Creating a 21 st Century Darwinian Paradigm

Here I summarize and expand an innovative paradigm that updates Darwin and the Modern Synthesis with a mechanism that explains how rapid evolutionary change in multi-cellular organisms can be initiated and implemented. This new theory hinges on the premise that thyroid hormone (TH, T3 and/or T4) plays a pivotal role in speciation and adaptation. In most animals TH can be absorbed from ingested food as if it were a vitamin but in vertebrates, it is also manufactured (like a classic hormone) in a distinctly rhythmic fashion. I argue that the rhythmic secretion of manufactured TH (or its analogue in non-vertebrates, including plants and insects) is an under-appreciated feature of TH metabolism that has fundamental biological and evolutionary significance. I contend that this hormonal mechanism can not only explain domestication as a natural speciation process (e.g. the transformation of wolves into dogs) but change over time in virtually all multi-cellular organisms, including our human ancestors. In part, this mechanism works through TH regulation of many developmental genes in a doseand time-dependent manner. I suggest that during colonization of new habitats, small founder populations possess a non-random subset of ancestral TH rhythms phenotypes and that this loss of hormonal variation initiates rapid heterochronic changes via shifts in developmental gene function. Increases in quantity of ingested TH, due to habit-associated changes in food sources, can compound these effects to generate the macro-evolutionary modifications seen in higher-level taxa. This integrative evolutionary theory takes into account all we have learned in recent decades about the unique attributes of TH (particularly its critical influence on development genes and other hormones) and the fundamental nature of all biological rhythms, all of which have profound implications for evolutionary biology. Most importantly, this unifying paradigm offers a longoverdue addendum to the exclusively genocentric view mandated by the Modern Synthesis: it not only addresses the so-called “species problem” but makes evolution uniquely personal. The concept thus provides a testable theoretic framework for a 21 century update in evolutionary biology and medical science.