Constraint, Inheritance, and Historical Development

Evolutionary change occurs within organisms that already possess genes, proteins, developmental pathways, regulatory networks, and anatomical structures. New variations therefore arise within an inherited biological framework rather than independently of what came before.

This history can influence which evolutionary pathways are available. Earlier changes may make later innovations possible, while developmental, genetic, physical, or functional constraints may make other changes difficult or inaccessible.

Existing biological organization can also affect the consequences of subsequent change. In hierarchical gene regulatory networks, for example, alterations at different positions can have very different effects upon development. Present-day organization may therefore reflect a long history of modification, retention, specialization, and constraint.

Experimental evolution provides direct evidence that evolutionary history can influence later possibilities. In the long-term E. coli experiment studied by Blount and colleagues, the ability to use citrate under the experimental conditions arose in only one lineage after tens of thousands of generations. Replay experiments indicated that earlier changes in that lineage had increased the potential for the later innovation.

Comparative evolutionary studies also show that organisms do not possess an unlimited range of possible variations. Genetic and developmental systems can bias the kinds of variation that occur, while natural selection acts upon the variation that is actually available.

Research on developmental gene regulatory networks further shows that inherited network architecture matters. Changes occurring at different levels of a regulatory hierarchy can produce different evolutionary consequences, linking later biological organization to structures already present in ancestral systems.

Historical contingency and constraint do not provide a single mechanism explaining every biological feature. They describe ways in which prior history and existing organization can influence subsequent evolutionary possibilities.

Constraint also should not be invoked merely because the same solution appears repeatedly. Similar features can arise because natural selection repeatedly favors them, because available variation is constrained, or through combinations of these processes. Distinguishing among these possibilities requires additional evidence.

Historical explanations can also be difficult to reconstruct. Many intermediate populations and molecular states no longer exist, so evolutionary histories often must be inferred from comparative evidence, fossils, genetics, experiments, and models.

For many complex biological systems, the detailed sequence by which present organization developed remains only partly known. Researchers continue to investigate which components were inherited, which were duplicated or modified, which changes were selectively favored, and which possibilities were limited by existing developmental or molecular organization.

Another question concerns the relative importance of contingency and repeatability. Evolution sometimes produces similar solutions independently, while in other cases small historical differences can lead lineages along different paths.