Another question
The Tibetan antelope example shows how phenotypic plasticity and the Baldwin effect can guide evolutionary direction. But this raises a broader question: how does this two‑step process fit within the Modern Evolutionary Synthesis (MES), the framework that has shaped evolutionary biology for nearly a century?
To answer that, we need to understand what the MES emphasised, why plasticity‑first evolution didn’t fit easily into it, and why the scientific community only recently began to recognise the evolutionary importance of plasticity and the Baldwin effect.
We’re NOT dismissing the Modern Evolutionary Synthesis! ….
The migration of Eurasian blackcaps (Sylvia atricapilla) has recently changed in a striking example of adaptation by the classic MES mechanism.
Image credit: Harry Beaman
What the Modern Evolutionary Synthesis emphasised
To recap, the MES, developed between the 1930s and 1950s, unified Darwin’s natural selection with Mendelian genetics. Its core assumptions can be summarised simply:
- Genetic variation arises randomly through mutation and recombination.
- Natural selection filters this variation.
- Phenotypes follow genotypes — genetic change comes first, phenotypic change second.
- Development and plasticity play little or no role in generating adaptive variation.
This framework was enormously successful. It explained adaptation using population genetics, clarified how traits spread through populations, and provided a mathematical foundation for evolutionary theory. But it also had limitations. By focusing almost entirely on genes and selection, the MES left little conceptual space for development, environmentally induced phenotypes, or plasticity‑first evolution.
Why plasticity‑first evolution didn’t fit easily into the MES
Plasticity‑first evolution challenges the MES not by contradicting it, but by reordering the sequence of events.
In plasticity‑first evolution:
- The environment induces a phenotype — a functional response to new conditions.
- Selection acts on genetic variants that stabilise or refine that phenotype.
- The phenotype becomes hereditary through the Baldwin effect.
This reverses the MES assumption that genetic change must come first.
It also means:
- The environment is not just a filter — it is a cause of phenotypic variation.
- The phenotypes produced are not random — they are directly relevant to the environmental challenge.
- Development becomes a source of adaptive variation, not merely a mechanism for expressing genes.
For these reasons, plasticity‑first evolution sat outside the MES framework for most of the 20th century.
Publication show a late surge in recognition
The scientific literature itself shows how plasticity‑first evolution has gained acceptance only recently.
Publications concerning phenotypic plasticity and Baldwin effects
Black columns: publications concerning animal evolution generally.
Red columns: publications specifically concerning plasticity and Baldwin effects.
Hover for commentary and literature search details.
The graph shows a clear lag in the emergence of phenotypic plasticity as an evolutionary research subject:
- Before 1990, papers explicitly linking plasticity to evolutionary direction were rare.
- After 2000, publication rates increased sharply.
- Key turning points include:
- Mary Jane West‑Eberhard’s Developmental Plasticity and Evolution (2003)
- Empirical demonstrations of plasticity‑first evolution in altitude adaptation, thermal tolerance, behaviour, and morphology
- The integration of developmental biology with evolutionary theory (evo‑devo).
The pattern is clear: the evolutionary role of plasticity has only recently been widely acknowledged, despite being conceptually proposed more than a century ago.
Why plasticity and the Baldwin effect were historically overlooked
Several factors contributed to the long delay in recognising plasticity’s evolutionary role:
- Disciplinary separation: genetics and development were studied in different scientific communities.
- Plasticity was seen as noise: a way organisms buffered themselves against change, not a source of adaptation.
- The Baldwin effect was ignored: proposed in 1896, it was sidelined during the rise of population genetics.
- Lack of tools: mid‑century biology lacked the molecular and developmental methods needed to study plasticity mechanistically.
- MES dominance: once the MES became the standard model, ideas that didn’t fit its assumptions were often dismissed as peripheral.
Only with the rise of evo‑devo, physiological ecology, and genomics did plasticity and the Baldwin effect re‑enter mainstream evolutionary thinking.
Plasticity and the Baldwin Effect: How They Work Together in Evolution
When environments change, organisms have two main ways to cope.
The first is phenotypic plasticity — the ability of one genotype to produce different phenotypes in different conditions. Plasticity gives organisms a quick, useful response without waiting for new mutations. Importantly, these plastic responses are often directly relevant to the challenge the organism is facing, because they are triggered by the environment itself.
The second part of the story is the Baldwin effect. This is the name for evolutionary processes that can turn a helpful plastic response into a hereditary trait over generations. The Baldwin effect includes two closely related mechanisms:
- Genetic accommodation — natural selection adjusts how a plastic trait is expressed (for example, making it stronger, weaker, or more reliably triggered).
- Genetic assimilation — a plastic trait that originally appeared only in certain environments becomes expressed even without that environmental trigger.
For our purposes, the fine‑grained differences between accommodation and assimilation are not essential. What matters is that both allow a plasticity‑based phenotype to become genetically fixed.
Together, plasticity and the Baldwin effect form a two‑step evolutionary pathway:
- Plasticity produces a phenotype that already fits the new environment.
- The Baldwin effect makes that phenotype hereditary.
This combination is powerful because it allows evolution to respond directly and appropriately to the specific environmental challenge. Plasticity does not replace mutation and selection — instead, it shapes the selective landscape, influencing which genetic changes become advantageous and which evolutionary directions become possible.
Phenotype to genotype
In the next sections we look at other ways in which phenotype shifts can become hereditary.