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Short Video: Using Derived Characters to Infer Phylogeny

by Pearson
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Using derived characters to infer phylogeny. Looking at the distribution of characters among species can help researchers infer their evolutionary relationships. It is especially useful to look at shared derived characters, which are evolutionary novelties, such as a vertebral column that are unique to a clade. This table shows the distribution of five derived characters in six taxa. An amnion, if you are not familiar with the term, is a specialized membrane that surrounds and protects a developing embryo. And the other derived characters probably are familiar to you. A "0" indicates that the character is absent in an organism, and a "1" indicates that the character is present. For example, a frog has a vertebral column or backbone, hinged jaws, and four limbs, but it does not have an amnion or hair. Which organisms in the table have hinged jaws but no amnion? The correct answer is the bass and frog, which have hinged jaws but no amnion. How can we use the information in this character table to infer phylogeny? To do this, we must select an outgroup, a species or group of species from an evolutionary lineage that is closely related to but not part of the group of species that we are studying, which is the ingroup. In this case, the ingroup includes a variety of vertebrates, and the appropriate outgroup is the lancelet. The lancelet is a chordate, like vertebrates, but unlike them, it does not have a backbone. The first branch point of the tree splits the outgroup from the ingroup. A derived character that's shared by all vertebrates is the vertebral column, or backbone. This character separates the vertebrates from lancelets, the outgroup. Next, we must use the information in the table to infer the sequence in which the other characters appeared over time. Based on the data shown in the character table, identify the derived character that marks the next evolutionary branch point. Note that one character, hinged jaws, is present in all members of the ingroup except lampreys. This suggests that the lineage leading to lampreys was the next to diverge from other members of the ingroup. (Remember that a branch in a phylogenetic tree represents the evolutionary lineage that leads to the taxon named at the end of the branch.) We can continue in the same way, seeing that the character "four limbs" is absent in bass but is shared by frogs, turtles, and leopards. That the amnion is absent in frogs but is shared by turtles and leopards and finally, that hair is a derived character in the lineage leading to leopards. Note that the characters in this table were chosen to make inferring phylogeny straightforward. Researchers would typically use many more characters, not only five, and some of the characters might even conflict with one another and suggest different relationships among taxa. Moreover, the absence of a character can in some cases be misleading. For example, over the course of evolution, a subgroup of a lineage may lose a character that's typically found in other members of that lineage. Consider snakes, which have a vertebral column, hinged jaw, and an amnion, like turtles. But do not have four legs or obvious limbs like turtles and other reptiles do. Snakes and turtles both evolved from a reptilian ancestor that had four limbs, but over time, these limbs became highly reduced in the snake lineage.
Using derived characters to infer phylogeny. Looking at the distribution of characters among species can help researchers infer their evolutionary relationships. It is especially useful to look at shared derived characters, which are evolutionary novelties, such as a vertebral column that are unique to a clade. This table shows the distribution of five derived characters in six taxa. An amnion, if you are not familiar with the term, is a specialized membrane that surrounds and protects a developing embryo. And the other derived characters probably are familiar to you. A "0" indicates that the character is absent in an organism, and a "1" indicates that the character is present. For example, a frog has a vertebral column or backbone, hinged jaws, and four limbs, but it does not have an amnion or hair. Which organisms in the table have hinged jaws but no amnion? The correct answer is the bass and frog, which have hinged jaws but no amnion. How can we use the information in this character table to infer phylogeny? To do this, we must select an outgroup, a species or group of species from an evolutionary lineage that is closely related to but not part of the group of species that we are studying, which is the ingroup. In this case, the ingroup includes a variety of vertebrates, and the appropriate outgroup is the lancelet. The lancelet is a chordate, like vertebrates, but unlike them, it does not have a backbone. The first branch point of the tree splits the outgroup from the ingroup. A derived character that's shared by all vertebrates is the vertebral column, or backbone. This character separates the vertebrates from lancelets, the outgroup. Next, we must use the information in the table to infer the sequence in which the other characters appeared over time. Based on the data shown in the character table, identify the derived character that marks the next evolutionary branch point. Note that one character, hinged jaws, is present in all members of the ingroup except lampreys. This suggests that the lineage leading to lampreys was the next to diverge from other members of the ingroup. (Remember that a branch in a phylogenetic tree represents the evolutionary lineage that leads to the taxon named at the end of the branch.) We can continue in the same way, seeing that the character "four limbs" is absent in bass but is shared by frogs, turtles, and leopards. That the amnion is absent in frogs but is shared by turtles and leopards and finally, that hair is a derived character in the lineage leading to leopards. Note that the characters in this table were chosen to make inferring phylogeny straightforward. Researchers would typically use many more characters, not only five, and some of the characters might even conflict with one another and suggest different relationships among taxa. Moreover, the absence of a character can in some cases be misleading. For example, over the course of evolution, a subgroup of a lineage may lose a character that's typically found in other members of that lineage. Consider snakes, which have a vertebral column, hinged jaw, and an amnion, like turtles. But do not have four legs or obvious limbs like turtles and other reptiles do. Snakes and turtles both evolved from a reptilian ancestor that had four limbs, but over time, these limbs became highly reduced in the snake lineage.