Geoscience Research Institute


Origins 14(2):43-44 (1987).

Brief summaries of the main conclusions of the leading presentations are given below for those who may find the complete articles too long or technical.


    DNA normally exists in double-stranded form, each strand composed of molecular subunits called bases. The strength of attraction between the two strands depends on proper matching of their bases. The better the matching, the stronger the attraction, and the higher the temperature required to melt them (separate the strands). DNA distance is a measure of the extent of base matching of DNA strands from different species, and can be estimated from the effect of base mismatching on the melting temperature. One unit of DNA distance is equivalent to a melting point depression of 1șC.
    DNA distances have been used to estimate the degree of genetic similarity between species. "Family trees" have been constructed from DNA distances for most families of songbirds. Species classified as closely related usually cluster together in a distinct group with DNA distances of less than about 5 to 7. Species groups often are separated from other such groups by distances of 10 or more units. At distances more than about 8 to 10, branches tend to be so close together that the relationships among groups may be difficult to interpret. In addition, the branching pattern at greater DNA distances often conflicts with branching patterns derived from other methods of classification, leaving one to wonder which system is best.
    The clustering of species into distinct groups suggests they may be related by common ancestry. DNA distances seem to be useful in grouping species into higher taxonomic categories. The method sometimes suggests hypotheses of relationship between groups of species, or for unique species, that may not have been seriously considered previously. This is interesting, but, as with other methods of systematics, difficulties remain. One important question is the limit of resolution of the method. It seems likely that the reliability of the method decreases as DNA distance increases, and the method is probably best used for grouping species rather than determining relationships among such groups. Nevertheless, DNA distance, along with other methods of comparing species, will continue to be of interest to all who are interested in the relationships among living organisms.


    Fossil reefs are reported from many parts of the world, especially for the Paleozoic era. While binding of reef components by carbonate-secreting organisms is an undisputed fact for modern reefs, such binding for Paleozoic reefs is not generally observed. Other major differences include the size, taxonomy and abundance of reef-building organisms and the composition and coarseness of matrix material. Such differences make use of the term "reef" for the Paleozoic structures highly controversial.

© 1987

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