Evolution by Natural Selection

Natural selection acts when heritable differences among organisms affect their reproductive success. Variants that function more successfully under particular conditions can become more common in later generations.

This process does not require that an entire biological system appear at once. Existing structures can be modified, duplicated, combined, specialized, or placed under new forms of regulation. Selection can preserve changes that improve the performance of the resulting system.

Applied to the patterns examined in this investigation, natural selection may contribute to the development and refinement of feedback control, robustness, modularity, regulatory networks, and other forms of biological organization. Evolutionary models have demonstrated that some complex network properties can arise as populations adapt to changing functional demands.

Natural selection and adaptation are supported by extensive theoretical, comparative, genetic, and experimental research. Studies of adaptation have identified genetic changes that affect organismal function and fitness.

Evolutionary modeling also provides examples in which network properties resembling those found in biological systems emerge during repeated cycles of variation and selection. Hintze and Adami, for example, obtained modularity, robustness, and fault tolerance in evolving artificial metabolic networks.

Convergent evolution provides another line of investigation. Similar environments sometimes produce similar adaptations in independent lineages, consistent with natural selection repeatedly favoring effective functional solutions.

Natural selection should not be treated as an automatic explanation for every feature of biological organization. Selection acts upon available heritable variation, while mutation, genetic drift, recombination, duplication, developmental processes, physical constraints, and historical inheritance also influence evolutionary outcomes.

Likewise, the existence of an apparently useful feature does not by itself demonstrate that natural selection produced it. Convergent patterns may support an adaptive explanation, but constraint and other processes can sometimes produce similar outcomes.

Models showing that particular organizational properties can evolve demonstrate possibility under the conditions of the model. Additional evidence is required to determine the actual historical origin of a particular biological system.

For individual biological systems, important questions remain about which intermediate stages occurred, what selective pressures operated, which features arose adaptively, and which reflect inherited structures or other evolutionary processes.

The origin of highly integrated systems can also require reconstructing changes involving many interacting components over long periods of biological history. Such histories are often only partially recoverable from living organisms, fossils, comparative genomics, and experimental studies.