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Lecture 8

BIO SCI 94 Lecture Notes - Lecture 8: Tiktaalik, Archaeopteryx, Marsupial

Biological Sciences
Course Code
Brandon Stuart Gaut

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Lecture 8: Evidence for Evolution
Evidence for Evolution
Prediction 1: There should be evidence of extinction
Fossils: pieces of physical evidence from an
organism that lived in the past
Background extinction --> "normal" level of
Species go extinct due to ongoing environmental
change, predation, diseases, etc.
98% of all species have gone extinct (98% of the
things that lived on the earth are no longer around)
oSome of these represent "dead-end"
evolutionary lineages
oOther extinct species have descendants that
are still around today
5 major episodes of mass extinction (marked
increase in extinction rates)
oDuring each mass extinction, at least 60% of
the species died out!
oLargest --> 250 million yrs ago: End-Permian
60% of families went extinct
80-96% of the species went extinct
Marine organisms were especially
hard hit
oLate Triassic --> many amphibians & reptiles
went extinct
205 million yrs ago
Mammals become more common
Set the stage for the rise of dinosaurs
oEnd-Cretaceous (aka K-P extinction) --> 65 million yrs ago
Dinosaurs went "extinct"
Birds evolved from dinosaurs
Many other terrestrial and marine species also became extinct
oNow; Ongoing Extinction Events
11% of 9040 known bird species endangered
4000 U.S. plant species are of conservation concern, w/ 680 of 20,000 U.S. plant species
expected to be extinct by end of this year
20% of freshwater fishes in the world have either gone extinct in the last hundred yrs or are
endangered, w/ 40 species going extinct in the U.S. over the last century
31,000 plant & animal species are endangered worldwide, w/ as many as 1/5 of all species
going extinct in the next 30 yrs
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oSummary: extinction is a reality, and Darwin’s Theory of Evolution predictions that there should be
much evidence of extinction in the fossil record. However, we’re not quite sure why there have
been periods of mass extinction.
Prediction 2: There should be evidence of transitional (or intermediate) forms in the fossil
Transitional feature: trait in a fossil species that is intermediate between those of ancestral (older) and
derived (younger) species
Dinosaurs evolved into birds
Archaeopteryx: intermediate between a dinosaur & bird
oHave a few characteristics like those of modern birds that most dinosaurs (and sometimes all
dinosaurs) didn't have
oDisplay many traits in common w/ dinosaurs
oRepresents transition from dinosaurs --> birds
Lots of transitional fossils for horses
Whales also have transitional fossils
oEvolved from land animals
oWe can trace the structures of whales through time to see which are homologous
Tiktaalik found in 2004
oTransition fossil
oRepresents a potential transitional link from fish to land-based amphibians
Some species seem not to have changed much over time
o"living fossils"
In Darwin's time, the lack of transitional fossils was 1 of the problems w/ his argument w/ evolution
Notes about transitional forms in the fossil record:
oIt's difficult to find transitional forms for some species b/c the fossil record is so sparse.
For most organisms, the evidence for transitional forms is incomplete
Possible interpretations of “gaps” in the fossil record are that the conditions for fossil
preservation are relatively rare or that intermediate or transitional forms were relatively rare
and in small populations which made them less likely to be preserved as fossils.
oSome organisms have remained relatively unchanged throughout time (i.e. living fossils)
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