Any trait that can be passed from one generation to the next through inheritance can help us learn about evolutionary relationships. These traits include:

• DNA and RNA sequences (as well as other molecular traits like the amino acid sequences of proteins). Because DNA technology is now widely available, relatively inexpensive, and generates a lot of useful evidence, many evolutionary trees being built today are based on DNA sequences. In recent years, many branching connections on the Tree of Life (including where turtles fit on the Tree, how whales are related to other mammals, and the basic relationships among plants, fungi, and animals) have been revised based on DNA evidence. As DNA sequencing becomes faster and cheaper, biologists are increasingly using data from entire genomes to build trees.

  

• Morphology (i.e., structure and form). Morphology includes all sorts of physical traits that may be encoded in an organism’s genetic material — the size and shape of different body parts, the presence or absence of different physical characteristics, coloration, pattern, and much more. Before 1980, most evolutionary trees were based on morphology alone; today, trees based on DNA alone or a combination of DNA and morphology are much more common. Nevertheless, morphology still has an important role in phylogenetics, in no small part because DNA is usually unavailable for extinct species and because fossils preserve many aspects of morphology. Morphology is often the sole line of evidence that can help us learn how ancient life is related to modern species.

  

• Physiology and biochemical pathways. Characteristics such as variations in photosynthetic pathway or in digestive system function are heritable and so intimately intertwined with the basic biological systems of life that they can reveal deep evolutionary relationships.

  

• Behavioral and ecological traits. Many behaviors, like complexity of a mating dance, diet breadth, and habitat preference, have an underlying genetic basis and so are heritable. While trees are rarely based on such traits alone, behavioral and ecological traits may be used along with other types of characteristics to build evolutionary trees.