Origins of Biological Diversity

Chapter 19

The Fossil Record Provides Evidence to Life’s History

• Fossils form when organisms are buried in sedimentary rock or other material and remains are preserved.

• Petrified (turned to stone)• Impressions (footprints, dung etc.)• Entire organisms preserved in ice (wooly

mammoth)• Amber (tree sap preserves insects)

Geologic Time Scale

• Boundaries between eras are characterized by major changes in the environment & extinctions

Organizes Earth’s history into 4 distinct Eras:

1. Precambrian

2. Paleozoic

3. Mesozoic

4. Cenozoic

4.6 bya to 600mya

Earliest

Most Recent

Geologic Time Scale

Dating FossilsScientists can only study fossils if they know their age!

By using the layered “rock strata” scientists can determine the relative age: reflects the order in which groups of species existed compared to each other.

Youngest

Oldest

What about absolute age?

Radiometric Dating

• Method used to determine the absolute age of rocks and fossils is based on the amount of radioactive isotopes in the object.

• An isotope’s half-life is the time (years) it takes for 50% (half) of an original sample to decay.– Isotope Half-Lives are unaffected by

temp., pressure, etc.– Uranium-238 (half-life = 4.5

billion yrs.)– Carbon-14 (5,730 yrs.)

Scientific American – Radiocarbon Dating

Mass Extinctions• Episodes of great species loss are known as

mass extinctions and are evident in fossil records.

• Example-Dinosaurs (65 million years ago)

Rate of Evolution • Gradualism - involves a

slow, steady change in a particular line of descent.

• The fossil evidence supports gradualism

Rate of Evolution• Punctuated equilibrium -

is the term used to describe long periods of no change that is interrupted by brief periods of more rapid change.

• The fossil record supports.

• Most new species are produced during periods of rapid change.

Convergent Evolution

• When evolution produces similar structures and characteristics in distantly-related organisms.

• Similar environmental pressures promotes convergent evolution

• Darwin’s observations of the large flightless birds.

Convergent Evolution – For example, mammals that feed on ants and

termites evolved five times in five different regions as shown in the figure below.

– They all developed the powerful front claws, long hairless snout, and tongue covered with sticky saliva that are necessary adaptations for hunting and eating insects.

Convergent Evolution

• http://mcat-review.org/evolution.php

Convergent Evolution can lead to Analogous Structures

• Analogous structures – structures that are similar amongst different species but have different evolutionary origins.

http://www.bio.miami.edu/dana/106/106F05_4.html

Divergent Evolution

• -Accumulation of differences amongst groups that can eventually lead to separate species

• -Often the result of the same species diffusing into isolated areas. This can restrict gene flow and lead to differences in populations and eventually different species.

Divergent Evolution

http://www.mcatzone.com/glosslet.php?letter=d