Developmental Change and Body Plans
Animal form is produced by developmental systems that control when and where genes act. Changes in these systems can alter anatomy, but major body-plan innovations involve coordinated developmental changes.
The Investigative Question
How can changes in development produce new anatomical forms, and what must be explained when major body plans arise?
What We Observe
Many genes involved in development are shared across very different animals. Differences in anatomy can therefore arise partly through changes in when, where, and how developmental genes and regulatory networks operate.
Scientific Background
Developmental biology has identified conserved signaling pathways, transcription factors, regulatory elements, and gene networks involved in constructing animal bodies.
Why It Matters
The reuse of conserved developmental components shows that morphological innovation does not necessarily require a completely new set of genes. It also focuses attention on how developmental networks themselves are modified and coordinated.
What Is Known
Changes in developmental gene regulation can alter morphology. Conserved developmental genes and pathways are used in different ways across animal groups. Comparative studies have associated changes in Hox gene expression, downstream regulatory targets, and other developmental processes with differences in segmentation, appendages, regional specialization, and other anatomical features.
What Is Proposed
Evolutionary developmental biology proposes that changes in regulatory networks, gene deployment, duplication, and interactions among developmental pathways contributed to the diversification of animal form. Some models distinguish changes to relatively peripheral portions of developmental networks from changes to more deeply embedded regulatory circuitry that helps establish major features of body organization.
What Remains Uncertain
The detailed historical pathways leading to the earliest major body plans are reconstructed from developmental, genomic, phylogenetic, and fossil evidence and remain incomplete in many cases. Developmental regulatory networks are hierarchical, so demonstrating that one kind of regulatory change can modify an existing feature does not by itself establish the sequence of changes that produced a major body-plan innovation.
Design Relevance
Major anatomical innovation is relevant to Intelligent Design because development requires coordinated spatial and temporal control. The investigation must ask what mechanisms can modify such systems and what evidence connects those mechanisms to particular historical transitions.
Assessment
Developmental systems are capable of significant evolutionary modification, and comparative evidence connects changes in developmental regulation with important anatomical differences. At the same time, developmental networks contain components with different roles, interactions, and degrees of evolutionary conservation. Explaining major body-plan origins therefore requires connecting known mechanisms of developmental change with evidence for the particular coordinated changes involved in each historical transition.