Evidence For Evolution

We have defined evolution as change over time. Through his studies of Lamarck and his voyages around the world, Charles Darwin created the

theory of natural selection which explained how this change might actually occur. We will look next at the evidence that supports these ideas.

During the first week of this unit, we completed a lab that introduced us to the three main lines of evidence supporting evolution. They were molecular biology, anatomical evidence, and

paleontology. Let’s take a closer look at each of these…

I. Evidence From Molecular Biology:

1. Watch Genetic Tool Kit Video Clip (Video 2)

2. Summarize what you learned in lab…

II. Anatomical Evidence

1. Homologous Structures:Structures that are similar in structure, not in function.Suggests common ancestryEx: Bat = flight, Penguin = swimming, but same bones in each.

2. Analogous Structures: Structures that are similar in function, not

in structure Does not suggest common ancestry Ex: Bird wing and fly wing

3. Embryological Evidence

• Embryo: Organism in EARLY developmental stages.

• Comparative Embryology: Study of embryos of different organisms at similar stages in development to note similarities and differences.

Watch Comparative Embryology Video Clip

III. Paleontology

A. Background Information on Fossils

1. What?

Most often found in sedimentary rock (sometimes metamorphic).

The remains or traces of a once-living organism. Formed by hard body parts like shell, bone, teeth, stems.2.

Where?

3. How?

Mold An imprint in rock in the shape of an organism. (Ex: the mold made for braces)

Cast A mold is filled with minerals leaving a rock-like model. (Ex: pouring plaster into a footprint)

Others Preservation (ice, amber, tar) Hard minerals replace soft tissue (petrified wood)

Living Fossils The coelocanth, a deep water fish with fin-like legs

The fossil record shows that the complexity of life has increased over time…

B. What Can Fossils Tell Us?

• Relative Dating Observing layers of rock in order to determine relative age (stratification).

Ex: Which rock layer is the oldest?

If fossils are found in these layers, the approximate age can be determined.

• Absolute Dating Determining an “exact” age through radiocarbon dating.

• Certain molecules—through collisions in the atmosphere—can become unstable, or radioactive.

• Let’s look at carbon, for example:

Carbon-12 (normal, 6 protons/6 neutrons)

Carbon-14 (radioactive, 6 protons/8 neutrons)

C-12 & C-14 are called isotopes. Isotopes are atoms with the same number of protons

but different number of neutrons!

1. Through photosynthesis, plants take in carbon (a constant ratio of C-12 to C-14). So, anything that eats plants also consumes carbon.

2. When an organism dies, no more carbon is consumed.

3. Over time, the amount of C-12 stays constant.

4. C-14 begins to decay.

5. By comparing the ratio of C-12 to C-14, the approximate age of a fossil can be determined.

• Half-life the amount of time required for half of a material to decay. The half-life of carbon is 5,730 years.

Example: Assume, for simplicity’s sake, that C-12 and C-14 are found in a 1:1 ratio (instead of 1:1,000,000,000,000). A

paleontologist finds a fossil containing 80g of C-12 and 40g of C-14. Since there were

originally equal amounts of C-12 and C-14…

What percent of the C-14 sample has decayed?

So, what is the age of the sample?

A few more ways to look at it…

If the half-life of a radioactive isotope is 4,000 years, how much of the isotope will be left after 8,000 years?

The half-life of C-14 is 5730 years. How many years will it take for a 36 gram sample to decay into a 9 gram sample?

5. By comparing the ratio of C-12 to C-14, the approximate age of a fossil can be determined.

6. Half-life the amount of time required for half of a material to decay. The half-life of carbon is 5,730 years.

Example: Assume, for simplicity’s sake, that C-12 and C-14 are found in a 1:1 ratio (instead of 1:1,000,000,000,000). A

paleontologist finds a fossil containing 80g of C-12 and 40g of C-14. Since there were

originally equal amounts of C-12 and C-14…

What percent of the C-14 sample has decayed?

So, what is the age of the sample?

Using these techniques, scientists estimate the age of the earth to be 4.5

billion years.