Problem 10 - Biology

Question

Consulting the modern phylogenetic tree of animals, which of the following would not constitute a clade? a. deuterostomes b. Iophotrochozoans c. Parazoa d. Bilateria

Step-by-Step Solution

Answer

Parazoa

Step 1: Understand Clades

A clade is a group of organisms that consists of a common ancestor and all its lineal descendants, representing a single branch on the tree of life.

Step 2: Identify Deuterostomes

Deuterostomes are a clade that includes phyla such as Chordata and Echinodermata, sharing a common ancestor.

Step 3: Identify Lophotrochozoans

Lophotrochozoans are a clade that includes several phyla such as Mollusca and Annelida, sharing a common ancestor.

Step 4: Identify Parazoa

Parazoans include organisms such as sponges (phylum Porifera). They do not share a common ancestor with other animals and are often considered a separate group.

Step 5: Identify Bilateria

Bilateria are a clade that includes most animals that have bilateral symmetry, excluding some groups like Cnidaria.

Step 6: Determine the Outlier

Since Parazoa do not form a clade but rather a separate group, they do not constitute a clade like the others listed. Parazoa do not include all descendants of a common ancestor.

Key Concepts

Clade

A clade is a fundamental concept in phylogenetics and evolutionary biology. It refers to a group of organisms that have all descended from a common ancestor. This means that a clade includes not just the common ancestor but all of its lineal descendants as well. It's like a branch on the tree of life. If you were to cut one branch off, you get all the species that fall under that branch. Typically, names of clades are based on shared characteristics, but the essential criterion is their common descent. Clades are essential for understanding how life has evolved over millions of years and how different organisms are related to each other.
This concept helps biologists make sense of animal diversity and the relationships among different species. It's a clearer method to classify life compared to older systems based only on similarities.

Deuterostomes

Deuterostomes are a major clade in the animal kingdom. This group includes organisms that share a specific developmental pattern during embryogenesis. In deuterostomes, the first opening that develops in the embryo becomes the anus, while the mouth forms later. This group contains many well-known animals, such as those in the phylum Chordata (which includes all vertebrates) and Echinodermata (which includes starfish and sea urchins).
Deuterostomes are characterized by several key features:

• Radial cleavage during early embryonic development.
• The blastopore (first opening in the embryo) becomes the anus.
• A coelom (body cavity) that forms from mesodermal pouches.

Understanding deuterostomes is essential for grasping the evolutionary lineage leading to complex animals, including humans.

Lophotrochozoans

Lophotrochozoans are another diverse clade within the animal kingdom. This group was identified relatively recently using molecular data. It includes animals such as mollusks (e.g., snails and octopuses) and annelids (e.g., earthworms and leeches). They are unified by key traits, including the presence of a lophophore (a feeding structure) or a trochophore larval stage.
Key characteristics of Lophotrochozoans are:

• Lophophore: A crown of ciliated tentacles used for feeding, seen in organisms like bryozoans.
• Trochophore larvae: A distinct larval stage observed in animals such as marine worms.
• Spiral cleavage during embryonic development.

Lophotrochozoans showcase the vast diversity of animal forms that have evolved through different developmental strategies.

Parazoa

Parazoa is a term used for a group of simple animals that do not have specialized tissues. This group primarily includes sponges (phylum Porifera). Unlike other animal groups, Parazoans don't fit neatly into the traditional clades of the animal kingdom because they lack true tissues and organs.
Key features of Parazoa include:

• Only three types of cells: choanocytes, amoebocytes, and pinacocytes.
• Asymmetrical or radially symmetrical body structures.
• Lack of nervous, digestive, and circulatory systems.

Parazoans are often considered a separate entity from other animal groups because they do not share common developmental pathways that form the basis for clades like deuterostomes or lophotrochozoans.

Bilateria

Bilateria refers to a large clade of animals that exhibit bilateral symmetry. This means they have a left and right side that are mirror images of each other. Most animals fall into this group, which includes creatures as varied as insects, fish, and mammals. They are all thought to have descended from a common ancestor that likely had bilateral symmetry.
Key traits of Bilateria are:

• Bilateral symmetry: Distinct left and right sides.
• Triploblastic: Three germ layers (ectoderm, mesoderm, and endoderm).
• Most have a centralized nervous system.

Bilateria is split into two main groups: Protostomes (like arthropods and mollusks) and Deuterostomes (like vertebrates). This evolutionary divergence is foundational for understanding the diversity of animal life on Earth and how complex body plans have evolved.