What makes a trait?
For the longest time throughout the 20th century, the
concept of a gene followed the conventional paradigm described above.
This stance suggested that genes exert their effects unidirectionally
(i.e., from DNA to mRNA to protein to phenotypic trait) and code for
particular traits in a one-to-one manner. Based on these premises, many
research programs were devoted to finding genes that coded ”for ”
a particular trait. These research agendas included complex traits such
as morphological, physiological, and behavioral characteristics.
However, the notion of a gene ”for ” a particular trait has turned
out to be far too narrow. This predicament was not missed by
philosophers of biology (Moss, 2004). Contemporary biologists
acknowledge that the relationship between genes and phenotypes is much
more complex and multifaceted than previously suggested. This way of
non-reductionist, context-dependent thinking represents a significant
shift from a gene-centric view to an organism-centric view that includes
the embedding of the organism in its specific niche (Odling-Smeeet al ., 2003). The epistemic shift occurred gradually over
several decades and was influenced by empirical discoveries, theoretical
advances, and, not least, conceptual contributions from philosophers of
biology (Jablonka & Lamb, 2005; Godfrey-Smith, 1996; Sterelny, 2003).
Key insights that fueled this epistemic shift came from the novel field
of evolutionary developmental biology (EvoDevo) that emphasized the
complex interactions between genes, regulatory elements, and the
environment, highlighting the importance of gene regulation,
epigenetics, and gene-environment interactions in shaping phenotypic
variation. In the meantime, in other fields of biology, the notion of a
gene ”for ” matured into the notion of a gene ”involved
in. ” Yet, the latter hardly provided any more explanations of how genes
contributed to the genesis of traits.
In its traditional definition, a trait is a specific characteristic or
feature of an organism that can be inherited, observed, and
measured. Research questions have largely focused on explaining the
inheritance of a phenotypic trait, but without explaining how that trait
develops. This confusion is rooted in the standard theory of evolution,
the Modern Synthesis, that grants genetic information the role of a
deterministic program to be executed or an instruction to be followed
(Laland et al ., 2014). However, the development of phenotypic
traits is governed not so much by nucleotide sequences as by
spatiotemporal patterns of gene expression—that are part of complex
interaction networks and are subject to intrinsic and extrinsic stimuli
(internal and external to the cell, respectively)—as well as by
epigenetic modifications and distinct DNA conformations.
A thought-provoking conceptual understanding about how genes contribute
to the phenotypic emergence of traits comes from EvoDevo rooted in
collaborative work by theoretical biologists and philosophers of biology
(Sultan, et al ., 2022). According to EvoDevo researchers, traits
are characterized as the outcome of developmental processes to which
genes and their products contribute important interactants (Moczek,
2020). Besides genes and gene products, other factors and processes play
equally important roles in the formation of traits, such as cell-cell
signaling, or reciprocal induction of tissues, or complex feedback loops
among components of organ system (Moczek 2015a). Naturally, genes and
their allelic variations contribute to these processes that take place
on various levels of biological organization, ensuring high
predictability and reliability of these interactions by
intergenerational, heritable transmission. However, when looking on the
organismal level, the role that genes play is significantly more
complex, versatile, and non-linear than when looking at their roles on
the molecular level. EvoDevo biologist Armin Moczek urges that research
requires “a reorientation away from an understanding of traits and
organisms as residing solely in genes and genomes, and toward an
appreciation of traits as products of developmental systems” (Moczek,
2020; p. 76).
Philosopher Evelyn Fox Keller convincingly argues along the same lines.
“[R]ecognizing that, however crucial the role of DNA in development
and evolution, by itself, DNA doesn’t do anything. It does not make a
trait; it does not even encode a program for development. Rather, it is
more accurate to think of DNA as a standing resource on which a cell can
draw for survival and reproduction, a resource it can deploy in many
different ways, a resource so rich as to enable the cell to respond to
its changing environment with immense subtlety and variety. As a
resource, DNA is indispensable; it can even be said to be a primary
resource. But a cell’s DNA is always and necessarily embedded in an
immensely complex and entangled system of interacting resources that
are, collectively, what give rise to the development of traits”
(Keller, 2010; p. 51).
Other important insights that challenge the gene-centric view and the
notion of a-gene-”for ”-a-particular-trait come from studies of
phenotypic plasticity. Phenotypic plasticity is the ability of organisms
to produce different phenotypes in response to environmental cues. This
phenomenon of plasticity is diametrically opposed to the idea of
“fixed” traits. Rather than being static, fixed characteristics,
traits unfold over time during the organism´s lifespan and in relation
to the organism´s niche. Hence, trait development is seen as the result
of interactions between genotypes and complex non-genetic environments
as well as of epistatic interactions between genes. The manifestation of
a given trait strongly depends on an organism´s capacity of plasticity.
To quote evolutionary biologist Armin Moczek, “[w]ithout explicit
consideration of plasticity, our understanding of any trait, any pattern
of variation within a population, and any reconstruction or prediction
of evolutionary trajectories would be incomplete” (Moczek, 2015b; p.
302). Plasticity is a hallmark of development. It enables species to
cope with environmental heterogeneity and allows organisms to adaptively
(and sometimes also non-adaptively) adjust their phenotype within a
range set by genetic and developmental constraints thereby coping with
variability in the conditions of the environment. In this sense,
plasticity is a bedrock capacity for a trait to come into being. This
dynamic view on traits expound that traits exhibit significant
variations at different life stages and under different environmental
conditions. Analogously, plasticity gains increasing significance for
organisms in environments that do not exert a stable, but a fluctuating
pressure on them. This is clearly the case for long-living species such
as humans that occupy a panoply of niches and can move between them.