Regulatory Linkage. A single, simple, input may trigger multiple complex outputs. On the left, the hypothalamus integrates cold sensations from the sensory system. It sends out hormonal and nervous system signals to different body parts to trigger thermoregulatory processes. The skeletal muscles will contract quickly to produce heat, causing shivering. Arrector pili muscles in the skin will contract to cause hair follicles to stand up. On the right, Distalless (Dll, red) transcription is regulated by thoracic (blue) and head (yellow) transcription factors. It's expression directs appendage development in both tissues - antennae and mouthparts in the head, and legs in the thorax.
Brain illustration by Alain Gallien http://svt.ac-dijon.fr/schemassvt/, skeletal muscle illustration by Open Staxx College http://cnx.org/content/col11496/1.6, both images from Wikimedia Commons and published under a CC BY license.
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evolvability, genetic regulation, gene regulatory network, development, evolution, evodevo