READING PHYLOGENETIC TREES
Is a crocodile more closely related to a bird or to a lizard?
PHYLOGENY The evolutionary history of a group of organisms.
PHYLOGENETIC TREE A branching diagram of relationships showing common ancestry.
ANSWER: The fact that all land vertebrates have four limbs and the same forelimb bones indicates that they all share a common ancestor. But how precisely are they related? In other words, who’s more closely related to whom? Biologists want not only to categorize organisms, but also to have those categories reflect phylogeny, the actual evolutionary history of the organisms. This history is represented visually by a diagram called a phylogenetic tree, which is similar in some respects to a family tree.
Phylogenetic trees can be drawn in a number of ways, but most have certain features in common. At the base, or root, is the common ancestor shared by all organisms on the tree. Over time, and with different selective pressures, different groups of organisms diverged from that common ancestor and from one another, leading to separate branches on the tree. The points on the tree at which these branch points occur are called nodes. A node represents the common ancestor shared by all organisms on the branches above that node. At the very tips of the branches we find the most recent organisms in that lineage, including living organisms and organisms that became extinct. We can thus establish relationships between living organisms (at the tips of the branches) on the basis of the ancestors they share. The more recently two groups share a common ancestor, the more closely they are related (INFOGRAPHIC 17.7).
Evolutionary history, or phylogeny, is represented visually by a phylogenetic tree. Trees have a common structure, with a root, nodes, and branch points. To determine evolutionary relationships among living or extinct organisms, consider the most recent common ancestors.
385
A phylogenetic tree is a visual representation of the best hypothesis we currently have for how species are related, based on a shared evolutionary history. The evidence for a phylogenetic tree comes from many sources, including the fossil record, physical traits, and shared DNA sequences. For many years, biologists relied solely on observable physical or behavioral features to construct phylogenetic, or evolutionary, trees. But with the genetic revolution, it’s become common to include DNA evidence. Typically, researchers compare sequence differences in a gene that is found in all living organisms, such as a ribosomal RNA (rRNA) gene.
Sometimes the new genetic information yields surprises. Modern genetic evidence shows, for example, that crocodiles are more closely related to birds than they are to lizards, appearances notwithstanding. Genetics, you might say, is shaking the evolutionary tree.