How evolutionary developmental biology is transforming our understanding of causation in biology
Each spring, a male songbird bursts into complex melody at the first light of dawn. What makes him sing?
A biologist might answer: "Rising spring hormones trigger his brain's song centers." Another might counter: "He sings to defend his territory and attract a mate." Both are correct, yet they represent fundamentally different kinds of biological explanations—the first dealing with immediate mechanisms, the second with evolutionary purposes.
For decades, biology maintained a strict division between these two types of causes, known as proximate and ultimate causation. This conceptual wall provided a useful framework for organizing biological inquiry until a revolutionary new science—evolutionary developmental biology (evo-devo)—began revealing that the wall itself might be an illusion. This article explores how evo-devo is transforming our understanding of one of biology's most fundamental distinctions and revolutionizing our approach to life's biggest questions.
Bird song exemplifies the proximate-ultimate distinction: hormones trigger singing (proximate) while mating advantages explain its evolution (ultimate).
In 1961, influential biologist Ernst Mayr formally cemented a distinction that had been brewing in biology for nearly a century—the separation of proximate and ultimate causation 2 . Mayr argued that biology actually comprised two largely separate fields: functional biology, concerned with how organisms work, and evolutionary biology, concerned with why they evolved the way they did 2 .
Question: "Why did it evolve?"
Timeframe: Historical (generations)
Focus: Evolutionary history, adaptation
Example: Singing improves mating success 7
These explanations delve into deep evolutionary time and natural selection's shaping power.
This distinction became biology's conceptual cornerstone, with researchers comfortably segregating into camps studying either mechanism or evolution, rarely crossing the divide. As Mayr himself argued, "The suggestion that it is the Darwinians to explain development... makes it evident that [critics] are unaware of the important difference between proximate and ultimate causations" 5 .
Just as the proximate-ultimate distinction seemed settled, an emerging field began revealing its limitations. Evolutionary developmental biology (evo-devo)—the study of how changes in embryonic development drive evolutionary change—started uncovering profound connections between developmental processes and evolutionary trajectories 3 . Evo-devo researchers made a crucial observation: development shapes evolution by generating the variation upon which natural selection acts.
"Evo-devo is that part of biology concerned with how changes in embryonic development during single generations relate to the evolutionary changes that occur between generations" 3 .
This field reopened a conversation that had largely closed after the nineteenth century, when evolutionary embryology flourished before being sidelined by genetics.
Development and evolution influence each other in complex feedback loops 5 .
Biological variation isn't equally likely in all directions 5 .
Similar genetic tools build diverse structures across species 6 .
The fundamental challenge evo-devo poses to the proximate-ultimate distinction is this: developmental processes themselves evolve 5 . The same genes and mechanisms that control development (traditionally proximate concerns) have histories and have been shaped by natural selection (traditionally ultimate concerns). As one analysis notes, "Since natural selection invariably operates on developmental processes, any account of an evolutionary transition is causally incomplete so long as it does not include the relevant developmental processes" 5 .
How do researchers test these complex ideas about development and evolution? Recently, scientists have turned to digital evolution systems that allow them to observe evolutionary processes in action under controlled conditions. A groundbreaking 2025 study published in Electronics journal created a 3D physical simulation to investigate how development and niche construction interact during evolution .
Researchers designed an elegant experiment: populations of digital creatures composed of interconnected blocks had to evolve the ability to cross two valleys to reach a target. These creatures existed in a sophisticated physics simulation that realistically modeled gravity, movement, and object interactions .
Body structure formed once at creation
Bodies could grow and change throughout their virtual lives
Ability to modify environment by placing blocks
Environmental modifications persisting across generations
The findings revealed fascinating interactions between developmental processes and environmental modifications. Researchers observed that lifetime development and niche construction played complementary roles: LD primarily helped creatures cross the first valley, while NC helped with the second .
| Condition | Valley 1 Success | Valley 2 Success | Key Insight |
|---|---|---|---|
| ID Only | Low | Low | Static bodies limited adaptability |
| LD Only | High | Moderate | Developmental plasticity aided navigation |
| NC Only | Moderate | High | Environmental modification overcame obstacles |
| LD + NC | High | High | Complementary strengths emerged |
| High EI | Variable | Variable | Often created maladaptive obstacles |
This experiment provides a compelling model for understanding how developmental processes (proximate) and evolutionary outcomes (ultimate) continuously influence each other. The digital creatures demonstrated that the capacity to change during life (development) and to modify environments (niche construction) creates complex feedback loops that blur traditional categorical boundaries .
To uncover these connections between development and evolution, researchers employ a sophisticated array of methods that span biological hierarchies from genes to whole organisms. These tools have enabled scientists to decipher how the same genetic "toolkit" can build wildly different structures across species.
Mapping the complex networks of genes that control development reveals how small genetic changes produce major morphological evolution 6 . By comparing GRNs across species, researchers can identify evolutionary hotspots—points in networks where changes have dramatic consequences.
Modern imaging and staining techniques allow detailed comparison of embryonic development across species, revealing both conserved processes and divergent pathways 3 8 . For instance, studies of galloanseran birds (2025) examine exceptions to von Baer's laws, which state that early embryos of related species resemble each other 8 9 .
This cutting-edge technology enables researchers to track the fate of individual cells during development and compare these patterns across species 8 9 . A 2025 study created a "morphological cell atlas" of the freshwater sponge, providing insights into the evolutionary origins of animal cell types 8 .
By precisely modifying genes in model organisms, scientists can test hypotheses about the function of specific genes in development and evolution 8 . Recent work has established gene knockout and knock-in protocols in corals, opening new possibilities for studying the evolution of symbiotic relationships 8 .
As demonstrated in the valley-crossing experiment, digital evolution systems provide testbeds for exploring evolutionary principles free from physical constraints . These models can incorporate developmental processes, environmental factors, and evolutionary dynamics simultaneously.
By reconstructing evolutionary relationships among species, researchers can determine the sequence in which developmental and morphological changes occurred through evolutionary history 4 .
These tools have collectively revealed that evolution often works by tinkering with developmental processes—reusing and modifying existing genetic programs rather than inventing entirely new ones from scratch. As one researcher notes, "genes are regulated differentially in different cells and places" during development, and these patterns of regulation have profound evolutionary consequences 3 .
The distinction between proximate and ultimate causation remains a useful framework for organizing biological questions, but evo-devo research has revealed its limitations as a strict boundary. As philosopher Massimo Pigliucci argues, we should maintain a "lean version" of the distinction while recognizing that developmental causes can carry explanatory force in both proximate and ultimate domains 5 .
Think in terms of which aspects of the complete causal story we foreground or background in specific research contexts 5 .
Complex interactions between developmental processes, environmental modifications, and evolutionary change .
The emerging picture suggests we should think not in terms of separate categories of causes, but in terms of explanatory abstraction—which aspects of the complete causal story we choose to foreground or background in specific research contexts 5 . Sometimes focusing on natural selection provides the clearest explanation; other times, understanding developmental constraints or niche construction is essential.
This more integrated view represents an exciting expansion of evolutionary theory, one that acknowledges the complex feedback loops between developmental processes, environmental modifications, and evolutionary change. As the digital evolution experiments show, when creatures can both develop during their lifetimes and modify their environments, they create complex evolutionary pathways that resist simple categorization .
The next frontier integrates ecology into the framework, recognizing that organisms simultaneously evolve, develop, and interact with their environments .
The next frontier lies in integrating ecology more thoroughly into this framework—developing what some researchers call "eco-evo-devo" . This approach recognizes that organisms simultaneously evolve, develop, and interact with their environments in ways that shape evolutionary trajectories. As we continue to unravel these connections, we move closer to a more complete understanding of life's magnificent complexity—one that honors both the mechanisms operating in the moment and the historical processes that shaped them.
The songbird's melody, we now understand, emerges not just from immediate hormones or ancient adaptations, but from the intricate dance between them—a dance that evo-devo is helping us learn to follow.