Speciation via Hormonal Changes to Growth & Development
Rapid speciation involves heterochronic change, which appears to be driven by rhythmic production of thyroid hormone
[This essay has been adapted from a pre-print paper, based on my Ph.D. dissertation and a follow-up book.]1
Speciation and Hormones
One category of information that is missing from the gene-centric Modern Synthesis involves the known effects of rhythmic patterns of hormone production, including well-known circadian rhythms.2
This rhythmic pattern of secretion is a feature of all hormones, which are released at least as often as every few minutes, in bursts that vary in length and intensity.
The production of hormones and their feedback mechanisms are now known to have a critical influence not just on how genes work but how the functions of multiple genes are coordinated in plants and animals.3
The rhythmic release of hormones — especially thyroid hormones — are known to exert profound effects on the initiation of genetically-controlled life history traits, such as stages of metamorphosis in amphibians, smoltification in salmon, the age of sexual maturity, and the time of year that animals mate or enter hibernation.4
This new awareness of the interplay between hormones and environmental cues has helped us understand, far better than before, how individual animals and populations of animals (i.e., species) adapt in response to environmental change.
Suddenly, the concept of rapid speciation now makes logical sense: organisms have to change almost as fast as environments do. If an organism cannot adapt, there must be a mechanism in place for rapidly generating a new species that better fits the new environmental conditions.
A process known as heterochrony seems to factor heavily into this rapid generation of new species. Heterochrony refers to the kinds of differences between species that result from changes in growth rates or in the timing of critical growth stages, or both, especially during embryonic or postnatal stages of development.
It’s become clear that all phenomena that impact growth and development via heterochrony, including both hormonal and genetic changes, must influence the speed, direction, and magnitude of evolution in response to environmental change.5
Heterochrony and Speciation
Heterochrony is a widespread phenomenon and has been identified in virtually all lineages of multicellular organisms, including vertebrate animals like mammals and fish, invertebrates like insects and clams, and even plants.6
Heterochrony often involves starting and stopping growth stages, changing the rate of foetal or postnatal growth of the ancestral species to various degrees, or all of these — at the same time or one after the other — making possible a wide variety of coordinated shape and size differences in descendant populations, i.e., new species.
In vertebrate animals, heterochrony has been implicated in a number of evolutionary novelties, such as bipedal morphology in human evolution, the reduced hind limbs of whales, the retained juvenile features of modern lungfish, the modified front limbs of bats, and the variation in jaw shapes of fishes, among many others.7
Heterochrony has also been implicated in the small differences that distinguish closely-related species, such as between domestic animals and their wild ancestors, between the chimpanzee and bonobo, and among closely-related species of deer, birds, and fish.8
The Modern Synthesis, however, cannot explain what initiates and controls heterochronic change and ensuing speciation. It has become increasingly apparent that the population genetic models that served well for explaining some cases of speciation and adaptation, including fruit flies and bacteria, are simply not adequate for addressing the phenomenon of heterochrony.
Heterochrony requires a different approach because the interactions between genotypes (i.e., the information encoded in genes) and phenotypes (i.e., the form of the individual organism from the moment an egg is fertilized on forward, including its behavior and hormone physiology), are not linear, one to one relationships but complex webs of interdependence.9
These interactions between genotypes and phenotypes, many of which involve hormone production and their feedback mechanisms, affect all aspects of an organism's shape, behaviour, and life history. While many biologists have acknowledged that this must be true and discussed the problems in depth, so far it has been a theoretical conundrum.10
No one has yet developed a comprehensive evolutionary model for speciation that melds the mathematical tenets of the Modern Synthesis (including the requirement of randomness) with the complex hormonally-driven processes involved in heterochrony. I’m not even sure, at this point, if such a thing is possible.
Twenty years ago I proposed a new paradigm for how new vertebrate species arise rapidly via a non-random process involving a non-genetic driver: thyroid hormone. I’ll lay out this paradigm in my next post.
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