January 30, 2012

Adaptations, large and small, are occurring at a regular rate in all species and demonstrate how evolution by natural selection works.    The changing shape and strength of the beaks of finches in the Galapagos; the darkening color of moth wings in England as the Industrial Revolution took hold; and the recent loss of sound produced by crickets rubbing their wings together in Hawaii are compelling examples of adaptations.

But what are adaptations and how do they “prove” natural selection?   According to  a most useful website “Understanding Evolution” sponsored by the University of California at Berkeley:

“An adaptation is a feature that is common in a population because it provides some improved function. Adaptations are well fitted to their function and are produced by natural selection. Adaptations can take many forms: a behavior that allows better evasion of predators, a protein that functions better at body temperature, or an anatomical feature that allows the organism to access a valuable new resource—all of these might be adaptations. “

Rosemary and Peter Grant are biologists working in the Galapagos Islands among the descendants of the finch populations which first attracted Charles Darwin’s attention in the 1830s.  Seeds are a primary food for the finches and during years of abundant rainfall seeds are plentiful; the birds use their beaks to crack the seeds open, to obtain the nutritious kernel within.  The Grants observed that over a prolonged period of drought however seeds were less abundant and their casings became harder to crack.  They noticed successive generations of the common ground finch developed shorter, stronger beaks. They had to work harder but they were able to attain food.  When the rains returned, beaks resumed their “normal” dimensions.  These and other findings by the grants are detailed in Jonathan Wiener’s book, The Beak of the Finch.

At about the time Darwin was circumnavigating the globe in HMS Beagle, the Industrial Revolution had been underway in England for many decades.  One of the many species of moth in England at the time was the peppered moth, with light coloring  to its wings and body,  As an increasing number of factories and mills were belching smoke and soot, the result of the coal they were burning, it was noticed succeeding generations of peppered moth were darkening.  Light colored moths alighting on surfaces that were light in color were less visible to predator birds but black, industrial soot was soon discoloring walls and trees, discoloring every surface on which it settled.  The moths adapted by taking on a brown coloration and this is been called industrial melanism.

The male of a species of cricket on the island of Kauai in the Hawaiian Islands  rubs its wings together to attract female crickets.  This action produces a sound familiar to anyone living in the temperate and tropical zones around the world and is a signal heard by predators as well.  On Kauai one predator is a fly which, drawn to the crickets by their sound, injects its larvae into them.  The larvae are parasitical and eventually kill the host.  The crickets have developed an adaptive defense and many of the males: continue to rub their wings together but the rubbing is now soundless.  According to Marilee Zuk, the principle researcher in this field, this adaptation has occurred over the course of 30 years, a little more than one generation for humans but many generations of crickets.  This is an astoundingly rapid adaptation.  A talk with Dr Zuk can be found on the “Ask a Biologist” website.  Click here and scroll down to the “Strange Cricket Silence” podcast.