Grouping Species Using DNA

For a long time, biologists sorted animals by how they looked: fins go with fins, wings go with wings. Cladistics changed that. It groups species by shared derived characters, traits that a group of species inherited from one recent common ancestor and that other groups don't have. Looking alike isn't enough on its own. What matters is which new traits appeared together on the family tree.

This same method explains why biologists now say birds are dinosaurs, not just their descendants. On a family tree built from shared derived traits, birds sit nested inside the dinosaur branch, alongside Tyrannosaurus and Velociraptor. A proper group in cladistics has to include an ancestor and every one of its descendants, so leaving birds out would break the group. That's different from convergent evolution, where unrelated species evolve a similar trait separately, like a bird's wing and a bat's wing. Parallel evolution is closer relatives independently evolving similar traits after they've already split apart.

Classification is not settled once and for all. Scientists keep gathering more DNA data, and new data can overturn an old grouping. There is another twist: some organisms, especially bacteria, can pick up DNA directly from unrelated species instead of only inheriting it from a parent. That is called horizontal gene transfer, and it means the tree of life is not a clean branching tree. Most DNA still travels down the branches from parent to offspring, but a few strands cross between them, so the shape is closer to a web.