Cladistics: Videos & Practice Problems

In biology, understanding the relationships among organisms involves classifying them into groups: monophyletic, paraphyletic, and polyphyletic. A monophyletic group, or clade, includes an ancestor and all its descendants. Paraphyletic groups contain the common ancestor but exclude some descendants, while polyphyletic groups do not include the common ancestor at all. These classifications are essential in cladistics, which focuses on evolutionary relationships, ensuring that only monophyletic groups are used in modern systematics for accurate scientific naming and classification.

Monophyletic, Paraphyletic, & Polyphyletic Group

In biology, understanding the relationships among groups of organisms is essential, and this is often represented through a phylogenetic tree. Key terms that describe these relationships include monophyletic, paraphyletic, and polyphyletic groups, which are crucial for classifying organisms based on their common ancestry, a practice known as cladistics.

A monophyletic group is defined as a clade, which includes an ancestor and all of its descendants. The prefix "mono" means one, and "phyletic" relates to a tribe or group, indicating that this group is a single, cohesive unit. For example, the group known as sauropsida encompasses turtles, lizards, birds, and crocodiles, all of which share a common ancestor. When visualizing this on a phylogenetic tree, a monophyletic group can be identified by drawing a circle that intersects the tree only once, encompassing the common ancestor and all its descendants.

In contrast, a paraphyletic group includes a common ancestor but only some of its descendants. The prefix "para" means beside or alongside, indicating that this group is incomplete. For instance, the group of reptiles includes turtles, lizards, and crocodiles but excludes birds. When circled on a phylogenetic tree, a paraphyletic group requires crossing another branch to exclude certain descendants, highlighting that while the common ancestor is included, not all descendants are.

Lastly, a polyphyletic group consists of organisms that do not share a common ancestor within the group. The prefix "poly" means many, suggesting that this grouping is based on traits that evolved independently, known as analogous traits. An example of a polyphyletic group could be swimming reptiles, which might include turtles and crocodiles but exclude their common ancestor. When circled on a phylogenetic tree, this group does not encompass the common ancestor, indicating that the traits shared among the members of this group are not derived from a shared lineage.

In modern systematics, it is important to use only monophyletic groups for classification, as they accurately reflect evolutionary relationships. Understanding these distinctions helps clarify the evolutionary history and relationships among different organisms, providing a framework for biological classification.

Cladistics Example 1

The study of avian classification reveals the concept of infra classes, which are groups smaller than a class but larger than an order. Within the infra class of Palaeonaths, the tree illustrates the evolutionary relationships among various birds, particularly highlighting the flight capabilities of these species. The ancestor at the root of this tree was a flying bird, but today, only the tinamous within this group retain the ability to fly.

Among the flightless birds in this group, known as ratites, we can analyze their classification. A monophyletic group includes all descendants from a common ancestor, but since ratites exclude the flying tinamu, they cannot be classified as monophyletic. When examining the tree, if we circle the ratites—such as ostriches, rheas, emus, and kiwis—without including the common ancestor, we identify this grouping as polyphyletic. A polyphyletic group is defined as one that does not include the common ancestor of the organisms within it.

In contrast, when considering the Palaeonaths with nostrils positioned near the midpoint of their beaks, we can define this group while recognizing that it excludes the kiwi, which has nostrils at the tip of its beak. This exclusion indicates that the group cannot be monophyletic. However, if we include the common ancestor in our circle but leave out the kiwi, we classify this as a paraphyletic group. A paraphyletic group contains the common ancestor but not all of its descendants.

In summary, understanding these classifications—monophyletic, paraphyletic, and polyphyletic—helps clarify the evolutionary relationships among birds. Monophyletic groups include a common ancestor and all its descendants, paraphyletic groups include the ancestor and some descendants, while polyphyletic groups exclude the common ancestor entirely. This framework is essential for studying avian evolution and taxonomy.

According to the tree below, “Fish” should most likely be considered what type of group?

• A. Monophyletic.
• B. Polyphyletic.
• C. Paraphyletic.

Answer: (C)

According to the tree below, “monkeys” should most likely be considered what type of group?

• A. Monophyletic.
• B. Polyphyletic.
• C. Paraphyletic.

Answer: (C)

Traditionally, scientists have identified 4 Kingdoms in the domain Eukaryota: the animals, the plants, the fungi, and the protists. Some scientists argue that the protists actually represent multiple kingdoms, and that there should be many more kingdoms in the domain Eukaryota. Based on the tree, why do some scientists argue for so many kingdoms?

• A. Protists represent a polyphyletic group, so they should not be grouped together.
• B. Protists are a paraphyletic group and modern systematics is usually based on monophyletic groups.
• C. Some protists are more closely related to the archaea and bacteria making them a polyphyletic group.
• D. Protists are a monophyletic group but do not represent a true clade according to modern classification.

Answer: (B)

Here’s what students ask on this topic:

What is the difference between monophyletic, paraphyletic, and polyphyletic groups in cladistics?

In cladistics, groups of organisms are classified based on their evolutionary relationships. A monophyletic group, or clade, includes a common ancestor and all its descendants, representing a complete branch on a phylogenetic tree. This is crucial for systematics, as it reflects true evolutionary lineage. A paraphyletic group includes a common ancestor and some, but not all, of its descendants. This type of grouping is often used to highlight evolutionary relationships where certain descendants have evolved distinct traits, like reptiles excluding birds. Lastly, a polyphyletic group does not include the common ancestor of the organisms within it. Instead, it groups organisms based on analogous traits that evolved independently, such as swimming reptiles like turtles and crocodiles. Understanding these classifications helps in studying evolutionary biology and the diversity of life.

Why are monophyletic groups preferred in modern systematics?

Monophyletic groups are preferred in modern systematics because they accurately represent evolutionary relationships by including a common ancestor and all its descendants. This comprehensive grouping ensures that the classification reflects true evolutionary lineage, which is essential for understanding the history and diversity of life. By using monophyletic groups, scientists can create a more accurate and consistent taxonomy, aiding in the study of evolutionary biology. In contrast, paraphyletic and polyphyletic groups can lead to misleading classifications, as they either exclude some descendants or the common ancestor, respectively. Therefore, monophyletic groups are fundamental in systematics for providing a clear and precise representation of the evolutionary tree.

How can you identify a monophyletic group on a phylogenetic tree?

To identify a monophyletic group on a phylogenetic tree, look for a group that includes a common ancestor and all its descendants. A useful method is to circle the organisms of interest on the tree. If your circle crosses the tree only once, encompassing the branch leading to the common ancestor and all its descendants, you have identified a monophyletic group. This group should form a single, unbroken branch on the tree, representing a clade. This method ensures that the group reflects a complete evolutionary lineage, which is crucial for accurate classification in systematics. Recognizing monophyletic groups helps in understanding the evolutionary relationships and diversity of life.

What are some examples of paraphyletic groups in biology?

Paraphyletic groups in biology include a common ancestor and some, but not all, of its descendants. A classic example is the group 'reptiles,' which traditionally includes turtles, lizards, and crocodiles but excludes birds. Although birds share a common ancestor with these reptiles, they have evolved distinct traits, such as feathers and a four-chambered heart, leading to their exclusion from the traditional reptile group. Another example is the group 'fish,' which includes various aquatic vertebrates but excludes tetrapods, even though they share a common ancestor. Paraphyletic groups are often used to highlight evolutionary relationships where certain descendants have evolved significant differences.

What role do analogous traits play in forming polyphyletic groups?

Analogous traits play a crucial role in forming polyphyletic groups, as these traits evolve independently in different lineages, leading to the grouping of organisms that do not share a common ancestor. These traits arise due to convergent evolution, where similar environmental pressures result in similar adaptations in unrelated species. For example, the ability to swim in turtles and crocodiles is an analogous trait that might lead to their classification in a polyphyletic group of 'swimming reptiles.' However, their common ancestor was not a swimming reptile. Recognizing polyphyletic groups helps in studying how different organisms adapt to similar environments, despite not sharing a direct evolutionary lineage.