The traditional tree of life (shown above) presents the three domains as distinct, monophyletic lineages. However, other hypotheses propose different views on the relationships among the Archaea, Bacteria, and Eukarya. In particular, the two-domain hypothesis—or eocyte hypothesis—is emerging as a well-supported alternative to the three-domain hypothesis. The eocyte hypothesis, illustrated below, suggests that eukaryotes evolved from eocytes (also known as the Crenarchaeota—a major lineage of the Archaea). Resolving the relationships among these ancient lineages is difficult, but it has profound implications on our understanding of the origin of eukaryotic cells.

Why are Archaea considered a monophyletic group according to the three-domain hypothesis?
a. Because this group includes all organisms except eukaryotes.
b. Because this group includes an ancestral population and all of its descendants.
c. Because all members of this group lack membrane-bound organelles.
d. Because this group evolved after the origin of bacteria.

Verified step by step guidance

1. Understand the concept of a monophyletic group: A monophyletic group, also known as a clade, includes an ancestral species and all of its descendants. This is a key concept in phylogenetics, which is the study of evolutionary relationships among biological entities.
2. Analyze the three-domain hypothesis: According to this hypothesis, life is divided into three domains: Bacteria, Archaea, and Eukarya. Each domain is considered a monophyletic group, meaning each includes an ancestral population and all of its descendants.
3. Consider the characteristics of Archaea: In the three-domain hypothesis, Archaea are considered a distinct domain that includes all organisms that share a common ancestor unique to Archaea, separate from Bacteria and Eukarya.
4. Evaluate the options given in the question: Option b states that Archaea are considered monophyletic because this group includes an ancestral population and all of its descendants. This aligns with the definition of a monophyletic group.
5. Conclude why Archaea are considered monophyletic: Based on the three-domain hypothesis, Archaea are considered a monophyletic group because they include an ancestral population and all of its descendants, which is the definition of a monophyletic group.