Question

Why are protists considered paraphyletic? a. They include many extinct forms, including lineages that no longer have any living representatives. b. They include some but not all descendants of their most recent common ancestor. c. They represent all of the descendants of a single common ancestor. d. Not all protists have all of the synapomorphies that define the Eukarya, such as a nucleus.

Short answer

Protists are considered paraphyletic because they include some but not all descendants of their most recent common ancestor. \( \boxed{\text{b}} \)

Step-by-step solution

Define paraphyletic

A paraphyletic group is an evolutionary group that includes a common ancestor and some, but not all, of its descendants. In contrast, a monophyletic group includes a common ancestor and all of its descendants. Now, let's analyze each answer choice.

Analyze answer choices

(a) They include many extinct forms, including lineages that no longer have any living representatives. While it's true that protists include many extinct forms, this is not the reason why they are considered paraphyletic. Their paraphyletic status is related to their evolutionary relationships, not their extinction history. (b) They include some but not all descendants of their most recent common ancestor. This choice describes a paraphyletic group, as mentioned in the definition. This answer choice is correct. (c) They represent all the descendants of a single common ancestor. This choice describes a monophyletic group, which is the opposite of paraphyletic. (d) Not all protists have all the synapomorphies that define the Eukarya, such as a nucleus. The presence or absence of synapomorphies doesn't determine if a group is paraphyletic or not.

Choose the correct answer

Based on our analysis, the correct answer is: (b) They include some but not all descendants of their most recent common ancestor.

Key concepts

Evolutionary Biology

Evolutionary biology focuses on understanding the processes through which organisms change over time. It studies how species evolve and adapt to their environment, exploring factors like natural selection, mutation, and genetic drift. These changes in organisms result in various evolutionary relationships like monophyletic and paraphyletic groups.
In evolutionary biology, a significant aspect is tracing the evolutionary history of species to determine their common ancestors and understand the branching of life forms. This field combines data from fossils, genetic sequences, and biological traits to unlock the mysteries of life's history.
Evolutionary biology provides a comprehensive framework to categorize organisms, such as protists, and understand their place in the tree of life.

Protists

Protists are a diverse group of mostly unicellular organisms that belong to the domain Eukarya. Unlike animals, plants, or fungi, they do not form a natural group of organisms, primarily because they show such varied characteristics.
While some protists are similar to plants and can perform photosynthesis, others resemble animals or fungi in their feeding and reproduction styles. This diversity makes them fascinating yet complicated to classify. They include well-known organisms like algae and amoebas.
Protists are considered paraphyletic because they consist of a common ancestor and some, but not all, of its descendants, as they do not represent all protists that have diverged from a common ancestor.

Common Ancestor

In evolutionary terms, a common ancestor is an ancestral species from which a group of organisms directly descends. This concept is fundamental in constructing evolutionary trees, or phylogenies, which illustrate how different species are related over time.
Tracing Lineages:

• Helps understand how traits evolved in different species.
• Aids in identifying the point of divergence of species.

The presence of a common ancestor doesn't necessarily mean all its offspring are included in a group. Some groups, like protists, might include functionally and morphologically diverse organisms, showcasing both ancestral and derived traits.

Monophyletic

A monophyletic group contains a common ancestor and all of its descendants. This type of grouping is often preferred in taxonomy because it fully reflects evolutionary relationships.
Characteristics of Monophyletic Groups:

• All members share a common evolutionary path.
• Contain all lineages derived from a single ancestor.

This is the opposite of paraphyletic groups, which, like protists, do not include all descendants from a common ancestor. By representing the complete lineage, monophyletic groups provide clearer insights into the evolutionary history and relationships among species.

Synapomorphies

Synapomorphies are shared derived characteristics that are unique to a particular evolutionary group. They arise due to an evolutionary trait that appears in an ancestor and is inherited by its descendants.
Importance in Classification:

• Help identify evolutionary relationships among species.
• Distinguish between ancestral and derived traits.

In the context of protists, not all members exhibit the synapomorphies that define Eukarya, such as a nucleus. Hence, they do not make up a monophyletic group. Recognizing synapomorphies is crucial in identifying true evolutionary paths and correctly categorizing organisms.