A cladogram is a diagram used in systematics to illustrate hypotheses about evolutionary relationships among groups of organisms. This visual representation helps scientists understand how different species or groups are related through shared ancestry and derived traits. Understanding cladograms is fundamental to grasping the “tree of life,” which maps the evolutionary history of all living things.
Understanding Key Cladogram Concepts
Cladograms rely on specific terminology to represent evolutionary hypotheses.
The individual groups of organisms being compared are called taxa (singular: taxon). An outgroup is a taxon closely related to the groups of interest but known to have diverged earlier, serving as a reference point.
A node represents a hypothetical common ancestor and marks a branching point where one lineage diverged. The lines extending from these nodes are called branches or lineages, illustrating the evolutionary path from an ancestor to its descendants. A clade, also known as a monophyletic group, consists of a common ancestor and all of its descendants.
Evolutionary relationships are determined by shared characteristics. A character refers to a heritable trait, such as the presence of fur or the number of limbs. The specific form a character takes, like “fur present” or “fur absent,” is known as a character state. A synapomorphy is a shared derived character that evolved in the common ancestor of a particular clade and is present in all its descendants but not in more distant relatives.
Gathering Your Data for Tree Building
Building a cladogram begins with careful data collection and organization. The first step involves selecting the taxa whose evolutionary relationships will be investigated.
Next, relevant characters must be identified that vary among the chosen taxa and are heritable. These can include morphological features, molecular data like DNA sequences, or behavioral traits. It is crucial to select homologous characters, meaning they are derived from a common ancestor, rather than traits that evolved independently through convergence.
For each character, the specific character state present in each taxon is observed and recorded, such as “feathers present” or “feathers absent.” This information is organized into a character matrix, a table where taxa form the rows and characters form the columns. Choosing an outgroup is important for rooting the cladogram and helping to determine which character states are ancestral (plesiomorphic) and which are derived (apomorphic).
Constructing Your Cladogram Step-by-Step
After preparing the data, cladogram construction can begin. The process starts by referring to the character matrix. The next step involves identifying shared derived characters (synapomorphies), as these are the indicators for grouping taxa into clades. These unique traits evolved in a common ancestor and are passed down to its descendants, providing evidence of shared evolutionary history.
A core principle guiding cladogram construction is the principle of parsimony, which suggests that the simplest explanation requiring the fewest evolutionary changes is generally the most likely hypothesis. This means that among several possible cladograms that fit the data, the one with the fewest character state transitions or evolutionary “steps” is preferred. To draw the tree, begin by placing the outgroup, which represents the most ancient lineage. Then, clades are progressively added based on the shared derived characters. For instance, if comparing a shark, frog, and wolf, a notochord is an ancestral trait shared by all. Lungs would be a synapomorphy for the frog and wolf, indicating their shared ancestry after diverging from the shark lineage.
The branching pattern is built by sequentially adding groups that share more recent synapomorphies. The final step involves mapping character changes onto the branches of the cladogram, indicating where each specific derived trait is hypothesized to have evolved. For example, the presence of hair and mammary glands would be mapped onto the branch leading to the wolf, distinguishing it from the frog within their shared lung-bearing clade. Computer programs are often used for this process, especially with complex datasets, as they can evaluate millions of possible trees to find the most parsimonious arrangement.
Making Sense of Evolutionary Relationships
Once a cladogram is constructed, it provides a visual hypothesis of evolutionary relationships. Each node represents a common ancestor from which new lineages diverged. The closer two taxa are on the cladogram, the more closely related they are.
Groups that share an immediate common ancestor are called sister taxa or sister groups, and the branching pattern illustrates evolutionary divergence, showing how lineages have split and evolved independently. It is important to avoid common misconceptions: the length of branches on a cladogram does not typically indicate the amount of evolutionary time or change, nor does it imply that organisms at the “end” of branches are more “evolved.” Cladograms simply depict a hypothesis about the pattern of shared ancestry and the divergence of traits.