In evolutionary biology the word “relationship” has a particular meaning, which is key to understanding and using the Tree of Life.
Phylogenetic trees come about through successive events of speciation (branching), in which one species gives rise to two.
“Phylogenetic relationship” refers to the relative times in the past that species shared common ancestors. Two species (B & C) are more closely related to one another than either one is to a third species (A) if, and only if, they share a more recent common ancestor with one another (at Time 2) than they do with the third species (at Time 1).
The model below shows that crocodiles and birds (the owl) are more closely related to one another than either is to mammals (the Gorilla). Why? Because scientists have inferred that the crocodiles and birds share a more recent common ancestor! Notice that no matter how you swivel the branches of this tree, the same relationships hold:
The information about relationships is not in where the species sit relative to one another at the tips of the branches; we don’t read trees across the top from left to right. Instead, we read trees downward from the tips, moving backward in time, or upward from the bottom, moving forward in time.
Starting from the bird, for instance, we can move back in time and ask whether the bird branch connects first to the crocodile branch or to the mammal branch. Starting from the bottom, we can ask which branching event occurred first, and which occurred later. The entire Tree of Life is made of small trees like this one.
Some relationships seem obvious: a cow and a sheep are more closely related than either one is to a sunflower. Others are not nearly so obvious. Is a mushroom closer to a human or to a water lily? Surprisingly, mushrooms are closer to us than they are to plants.
Organisms that look very different can be quite closely related, while very similar organisms, might be very distantly related. Remember, “phylogenetic relationship” refers not to the similarities and differences among organisms, but to the relative times that they shared common ancestors in the past.
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