^A Neurons communicate information by sending spikes or action potentials along their output axons. A nerve fiber (such as the ones that connect your spinal cord to the brain) is just a bunch of axons all lining up in the same direction. The speed of travel of these action potentials along a nerve fiber depends on several properties, including whether or not the axons are surrounded by myelin, a form of glial “helper” cells.³

^B A wonderful example of the speed of evolutionary change moving phenotypes within populations is the black and white moths that changed their distributions in response to smog in industrial England, where a population of peppered moths changed from being mostly white to mostly dark as coal pollution darkened the skies and trees of England in the late 1800s, and then changed from being mostly dark back to mostly white as the pollution was controlled.¹⁰ More detailed examples of how quickly evolution can change populations can be seen in the studies by Rosemary and Peter Grant measuring the shape and length of beaks of finches in the Galapagos Islands. The Grants’ finch studies are described in detail in Jonathan Wiener’s The Beak of the Finch.