DNA Evidence
One method that scientists can use to definitively order species based on how closely related they are is by using DNA evidence.
As explained in the Genetics Unit, we can isolate the DNA of different individuals in order to compare their DNA sequences. This is a method called gel electrophoresis. An example of a gel electrophoresis experiment is shown below:
As explained in the Genetics Unit, we can isolate the DNA of different individuals in order to compare their DNA sequences. This is a method called gel electrophoresis. An example of a gel electrophoresis experiment is shown below:
We cannot only use gel electrophoresis to solve crime scenes, but we can also use it to track the evolution of species. As shown by this example, we can see which species of deer is most closely related to an ancestral species:
What we can conclude is that organisms that are most closely related have the most similar DNA. For example, based on the chart below, you can see that humans only have 8 amino acid differences when compared to the macaque (a type of primate), 32 amino acid differences when compared to a domestic dog, and 67 amino acid differences when compared to a frog. The more differences, the further related the species are to each other.
In another example, you can see the similarities and differences of the following species based on their DNA sequences. By comparing the number of DNA sequences that are similar and different, scientists can create a family tree that tracks the evolution of different species.
Let's get back to whales. So when we are comparing the DNA of whales to every other living mammal today, it comes as no surprise that the animal that has the closest DNA to the modern day whale, is the hippo. This does not mean that whales directly evolved from hippos! Rather, whales and hippos both evolved from a common ancestor, and are the closest living relatives on earth today.
When looking at a faminly tree tracing the evolution of different mammals, you can see that the split of whales and hippos was the most recent development. There is also a space for the (now extinct) transitional form between whales and land mammals.
So, how do we know what organisms are the oldest, and where they actually lived? Click on to Fossil Dating to find out!