Question

A taxon composed of all species derived from a common ancestor is referred to as a. a phylum. b. a monophyletic group or clade. c. a genus. d. an outgroup. e. all of the above.

Short answer

The correct answer is b. a monophyletic group or clade.

Step-by-step solution

Understanding key terms

Understand what the question asks for. You are asked to identify a taxon that contains all species derived from a common ancestor. In biological taxonomy, all these terms - phylum, genus and outgroup are used, but they all refer to different levels or groups of organization. A phylum is a rank below kingdom and above class in the hierarchy of biological classification. A genus is a rank below family and above species in the hierarchy. An outgroup refers to an organism or group of organisms that act as a reference group when determining the evolutionary relationship among three or more monophyletic groups of organisms.

Identify the correct answer

Bearing in mind the definitions of all the terms, recall the definition of a monophyletic group or clade. This is the only group that consists of a common ancestor and all its descendants. Monophyletic groups are single branches on the tree of life. Thus, b. a monophyletic group or clade is the correct answer.

Key concepts

Monophyletic Group

A monophyletic group, also known as a clade, is a fundamental concept in the study of evolutionary biology and biological taxonomy. This group is composed of an ancestral species and all its descendants.

The idea is to illustrate evolutionary relationships by forming singular branches of the tree of life. In the realm of taxonomy, highlighting these clusters is crucial for understanding how species have diversified from common ancestors.

• Definition: A group of organisms that includes an ancestor and all of its descendants.
• Significance: Helps in mapping out the evolutionary pathways and genetic relationships among species.

Monophyletic groups stand in contrast to other groupings like paraphyletic and polyphyletic groups, which either omit some descendants or combine species with different ancestors respectively.

Phylum

A phylum is an important rank in the biological classification hierarchy. Known as one of the major taxonomic categories, it sits just below the kingdom level.

Phyla represent broad groups of organisms that share a similar general body plan and consistent evolutionary features. This classification helps in categorizing life forms that exhibit fundamental similarities in structure or developmental patterns.

• Definition: A level of classification or taxonomic rank below kingdom and above class.
• Role in Taxonomy: Unites organisms into groups that have similar characteristics and biological traits.

For example, in the animal kingdom, phyla include categories like Chordata, which encompasses animals with a notochord, and Arthropoda, which includes insects and crustaceans.

Genus

In biological taxonomy, a genus is a rank used in the classification of organisms that is below family and above species. The genus level of classification helps in organizing species into groups that are closely related, making it easier to study their evolution and distribution.

Genera (the plural of genus) gather species that are thought to have evolved from a common ancestor. This classification reflects both historical and morphological similarities among members.

• Definition: A rank in the taxonomic hierarchy that is above species and below family.
• Importance: Groups together species that are very similar and share a common ancestry.

For instance, the genus "Homo" includes modern humans (Homo sapiens) and closely related species like Homo neanderthalensis.

Outgroup

An outgroup is a concept used in phylogenetic analysis, which involves determining the evolutionary relationships among organisms. It comprises a group or species that doesn't belong to the main group under study but is closely related to it.

Outgroups serve as a reference point to help researchers infer the evolutionary traits of the study group and can help in determining which characteristics are ancestral and which are derived.

• Definition: A species or group used as a reference point in evolutionary studies, typically closely related but not part of the targeted group of interest.
• Application: Helps pinpoint evolutionary branches and validate the evolutionary pathway of the main group being studied.

By providing a comparison, outgroups assist in constructing more accurate phylogenetic trees and understanding the divergence of the main study group.