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Building phylogenetic trees

How do scientists construct phylogenetic trees? After the homologous and analogous traits are sorted, scientists often organize the homologous traits using a system called cladistics    . This system sorts organisms into clades: groups of organisms that descended from a single ancestor. For example, in [link] , all of the organisms in the orange region evolved from a single ancestor that had amniotic eggs. Consequently, all of these organisms also have amniotic eggs and make a single clade, also called a monophyletic group    . Clades must include all of the descendants from a branch point.

Art connection

The illustration shows the V-shaped Vertebrata clade, which includes lancelets, lampreys, fish, lizards, rabbits, and humans. Lancelets are at the left tip of the V, and humans are at the right tip. Four more lines are drawn parallel to the lancelet line; each of these lines starts further up the right arm of the V than the next. At the end of each line, from left to right, are lampreys, fish, lizards, and rabbits. Lizards, rabbits, and humans are in the clade Amniota, which form a small V nested in the upper right-hand corner of the V-shaped Vertebrata clade.
Lizards, rabbits, and humans all descend from a common ancestor that had an amniotic egg. Thus, lizards, rabbits, and humans all belong to the clade Amniota. Vertebrata is a larger clade that also includes fish and lamprey.

Which animals in this figure belong to a clade that includes animals with hair? Which evolved first, hair or the amniotic egg?

Clades can vary in size depending on which branch point is being referenced. The important factor is that all of the organisms in the clade or monophyletic group stem from a single point on the tree. This can be remembered because monophyletic breaks down into “mono,” meaning one, and “phyletic,” meaning evolutionary relationship. [link] shows various examples of clades. Notice how each clade comes from a single point, whereas the non-clade groups show branches that do not share a single point.

Art connection

Illustrations show a phylogenetic tree that includes eukaryotic species. A central line represents the trunk of the tree. From this trunk, various groups branch. In order from the bottom, these are diplomonads, microsporidia, trichomonads, flagellates, entamoebae, slime molds, and ciliates. At the top of the tree, animals, fungi and plants all branch from the same point and are shaded to show that they belong in the same clade. Flagellates are on a branch by themselves, and they also form their own clade and are shaded to show this. In another image, Flagellates and ciliates are shaded to show that they branch from different points on the tree and are not considered clades. Likewise, a grouping of animals and plants but not fungi would not be considered a clade cannot exclude a branch originating at the same point as the others.
All the organisms within a clade stem from a single point on the tree. A clade may contain multiple groups, as in the case of animals, fungi and plants, or a single group, as in the case of flagellates. Groups that diverge at a different branch point, or that do not include all groups in a single branch point, are not considered clades.

What is the largest clade in this diagram?

Shared characteristics

Organisms evolve from common ancestors and then diversify. Scientists use the phrase “descent with modification” because even though related organisms have many of the same characteristics and genetic codes, changes occur. This pattern repeats over and over as one goes through the phylogenetic tree of life:

  1. A change in the genetic makeup of an organism leads to a new trait which becomes prevalent in the group.
  2. Many organisms descend from this point and have this trait.
  3. New variations continue to arise: some are adaptive and persist, leading to new traits.
  4. With new traits, a new branch point is determined (go back to step 1 and repeat).

If a characteristic is found in the ancestor of a group, it is considered a shared ancestral character    because all of the organisms in the taxon or clade have that trait. The vertebrate in [link] is a shared ancestral character. Now consider the amniotic egg characteristic in the same figure. Only some of the organisms in [link] have this trait, and to those that do, it is called a shared derived character    because this trait derived at some point but does not include all of the ancestors in the tree.

Questions & Answers

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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