The main line of a cladogram represents the passage of time. The start of the line is the furthest back in the past, typically millions of years ago. The nodes of a cladogram represent common ancestors that evolved a particular physical trait or characteristic. All of the animals that branch off at or after a node share that trait or characteristic (and that common ancestor). The branches of a cladogram show when each animal split off from the main line. The node where an animal branches off is the last characteristic that animal has in common with the animals that branch off further up the line.
You read cladograms and phylogenetic trees the same way, but phylogenetic trees are often more complex than cladograms. While you can easily make a cladogram on your own, you couldn’t make a phylogenetic tree without access to DNA or other molecular data for the animals you’re studying. To make things somewhat confusing, many biologists use these two terms interchangeably, or refer to both types of diagrams as “evolutionary trees. "
On a phylogenetic tree, animals that are closer together are also more genetically similar. This isn’t necessarily true on a cladogram that doesn’t use any molecular evidence.
The starting point is usually the bottom-left, but it might be a different spot depending on the orientation of the cladogram you’re looking at. Orientation doesn’t matter with cladograms—they still convey the same information regardless of how they’re turned.
Lines on a cladogram aren’t drawn to scale. In other words, the main line doesn’t start or end on any particular year and the distance between nodes doesn’t correspond to any particular range of years. Since the lines aren’t drawn to scale, you can’t infer how long ago any particular characteristic evolved. You can only infer whether a characteristic evolved before or after another characteristic represented on the cladogram.
For example, if the node closest to the starting point of the main line represents “teeth” and the node halfway up the main line from the starting point represents “lungs,” you can infer from the cladogram that animals evolved teeth before they evolved lungs. Remember that since the lines don’t represent specific dates or ranges of dates, you can’t say exactly how long it took for these evolutionary developments to happen—only the general order in which they happened.
For example, imagine a simple cladogram that includes the characteristics “backbone” and “four limbs. " Two of the animals are sharks and humans. Both sharks and humans have backbones, but sharks don’t have four limbs. Sharks would branch out from the main line at the backbone node.
For example, suppose you’re looking at a cladogram with nodes for four limbs, hair, and retractable claws. The animals represented on the cladogram include reptiles, kangaroos, dogs, and cats. Of these animals, you could say that kangaroos, dogs, and cats all share 2 of the characteristics (four limbs and hair). In this example, reptiles would be considered an outgroup—they only have 1 characteristic in common with the other groups, and likely have significantly more differences. On a cladogram, they’re the first animal that branches off the main line. [7] X Research source
For example, you might choose dogs, cats, horses, and turtles.
For example, dogs, cats, horses, and turtles all have a backbone, so that could be the first node on the main line of your cladogram—the common ancestor all of these animals have in common that developed a backbone.
For example, you might list backbone, hooves, walks on 4 legs, carnivorous, hair/fur, warm-blooded, mammary glands, and retractable claws. Then, you would place checkmarks in the “horse” column for all of those traits except for retractable claws.
To return to the previous example, you can see that only horses have hooves, so you can eliminate that characteristic—since no other animals have it, you have nothing to compare them to. Cats, on the other hand, are a strong contender for the final slot because they have all of the traits (except hooves, which you’ve eliminated). Since all animals have 4 legs and all animals have a backbone, you only need one of these characteristics for your first node—you can eliminate the other one.
For the example with dogs, cats, horses, and turtles, your largest circle would be the “backbone” circle, because all of your animals have one of those. Inside that circle, you’d draw a circle for warm-blooded, which captures all of your animals except turtles. Then, you’d have a smaller circle for “carnivorous,” with the smallest circle going to “retractable claws. " Label each of your animals in the circle that represents the last characteristic it has in common with the other animals before it diverges. In the above example, turtles would go in the largest circle, then horses in the next-largest, then dogs, with cats in the smallest circle.
In the example, turtles would branch off first, from the node that represents “backbone. " Your next node would be “warm-blooded, " with horses branching off from it. Then you would branch of dogs from the “carnivorous” node, leaving only cats after the “retractable claws” node.
For example, since both cats (Felidae) and orchids (Orchidae) are family-level groups, you might think the members of each group have similar things in common. But, in fact, orchids are tens of millions of years older than cats and are more similar to one another biologically. Switching from the Linneaean system to the phylogenetic system doesn’t actually change biological names you might be used to all that much. For example, humans are still known as Homo sapiens. Modern biologists classify plants and animals according to clades, the groupings depicted on cladograms. Some traditional classifications don’t make the cut. For example, reptiles don’t form a clade.
