I am not too sure, but I believe it is because DNA is actually apart of living organisms along with Protein. So it can't only be compared with dead organisms but they can be living.
<u>Hope this helps!</u>
Ok. Lets pretend you see a habitat in the wood. You see the food chain of all the animals. mouse, fox, bear, human. Lets say a zoo decides to get rid of their foxes and dumps them in this forest. Then, instead of 20, you have 30 foxes. All of the mice are gone in a matter of weeks. So the Foxes start to slowly drop out. Then, theres not much for the bears to eat, so the bears start dying. Then, the humans can't hunt bears. It sets the whole chain of the forest off. Next, the humans will switch to dears, and all the dears will be killed off... etc... etc... Make sense? Got a question? Message me for more details and more questions!
From One Smartie to Another- BubbleSmartie11
In the chart provided below, you can see that the first species belongs to the plant kingdom, and the other three belong to the animal kingdom. The kingdom is a very high level of classification, right below the level of domains. Therefore, organisms that belong to different kingdoms are very distantly related.