Toward mechanisms underlying the developmental hourglass: evolvability and phylotypic conservation

Understanding the general relationship between animal ontogeny and phylogeny remains a long-standing issue in evolutionary developmental biology. Historically, observation of these two phenomena, which involve increasing complexity, led to a series of hypotheses culminating in Haeckel’s recapitulation theory in the 1860s. These classical theories posited the conservation of earlier developmental stages in what is sometimes referred to as the ‘funnel model’ of development. However, in the 21st century, a body of evidence accumulated that robustly supported the “developmental hourglass model”, first proposed by Duboule, in which the mid-embryonic phylotypic period is highly conserved. Despite this empirical progress, the mechanisms underlying such conservation have remained elusive. Recent empirical studies have begun to indicate that this conservation may stem from the reduced potential of embryos at this stage to create phenotypic variations. Nevertheless, the precise mechanisms underlying this lower evolutionary potential remain unclear. In this review, we outline historical and recent perspectives and findings on the relationship between ontogeny and phylogeny, contrasting historical assumptions with emerging evidence concerning the mechanisms of embryonic evolution. We particularly highlight the possible role of lower evolvability, referring to intrinsic properties of developmental systems that pontentially constrain phenotypic diversification.