Ontogeny recapitulates phylogeny doesn’t exactly flow off the tongue unless you are familiar with scientific terminology. However, what appears to be a somewhat intimidating phrase is actually marvelously succinct and elegant.
Ontogeny is “the development or course of development especially of an individual organism.” This could refer to the development of a plant from embryo to seed to seedling to mature, reproductive plant. Or it could refer to an animal growing as an embryo to an infant to an adult.
Phylogeny is “the evolutionary history of a genetically related group of organisms, as distinguished from the development of the individual organism.” Sometimes these relationships are illustrated as trees of information, with groups of closely related organisms called clades Studying and depicting shared evolutionary history is known as cladistics. Have you seen Darwin’s tree of life? If so, then you understand the basic idea of phylogeny. It’s all about the study of relationships.
More than just a catchy phrase, “ontogeny recapitulates phylogeny” is the foundation of Recapitulation theory. Recapitulation theory posits that the development of individual organisms (ontogeny) follows the same phases (recapitulates) of the evolution of larger ancestral groups of related organisms (phylogeny). Following this theory loosely, a seedling of a recently-speciated flowering plant (something that recently evolved to be a species) would, throughout its embryological development, mimic the morphology of more ancestral plants—or plants that evolved in earlier times. In theory, then, a young flowering plant might go through developmental stages that look like a moss (or early land plant), then a fern (or other vascular plants), then a gymnosperm (or other seed plants), and so on. Of course, this is not really true in terms of gross morphology, but the implication is that there may be embryological similarities.
Although there are often developmental similarities which do reflect shared evolutionary history, development (ontogeny) does not necessarily reflect (recapitulate) shared evolutionary history (phylogeny). Even ancient lineages can continue to change over time or may have been radically different from ancestors even early on. Also, flowering plants did not evolve from modern gymnosperms, seed plants did not evolve from modern ferns, and vascular plants did not evolve from modern bryophytes. These groups all share common ancestors, which do share features in common, just not necessarily obvious, modern-day appearances.
Differentiating between homologous similarity based on shared evolutionary history (synapomorphy), similarity based on features retained from even earlier ancestors (plesiomorphy), and artificial similarity based on convergence or loss (homoplasy), is one of the hallmarks of the modern phylogenetic cladistics approach, as shared derived features (synapomorphies) are the most informative with respect to relationships. For instance, humans share many features in common with all life (DNA, cellular respiration, etc.) and over a thousand genes in common with bacteria, but even more and more in common with progressively more ‘advanced’ animals (spinal cord, bones, four limbs, hair, etc.). Ontogeny can reflect phylogeny, but embryological development is itself subject to evolutionary change over time.
Recapitulation theory is just that—a theory. The idea that ontogeny recapitulates phylogeny has been used not only by some in the biological sciences, but also by those in the social sciences. It’s easy to understand why the phrase has such pull. It’s succinct and melodic in a way we want the natural world to be. The theory gives a certain amount of order to chaos, and perhaps that’s the catch—it’s a bit too orderly, too clean. Biology is nothing if not messy.
Ontogeny might recapitulate phylogeny, at least at times, but whether it does or it doesn’t, the recapitulation theory is a fascinating, controversial topic, and one that is sure to make you a hit with stimulating conversation at holiday parties.