Conclusion
Philosophers of biology have long challenged the classical
genes-”for ”-paradigm. The field of EvoDevo has traditionally
benefitted from intense exchange between philosophers of biology,
theoretical biologists, and empirical researchers. EvoDevo scientists
now widely agree that the emergence of complex traits derive from
developmental processes, often regulated by sensitive periods.
Accordingly, “[neurotypic] brain function results from a conserved
sequence of developmental processes of cell division, migration, network
formation and maturation, directed by intrinsic genetic programs as well
as by environmental and systemic cues, extrinsic to the nervous system.
Within this sequence, appropriate stimuli induce events of heightened
plasticity that are required to develop a given function” (Dehorter &
Del Pino, 2020). These phases of heightened plasticity are particularly
important in linking genetically guided developmental processes with
intrinsic and extrinsic cues. By shaping traits in response to
environmental inputs, sensitive periods can critically influence the
trajectory of trait development. Alterations of neuroplastic processes
can have advantageous effects as exemplified by heterochronic effects in
hominid phylogeny that led to an evolutionarily driven expansion of the
human neocortex. The dynamic aspects of traits are attributable to
genetic and epigenetic regulation as well as environmental factors
encountered in the species-specific niche. Complex interactions between
genetic predispositions and environmental factors, such as upbringing,
culture, or life experiences can have a tremendous impact on the
manifestation of certain traits. Moreover, some behavioral traits might
be more pronounced during stressful periods and manifest themselves in
an extreme manner (Sullivan et al ., 2006). Yet, irrespective of
extrinsic factors, traits can also exhibit significant variation due to
varying expression that are intrinsically driven and occur at different
developmental stages.
Developmental neuroscientists and developmental cognitive scientists
have accumulated a wealth of data that corroborate the aforementioned
framework. I believe that the time is right to get many more
neuroscientists onboard. Rethinking the classical, deterministic
paradigm of the gene–trait concept and working towards a philosophical
reconceptualization of traits as dynamic attributes tied to ongoing
developmental processes, will provide a three-fold advantage:
- First, it allows a re-interpretation of a gene´s pleiotropic effects.
Instead of focusing on the “intrinsic” functions of the respective
gene, the dynamics and conditions of the spatiotemporal expression of
the respective gene products (coding and non-coding sequences alike)
become essential. This is an important conceptual shift from viewing
genes driving processes to genes being recruited during
ontogeny. Moreover, it is the specific developmental pathway that is
of interest and less the individual component(s) of the pathway. In
fact, some of the components of the pathway may be exchangeable.
Because multiple genes contribute to a given process, gene products
can compensate for each other at different steps. As a consequence,
modular processes are better captured by systems biology rather than
by mechanistic approaches that investigate linear effects. For
example, positive feedback loops may be causally involved in the
emergence of extreme traits when (small) perturbations in a system
lead to amplification of the initial disturbance, resulting in
increasingly exaggerated or self-reinforcing responses.
- Second, it will encourage a rethinking of manipulability and the
effectiveness of interventions. Since most developmental processes
enable compensatory effects (buffering effects or canalization) due to
degeneracy, redundancy, or feedback mechanisms, many interventions
will prove rather ineffectual. The challenges of compensatory effects
are well known. By shifting the focus from fixed traits determined by
genes to traits that unfold and change due to developmental processes,
other means of interventions can be studied. These include changing
the duration of a given process or shifting the start point or the end
point of a given process.
- Third, by putting developmental processes in the center of the
investigation, environmental variability will gain heightened
importance. Rather than looking for the extrinsic trigger to express a
certain gene, the whole developmental process needs to be taken into
account. Environmental fluctuations can create selective pressures on
developmental trajectories that vary over time. Environmental
conditions can produce ontogenetic shifts that may be advantageous or
neutral at some time point (e.g., the beginning of a developmental
process). At a later timepoint, however, (e.g., at the cessation of
the developmental process), environmental conditions may have changed
and the outcome may be deleterious for the organism. There are many
examples for stress-induced, altered developmental trajectories (e.g.,
often triggered by stressful environmental cues that are encountered
prenatally or shortly after birth) that become maladaptive at a later
developmental stage (Packard et al ., 2021; McEwen, 2013). This
alternative view also concerns therapies for neurodevelopmental
disorders that might focus more closely on how genetic and
environmental variations influence the rates and timing of
developmental trajectories and milestones in children and adolescents,
and causes of variability among individuals.