TICK, TOCK - TORTOISE CLOCKS

The ancestors of some Malagasy animals and plants began living on the land that would become Madagascar when it was still attached to other land-masses millions of years ago. Others got here by swimming or flying after it was already an island. How do scientists discover when different kinds of animals arrived? Some scientists argue that we can deduce how long in the past two animals shared a common ancestor by looking at the rate of mutation in their genes over time. This method of dating species divergence is called using a 'molecular clock'.

The idea is simple and elegant: count the number of differences in the genetic code of two species, multiply it by the rate at which changes occur and voila! - you'll have the number of years that have passed since the two species shared a common ancestor.

The problem is accurately determining the rate of change. Different species have different rates of DNA mutation. The rate of change in turtle mitochondrial DNA, for example, is about _ to 1/3 the rate of change in mammal DNA. Also, some aspects of the genetic sequence are more likely to mutate than others. (In general, mitochondrial DNA has fewer mutations over time than nuclear DNA). Also, using this method, the date of divergence is based on the assumption that the rate of change was constant over time, and didn't happen as sudden, short-term events.

Despite these problems, molecular clocks have proven relatively accurate for many species when compared with fossil data. Let's take the information we learned when we analyzed tortoise DNA to see how recently the various species may have shared common ancestors. And then we can use that information to answer the question: did they walk or float?

We can take the percent differences from our chart and plug those numbers into the following equation to figure out when two species diverged:

Number of Years (in millions) = Percent Difference .5

Use the chart on the left to fill in the chart on the right in your field notebook (you will find the chart in your student guide). only the number in black would be filled in

Example:
Let's start with sp. B (P. arachnoides) and C (P. planicauda).
The % difference for these two species is 5%. The rate of divergence for tortoises is .5% per million years. If you divide the % difference by the divergence over time, you get time.
5% / .5% or .05 / .005= 10
Therefore, the sp. B and sp. C diverged around 10 million years ago.

According to your data, when did tortoises on Madagascar and tortoises in Africa share a common ancestor? (between 14 and 20 mya)
Did this ancestor arrive in Madagascar by walking or floating?
Madagascar separated from Africa in the late Jurassic period, around 160 million years ago, and separated from India around 88 mya.
So they must have FLOATED!

© 2007 Wildlife Conservation Society