hundred or more light and dark bands of varying sizes arrayed along their lengths.
On rare occasions a chromosome within an individual fly will undergo a particular kind of rearrangement. The chromosome breaks at two random points along its length. Usually these breaks are correctly repaired, so that the chromosome sequence is unchanged. But in some cases, the DNA breaks are repaired in a way that causes a section of DNA to be reversed in orientation. In that case, a portion of the chromosome, with its characteristic light and dark bands, appears to have been rotated 180 degrees in the chromosome. These rare chromosomal inversions can be of varying lengths, can occur on any chromosome, can occur within other inversions, and generally have no effect on the behavior or morphology of the fly. Because these inversions occur at random along the chromosomes and are of different sizes, each one is essentially unique.
If this kind of inversion mutation occurs in the sperm or egg cells of a particular fly, that fly can pass the rearranged chromosome to the next generation. Sometimes the inversion will become more and more common with each new generation, until an entire species of flies has the inversion. Therefore, if two species share an identical inversion, they must be descended from a common ancestral species that also had that inversion. If one species has an inversion that is not present in another species then that inversion must have occurred after the two species diverged from a common ancestor.
These inversions can be used to trace the evolution of many of the Hawaiian drosophilid species from common ancestral species. In Hawaii, populations of flies have speciated as they have adapted to new kinds of habitats or have acquired different mating behaviors. Speciation also has often followed founder events, when a single fertilized fly or several flies either traveled or were transported from one island to another island or between habitable but geographically isolated portions of the same island. Successful colonization is more likely when founders from older islands in Hawaii move to younger islands, since younger islands generally contain fewer competing species of drosophilids.
The Big Island of Hawaii is home to 26 distinct species of picture-winged Drosophila flies that have been found to live only on this island. The Big Island is the youngest of the Hawaiian islands, so these species likely formed since the volcanoes of the island emerged above the ocean and became vegetated less than half a million years ago. A major objective of this exercise is to explore the relationships between these “new” species and the species living on the older Hawaiian islands.
WHAT STUDENTS NEED TO KNOW
To perform this exercise, students will need a basic understanding of chromosome structure, banding patterns, and chromosomal inversions. They also will need to be familiar with the concept of biological species, the basic mechanisms of speciation, the relationship of geographic isolation to speciation, and the construction of phylogenetic trees. They can acquire some of this information from the Student Reading and Student Worksheet in this exercise. More information can be found in Teaching About Evolution and the Nature of Science (National Academy of Sciences, 1998).