Each ✓ in the table means a taxon has a derived trait: a feature that is new compared with the outgroup. Taxa that share more derived traits share a more recent common ancestor.
Tap a taxon, then a place on the tree. Place 1 (top) branches off first.
Tap a trait, then tap a + on the branch where it first appeared. Every taxon on the tip side of the mark has it.
Count the ✓ marks for each taxon. The taxon with the fewest derived traits branches off first, and the one with the most branches off last. A trait that every taxon has, the outgroup too, is ancestral here and does not help.
Read a tree by following the branches back to where they meet, not by reading across the tips. Pick a tree and a task.
Tap any node (a dot). Its two branches swap places, like a hanging mobile turning around. Watch the order of the tips change.
The relationships do not change. Every node still joins the same taxa, so it is still the same tree. Two drawings that differ only by rotating branches show exactly the same evolutionary history.
Follow the branches back from each taxon to where they meet. The pair whose branches meet at a more recent node (closer to the tips) shares a more recent common ancestor, so it is more closely related.
Tap tips to pick a group (tap again to drop one), then test it. A clade is one common ancestor and all of its descendants.
Or try a well-known group:
Learn the words used to read every tree. Pick Basic (6 labels) or Advanced (8), then try the blank diagram, the label quiz and match the jobs. On a phone, swipe the diagram sideways.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back.
Each line describes the job of one part. Tap a part name, then tap the box next to the job it does. Tap a filled box to take the word back.
Use this tree. Tap a card, then tap the box it belongs in. Press Check when all 10 are placed.
A worksheet with an answer key: label a cladogram, build one from a trait table, answer “who is more closely related?” questions, decide clade or not, and (AP) make a difference table and a tree from DNA.
Trees can be built from molecules as well as from body features. Related species inherited their DNA from a common ancestor, and mutations build up in each lineage after they split, so fewer differences usually means a more recent common ancestor. The short sequences below are made up for practice and exaggerate the differences: real dog and gray wolf DNA, for example, is more than 99% identical.
Tap two names: the positions where their bases differ light up.
Write the number of differences for each pair. (The table is a mirror image, so only the lower half is needed.)
This simple method is called UPGMA. In plain words: (1) find the smallest number in the table and join that pair; (2) treat the pair as one group, and work out its distance to every other taxon as the average of the distances from its members; (3) repeat until everything is joined. Each branch point is drawn at half the distance between the groups it joins, so the tips line up.
Molecular clock (with caveats). If mutations in a gene build up at a roughly steady rate, the number of differences tells you roughly how long ago two lineages split, once the clock is calibrated with dated fossils. But rates differ between genes and between lineages, and short sequences give rough answers. UPGMA assumes the same rate in every lineage; modern studies use long sequences and statistical methods (maximum likelihood and Bayesian) that allow different rates.
You and your cousins share grandparents. You and your brother or sister share parents, a more recent common ancestor, so you are more closely related to each other than to your cousins. A cladogram works the same way for species. The “parents” at each node are ancestral populations that lived long ago and split into separate lineages.
A derived trait is a new feature that appeared in an ancestor and was passed on to its descendants, like the amniotic egg in the ancestor of reptiles, birds and mammals. An ancestral trait is older and shared more widely. All the vertebrates in the builder have a notochord at some stage, and so does the lancelet, so the notochord cannot show which vertebrates are closest. Only shared derived traits (synapomorphies) group taxa into clades. The same trait can be derived at one level and ancestral at another: hair is new for mammals compared with lizards, but every mammal has it, so it does not help sort mammals from each other.
To tell old traits from new ones, compare with an outgroup: a taxon that we already know branched off before the rest (the ingroup). A trait that the outgroup also has is probably ancestral; a trait that only some of the ingroup have is a candidate derived trait. The outgroup also shows where the root of the tree goes.
Real data are messy. Some traits evolve more than once (wings in bats and in birds, which is convergent evolution), and some are lost (snakes lost their legs). Then the traits do not fit one tree perfectly. Scientists compare the possible trees and often prefer the one that needs the fewest evolutionary changes: the principle of parsimony. Today they also use statistical methods, such as maximum likelihood, on large DNA data sets.
Morphological evidence is body structure, including fossils. Molecular evidence is DNA, RNA and protein sequences. Molecules give thousands of characters and can compare organisms that look nothing alike, such as bacteria and plants. DNA has also tested older ideas: it supports birds as the closest living relatives of crocodilians and hippos as the closest living relatives of whales. When both kinds of evidence point to the same tree, scientists trust it more.
Build the plant tree, check it, then tap each node to see its clade. On the reptiles-and-birds tree, rotate two nodes and answer three “who is more closely related?” questions. Test “Reptiles without birds” with AP terms on. In the DNA section, build the reptile tree and watch which taxon joins the bird first.
Trees are not just for classifying animals. Scientists use them whenever they need to know who is related to whom.
| Use | How a tree helps |
|---|---|
| Choosing flu vaccines | Flu viruses from around the world are sequenced every year, and trees of their genes show which strains are spreading. This helps experts choose the strains for the next season’s flu vaccine. |
| Tracing where a disease came from | Trees of virus genes showed that HIV-1 came from a related virus of chimpanzees (SIV) that passed into humans in Central Africa. During the COVID-19 pandemic, public health teams used virus trees (for example on the Nextstrain project) to follow new variants as they spread. |
| Conservation | Some species are the last survivors of a long, separate branch, like the tuatara of New Zealand. Losing them would lose a lot of unique evolutionary history, so programs such as EDGE of Existence use trees to set conservation priorities. |
| Finding new medicines | Relatives of a plant that makes a useful chemical often make similar chemicals. The cancer drug paclitaxel was first found in the bark of the Pacific yew; today it is made with chemicals from the needles of other yews or in plant cell cultures, which saves the trees. |
| Catching food fraud | Comparing a short DNA sequence with known species (DNA barcoding) shows which species a fish fillet or a piece of meat really came from. |
| Understanding where traits came from | Mapping traits on a tree shows whether a feature evolved once or several times, for example eyes, flight or warm blood. |
Start at one tip and follow its branch back toward the root. The point where two branches meet is a node: the most recent common ancestor of those taxa. Taxa whose branches meet at a more recent node, closer to the tips, are more closely related. Do not read across the tips, because the order of the tips can be rotated without changing the tree. A mark on a branch is a trait that appeared there and is shared by every taxon descended from that branch.
Key takeaway: follow the branches back to where they meet.Both show how taxa are related. A cladogram shows only the branching order: which taxa share more recent common ancestors. Its branch lengths mean nothing. A phylogenetic tree (phylogeny) is the more general name, and its branch lengths often show the amount of genetic change or time. Many textbooks use the two words almost the same way.
Key takeaway: a cladogram shows order only; a phylogenetic tree may also show change or time.A clade is a group made of one common ancestor and all of its descendants, nothing more and nothing less. It is also called a monophyletic group. Mammals are a clade, and so are birds. Reptiles form a clade only if birds are included, because birds descend from the same common ancestor as lizards, snakes, turtles and crocodiles.
Key takeaway: one ancestor plus all of its descendants.An outgroup is a taxon that is related to the group being studied but branched off before all of its members. Traits that the outgroup also has are probably ancestral, so they do not show which members are closest. Traits found in only some of the group, but not in the outgroup, are likely derived. The outgroup also shows where the root of the tree is.
Key takeaway: the outgroup tells old traits from new ones and roots the tree.A node is a branching point. It stands for an ancestral population that split into two (or more) lineages, which is the most recent common ancestor of all the taxa above that node. The ancestor itself is usually not a known species; scientists infer that it existed from the shared traits of its descendants.
Key takeaway: a node is a common ancestor where lineages split.Only in a limited way. Along one path from the root to a tip, a node closer to the root is older than the nodes beyond it, toward the tips. But a basic cladogram has no time scale: branch lengths mean nothing, and you cannot compare the ages of nodes on different paths. Trees that are drawn to a time scale, using dated fossils and molecular clocks, are called time trees (chronograms).
Key takeaway: order yes, dates no, unless the tree is drawn to a time scale.A shared derived trait is a feature that first appeared in a common ancestor and was inherited by its descendants, so they share it but more distant relatives do not. Jaws are a shared derived trait of sharks, bony fish and four-limbed animals; lampreys and lancelets lack them. Shared derived traits are the evidence used to group taxa into clades.
Key takeaway: new traits shared by a group are the evidence for a clade.Count how many derived traits each taxon has. The outgroup has none (or the fewest) and branches off first; the taxon with the most branches off last. Draw a ladder in that order, then put each trait on the branch leading to the group that has it, so every taxon on the tip side of the mark has the trait and every taxon that branched off before it does not. If two taxa have the same set of traits, the table cannot tell which branches off first.
Key takeaway: fewest traits branch off first; each trait marks one branch.Monophyletic: an ancestor and all of its descendants (a clade), for example birds plus crocodiles. Paraphyletic: an ancestor and some, but not all, of its descendants, for example the traditional reptiles that leave out birds. Polyphyletic: taxa from separate lineages, grouped by a feature that evolved more than once, for example warm-blooded animals (mammals plus birds). Modern classification aims to name only monophyletic groups.
Key takeaway: mono = all descendants; para = some cut out; poly = separate lineages.No. Whales and hippos share a common ancestor that lived more than 50 million years ago. After the split, both lineages kept evolving for the same length of time. Hippos alive today are the closest living relatives of whales, like cousins, not their ancestors.
Key takeaway: whales and hippos share a common ancestor; neither came from the other.