Abstract
Vestigial features of humans and other organisms are well known and have long been used as key evidence for evolution. Such features include not only anatomical structures but also physiological processes, biochemical reactions, and even behaviors. In addition to remnants of evolutionary change, traces of developmental change likewise exist, and it is important to distinguish embryonic remains from true evolutionary vestiges because people confuse evolutionary and developmental changes. To avoid confusion, the term vestige should be reserved for true evolutionary holdovers. This paper presents examples of developmental remnants, which often involve circulatory and reproductive alterations, and discusses numerous other retained or reappearing historical holdovers in ways that reveal the unfolding dynamic interaction between genotype and phenotype. These holdovers are useful in differentiating and relating concepts of phylogeny and ontogeny as well as revealing benefits of historical reasoning in understanding patterns and processes of organismal change.
Highlights
Life is a dynamic process, and organismal structures disclose the history of the two principal levels of biological change: evolutionary and developmental
The term vestige should most properly be reserved for true traces or holdovers of evolution, not for remnants of embryonic processes
Whereas the compound atavistic and homologous clues can be seen as revealing historical changes within a single evolutionary lineage, others are best viewed through the prism of divergent lineages
Summary
Life is a dynamic process, and organismal structures disclose the history of the two principal levels of biological change: evolutionary and developmental. In many cases there are features of organisms that may (unlike vestiges) retain an original function but reveal glimpses of history via comparative cladistic analysis of a species and its lineage (Table 1) These can be divided into traces from the past that indicate prior states of evolution or development. Whereas the compound atavistic and homologous clues can be seen as revealing historical changes within a single evolutionary lineage, others are best viewed through the prism of divergent lineages The latter kind of relationship—not merely phyletic but phylogenetic (literally, referring to the origin of a new phylum or major taxon)—reveals a history that can be concurrently approached from two directions, as each clade relates to the other and as both depart from their common ancestor, best studied as corresponding divergences of developmental programs. Biologists must be careful not to speak of the development (rather than evolution) of horse or fern lineages
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