Question

A monophyletic taxon is one that includes: a. an ancestor and all of its descendants. b. an ancestor and some of its descendants. c. organisms from different evolutionary lineages. d. an ancestor and those descendants that still resemble it. e. organisms that resemble each other because they live in similar environments.

Short answer

The correct answer is a. a monophyletic taxon includes an ancestor and all of its descendants.

Step-by-step solution

Understanding the Concept of Monophyly

Monophyly is a term from cladistics that describes a group of organisms that forms a clade, which consists of all the descendants of a common ancestor. A monophyletic taxon is therefore one that includes an ancestor and all of its descendants, without excluding any descendant lineages.

Eliminating Incorrect Options

Based on the definition of monophyly, we can eliminate options that do not include all descendants of a common ancestor. This immediately allows us to remove options b, c, d, and e as they describe groups that either do not include all descendants or are based on criteria other than common ancestry.

Identifying the Correct Answer

Now that incorrect options have been eliminated, the only remaining option that fits the concept of a monophyletic taxon is option a: an ancestor and all of its descendants.

Key concepts

Cladistics

Cladistics is a method of classification in biological taxonomy that arranges organisms by their order of branching in an evolutionary tree. Unlike traditional Linnaean taxonomy, which groups organisms by overall similarity, cladistics focuses purely on the relation to a common ancestor and the evolutionary branches that have occurred since. This evolutionary tree is known as a 'cladogram'.

Within cladistics, the goal is to work out the lineages of living and extinct species by looking at shared characteristics that come from the most recent common ancestor. The fundamental premise is that members of a group share a closer common ancestry with each other than with members of any other group. By utilizing DNA, RNA, and sometimes morphological data, researchers can determine these evolutionary relationships and clarify the path of descent.

Common Ancestor

The concept of a common ancestor lies at the heart of understanding evolutionary relationships. A common ancestor refers to an ancestral species from which two or more descendant species evolved. This ancestor may have lived millions of years ago, and it is the genetic source of at least one characteristic shared by its descendants.

When we talk about monophyletic taxa, we imply that there exists a common ancestor from which all members directly descend. This is different from other groups, where similarities might arise due to convergent evolution – where species independently evolve similar traits due to similar environmental pressures, not because they share a common ancestry.

Descendant Lineages

Descendant lineages refer to the sequence of species that have evolved from a common ancestor over time. Each species in the lineage is the product of speciation, where one species branches out to become two or more species that follow separate evolutionary paths.

In a monophyletic taxon, all the descendant lineages from a single ancestor are included. This group is also known as a clade. An important point is that, in monophyletic groups, no descendants are left out. As a straightforward analogy, imagine a family tree where you include not just your immediate relatives but every branch including third cousins, and so on, tracing back to a known common ancestor.