Implications of CladisticsUNDERSTANDING BRANCHING DIAGRAMS
The output from a phylogenetic analysis is a hypothesis of relationship of different taxa. This hypothesis can be represented as a cladogram, a branching diagram. Cladograms bear a lot in common with the notion of family trees. In a family tree we trace back our ancestry. For example, in the family tree on the right, the ancestors of all the rest of the family are the initial black dot and yellow square. These ancestors give rise to three children, one of which mates and has two children. We can all trace our lineages back to one set of ancestors.
All species have ancestors too. So, for example, sometime in the past an ancestral species (father) of Homo sapiens walked the earth. This ancestor went extinct (died), but left descendent species (children).
In family trees, we can talk coherently about real ancestors. In biology, the ancestors are often gone sometimes without a trace. All we have left are the children. Reading cladograms is much like reading a family tree. Both are rich in information. Cladograms, like family trees, tell the pattern of ancestry and descent. Unlike family trees, ancestors in cladistics ideally give rise to only two descendent species. Also unlike family trees, new species form from splitting of old species. In speciation, it does not take two to tango. The formation of the two descendent species is called a splitting event. The ancestor is usually assumed to "die" after the splitting event.
In the first tree, labelled Cladogram A, notice the small circles. These mark the nodes of the tree. The stems of the tree end with the taxa under consideration. At each node a splitting event occurs. The node therefore represents the end of the ancestral taxon, and the stems, the species that split from the ancestor. The two taxa that split from the node are called sister taxa. They are called sister taxa because they are like the siblings from the parent or ancestor. The sister taxa must each be more closely related to one another than to any other group because they share a close common ancestor. In the same way, you are most closely related to your siblings than to anyone else since you share common parents. Lets focus on node C in Cladogram A. At the node, the ancestor goes extinct but leaves two siblings hypothesized to be humans and gorillas. Humans and gorillas are sister taxa and are more closely related to one another than either is to chimpanzees or baboons.
Working down the tree we come to node B. At this node the ancestor of the humans and gorillas split from the chimpanzees. Therefore the chimpanzees sister taxon is the human/gorilla ancestor. A sister taxon can be an ancestor and all its descedents. We call an ancestor plus all its descendents a clade. A cladogram shows us hypothesized clades.
Finally we come to node A. Here, we find the splitting event that led to the baboons and the ancestor to the chimpanzees, humans and gorillas. By working our way down the cladogram we have learned the pattern of splitting. We have found out that chimpanzees, humans and gorillas are more closely related to each other than to baboons. In this example, baboons are the outgroup.
Now, how in the world did we manufacture Cladogram A? We mentioned that it was a hypothesis. What if it we chose another hypothesis like Cladogram B or Cladogram C? We would change the pattern of speciation events. In Cladogram B, humans and chimpanzees are sister taxa and in Cladogram C, chimps and gorillas are sister taxa.
Which of the three cladograms presented above is correct? None of the cladograms can be proved correct, but Cladogram B is the best supported of the three based on character data and is therefore hypothesized to best reflect the true branching pattern.
Manufacturing cladograms which show hypotheses of ancestry and descent requires that we analyze characters and find those characters that unite clades.
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Implications of Cladistics