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The Missing Links: Sibling Species Reveal Evolution in Action

 If you seek to expand your skills as a naturalist and learn more about identifying animals and plants, you will encounter one of the more interesting phenomena in biology.  This is the fact that there are considerable numbers of species that differ very little from their closest relatives and are quite difficult to tell apart.  They are clearly separate species based on  genetics, behavior, range, food, etc. yet can appear to the eye as almost identical.  This is a quite different circumstance from mimicry in which species that are not necessarily closely related may look similar.   Such closely related and similar appearing taxa are called sibling species, since they are relatively recently derived from a common ancestor.  I encounter such sibling species all the time during my nature rambles and they definitely pose challenges in identification.  Here are just a few examples to illustrate this point; all of these sibling species are in the same genus. 

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One of the largest and most distinctive summer butterflies that lives from N GA to Canada is the great spangled fritillary.  It is common on our VA farm but there is a sibling species, the aphrodite fritillary that lives nearby at higher elevations.  Both species feed mainly on violets as caterpillars and you may find it quite difficult to tell them apart.  When the wings are open there is a dark spot near the base of the forewing of the aphrodite that is absent in the great fritillary.  When the wings are closed there are also some subtle differences in coloration. 

 

The question mark and eastern comma are another example of sibling species.  They are found throughout most of the eastern US.  There are some differences in that question marks migrate and the commas are generally smaller, but they differ only slightly in coloration.  I always look for the extra black bar in the outer part of the forewing of the question mark, but it can be a difficult call in the field.  

 

Many vertebrates also have sibling species groups.  The very widespread bullfrog (extending down to central FL) has a southern relative the pig frog (southern coastal plain to southern FL),  which can be difficult to distinguish by sight.  But if you get a close look there is a subtle difference in the amount of webbing on the fourth toe.  In the pig frog the webbing extends almost to the tip whereas in the bullfrog it is much less extensive.  The territorial and breeding calls are quite different, illustrating how reproductive isolation is achieved in these closely related species.   

 

A famous example of sibling species in birds is a group of four small eastern flycatchers in the Empidonax genus, the willow, alder, least and Acadian.  These are very similar in appearance but have distinctive songs.  We have willow flycatchers breeding all around our farm house and encounter the other species in different habitats nearby.  But unless they sing, they are difficult at best to separate.

 

So for those who like a challenge, studying the sibling species groups will provide plenty of intellectual stimulation.  But the larger concept is that these species illustrate perfectly how evolution is constantly in action as conditions change and species evolve to adapt to such changes in climate, geography, food, predation, competition, etc.     

 

Bill Dunson, Englewood, FL, and Galax, VA 
wdunson@comcast.net 

http://www.galaxgazette.com/blogs 
http://lemonbayconservancy.org/dunson_archives.htm