Distinguishing Evolutionary Conservation from Derivedness

While the concept of evolutionary conservation has enabled biologists to explain many ancestral features and traits, it has also frequently been misused to evaluate the degree of changes from a common ancestor, or derivedness. We propose that the distinction of these two concepts allows us to properly understand phenotypic and organismal evolution. From a methodological aspect, conservation mainly considers genes or traits which species have in common, while derivedness additionally covers those that are not commonly shared, such as novel or lost traits and genes to evaluate changes from the time of divergence from a common ancestor. Due to these differences, while conservation-oriented methods are effective in identifying ancestral features, they may be prone to underestimating the overall changes accumulated during the evolution of certain lineages. Herein, we describe our recently developed method, transcriptomic derivedness index, for estimating the phenotypic derivedness of embryos with a molecular approach using the whole-embryonic transcriptome as a phenotype. Although echinoderms are often considered as highly derived species, our analyses with this method showed that their embryos, at least at the transcriptomic level, may not be much more derived than those of chordates. We anticipate that the future development of derivedness-oriented methods could provide quantitative indicators for finding highly/lowly evolvable traits.

Automated summary

The concept of evolutionary conservation is used to explain ancestral features and traits, but it has been misused to evaluate derivedness. Distinguishing these two concepts can help understand phenotypic and organismal evolution. Conservation focuses on common genes or traits, while derivedness covers novel or lost traits and genes to evaluate changes from a common ancestor. Conservation-oriented methods are effective in identifying ancestral features, but may underestimate overall changes. A new method, the transcriptomic derivedness index, uses whole-embryonic transcriptome as a phenotype to estimate phenotypic derivedness.