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.