October 30, 2009 | Leave a Comment
There are the interpretations of evolution that emphasize mutation. Evolutionary developmental biologists are exploring ways life may be evolving that are unrelated to mutation, pathways influenced by the environment. My work concentrates on how the rates and timing of maturation are influenced by hormones, with sexual selection or environmental changes transforming individuals and then species over time.
In the river analogy that preceded this piece, there is no explanation for how a species might leave the groove provided by the trajectories set up by maturational delay and acceleration. This river chatters, making music as it flows downstream. What might be the music of evolution?
A species could evolve over time, prolonging infant features into adult descendants, and then it could reverse that trend by withdrawing adult features into descendant infants. Then repeat. It would seem, like a teeter totter, that over great periods of time there would be no real movement, just variation between two polarities.
Indeed, in some cases this may be what occurs. Nevertheless, unique variations keep emerging, species that have never been observed.
Some reasons for this come to mind. What might these reasons sound like?
Species don’t just prolong infant features to adult descendants. Aspects of embryonic stages can be carried into postbirth life. This provides a constant source of new material as the highly responsive features of fetal ontogeny, appearing later in development, offer unique ways of behaving and being in the world. We might look at fetal stages of development as a source of constantly new phenotypes and behaviors when manifested as the young and then adults of species.
When a mutation does appear, the feature or the behavior emerges in a specific ontogenetic position. From there, neoteny and acceleration can introduce that feature or behavior to earlier or later ontogenetic stages. In other words, the rate and timing adjustments staged by changes in hormones, hormones influenced by sexual selection and the environment, can carry exhibitions of mutations to new ontogenetic locations, thus transforming species.
Haeckel’s almost sole focus was acceleration, what he called recapitulation, the withdrawing of adult features backward over time. When he declared that ontogeny recapitulates phylogeny, Haeckel was stating that evolution was driven by the embryonization of species, revealed by what he described as ancient adult progenitors appearing in descendants’ earliest ontogeny. I would consider this half the story. At the same time that adult features move backward, infant features move forward. This may occur while taking turns or at the same time. Perhaps some species’ lineages take turns; some mature and delay at the same time and some do both at different times. How exactly this process unfolds, and the juxtaposition of this process with mutations, may have a lot to do with how unique new traits and behaviors emerge.
And then there are those species that exhibit two different sexes on two different maturational trajectories. One sex may be accelerating while the other delays. Then, they might switch. An emerging mutation then could be sliding off in two different directions over the same period of time within the same species.
Consider that in early ontogeny, the period that the evolutionary developmental biologists emphasize, information is integrated from the environment that compels an individual to develop in a direction that his or her forebears have not engaged in. If an environment is radically different from that which a species has been exposed to, that information may compel the emergence of features or behaviors that are unique. These are characteristics outside the maturation delay/acceleration frames, features nonetheless adjustable by a genetic/environment interface with built-in flexibility.
One more thing. Where does the feature content come from when a lineage accelerates without ceasing over time? If adult features keep withdrawing, accelerating, recapitulating, then consider this effect on the adult stage of a lifetime. Whereas in neoteny embryo features keep prolonging forward in species time, in acceleration the hormonal constellation that features high testosterone can keep growing more concentrated or intense, evidencing unique features perhaps only curtailed by the poisonous effects of steroidal hormones. In the meantime, brand new features may be emerging in association with extreme endocrinological conditions. The end of ontogeny may birth new features.
Both the beginnings and endings of ontogeny may be the source of new features and behaviors. Add to that mutations. Add to that new features formed from environmental impacts in the womb, features unrelated to maturation. No doubt other sources of unique features have been discovered but are still not understood.
This stuff is complex, but not so complex that it cannot be made into a song. This feels to me like music. Perhaps we can describe an evolution of species by correlating impacts and trajectories with harmonies, melodies, rhythms and beats. I am speechless. Perhaps this is when we should choose to sing.