Human lactation / nursing is an example of a ___ .

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To begin, let's examine what each term means in evolutionary biology and how it applies to human lactation/nursing.
Option a: synapomorphy. A synapomorphy is a shared derived character that defines a particular clade. Human lactation is a specialized trait that evolved in mammals and is shared by all (and only) living mammals, thus it serves as a derived feature of the mammalian lineage. This makes it a hallmark of a group rather than a general ancestral condition, aligning well with the concept of a synapomorphy.
Option b: phylogeny. A phylogeny is the evolutionary history or the branching pattern of a group of organisms. It is a representation, not a trait itself. Since lactation is a trait, not a description of evolutionary relationships, this option is not the correct labeling for the trait in question.
Option c: clade. A clade is a group consisting of an ancestor and all its descendants. While lactation is a characteristic that helps define a clade (mammalia), the term clade refers to a group, not to a particular inherited feature. Thus this option misidentifies the concept being described.
Option d: derived trait. A derived trait is any feature that evolved in a lineage and is different from the ancestral state. While lactation is indeed a derived trait within mammals, the more precise term that captures its role in defining a monophyletic group is synapomorphy. The distinction is subtle but important: synapomorphy specifies a shared derived character defining a clade.
Option e: plesiamorphy. The correct term is pleisiomorphy (with an 'i'), which refers to an ancestral ("primitive") trait shared by both the group and its distant relatives. Lactation is not an ancestral trait for all vertebrates, but a derived mammalian feature, so this is inaccurate.
Option f: taxonomy. Taxonomy is the science of naming and classifying organisms. It describes organization, not a specific trait, so it is not an appropriate label for a particular feature like lactation.
In summary, option a correctly identifies lactation as a synapomorphy (a shared derived trait that defines the mammalian lineage), while the other options mischaracterize the concept or the nature of the trait.

BIO 181 (001&002) FALL 2025 FA25: Exam 3: Phylogeny & Biodiversity (Breathe in, breath out)

Human lactation / nursing is an example of a ___ .

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