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FROM LECTURE 17:   LINEAGE SPLITTING (SPECIATION)

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Hawaiian Picture-Wing Drosophila


    The architypal example of diversification by allopatric speciation on island archipellagos are the Hawaiian picture-wing Drosophila.    In the Hawaiian islands, there are approximately 100 species of picture wings, among which  there is a tremendous diversity of form and sizes.  Some species are quite small and non-descript, like the typical Drosophila melanogaster with which most of you are proably at least passingly familiar.  On the other hand, many species are much larger.  The picture wings also have a variety of different coloration patterns on their wings, which are used as species-specific displays during courtship.  Many  species have greatly modified mouthparts, while others have greatly modified legs and feet.
They occupy a great variety of ecological habitat: some lay eggs in rotting tree branches, others in fermenting bark, still others in slime fluxes.  Some inhabit upland rain forests, others in lowland dry forests.

   Species of Hawaiian "picture-wing" Drosophila.   Many species are quite large (No. 4 is about the same size as a D. melanogaster).   They get their common name from the conspicuous markings on the wings, which are used as displays during courtship.  (From J. S. Yoon.  1989.  Chromosomal evolution and speciation in Hawaiian Drosophila.  In Genetics, Speciation and the Founder Principle,Oxford Univ. Press, New York.)
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Male-male competition in Hawaiian Drosophila.  The left picture portrays two males of D. heteroneura butting heads to establish ownership of a display territory.  The right picture shows two males of D. sylvestris grappling in a contest over possession of a display territory.   (From S. Freeman and J. C. Herron. 1998.  Evolutionary Analysis.  Prentice Hall, Upper Saddle River, NJ.  See p. 331 for clearer pictures.)

    Clearly, there has been a tremendous amount of lineage splitting that has gone on in this group.  Work by Hampton Carson and his co-workers has provided a pretty good picture of how this speciation has taken place.  Carson's group has done two important things: (1) they have characterized the habits and distribution of each of the species of picture-wing Drosophila, and (2) they have characterized the banding patterns on the polytene chromosomes for each of the groups.  Using banding pattern characteristics of a species as a taxonomic character, it is possible, using methods that will be discussed later in the course, to determine the phylogeny of the picture-wing species group.  The phylogeny is simply a diagram that depicts the pattern of lineage splitting that produced the living species:

Phylogeny of Hawaiian Drosophila, based on chromosomal banding patterns.  The left picture potrtays the phylogeny of the picture-wing species group.  The right picture is a blow-up of the upper-right corner of the left picture.  Note that in general individual species are restricted to a single island and closely related species tend to occur on different island.
 


 

   In this phylogeny, each species is listed as a name (e.g. murphyi in upper left corner), and below the name is listed the island or islands on which the species is found.  This phylogeny shows some remarkable patterns:

        1.  First, over 95% of the species are single-island endemics, which is to say that they occur only on one island.

        2.  If one looks at closely related species, they tend to occur on different islands.  For example, if one looks at the
             species group composed of limitata, sejuncta, ochracea, and claytonae (lower part of right figure above), one
             finds they are on the islands of Maui, Kauai, Hawaii, and Hawaii respectively.  If one looks, say, at relicta,
             silvarentis and turbata (upper part of right figure), they occur on Maui, Hawaii, and Oahu respectively.

    These two patterns are exactly what is expected if occasional colonization of an island leads to speciation as described previously.  The Hawaiian picture-wing Drosophilids thus provide a textbook example of the kind of distributional evidence used to infer the importance of allopatric lineage splitting in nature.
 

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