nature ‘selects’ most best, or ‘most fit’ traits
Artificial Selection Examples
Hundreds of varieties of
peppers – all originated
from an initial species
Different breeds
of dogs and cats
‘created’ by man
by selecting for
desirable traits
(fast, sociable,
good smell, short
fur, etc.)
Homologous Structures

Similar structures in a variety of different
organisms
 Shared trait provides evidence that the
organisms had a shared common ancestor
with that trait
Homologous
bone
structure in
forelimbs of
all mammals
Vestigial Structures
 As
species evolve over time… some
structures become unnecessary to survival
of organism
 Over time become reduced (vestiges)
 Eventually disappear
http://www.livescience.com/animalworld/top1
0_vestigial_organs.html
Embryology
 Early
stages of related animals are very
similar

More closely related = more similar
 Similarities
in the steps of development
are evidence of common ancestry
Comparative Embryology of Vertebrates
Comparative Analysis of DNA
If all organisms evolved from same original
ancestors (3.5 bya), then all should share
come common DNA
 YES! all life shares same general
machinery for regulation of cell division,
building cell parts, etc.
 Greater similarity between 2 species =
more recent common ancestor