IN A FEW WORDS

Origins 11(2):63 (1984).

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.

CHROMOSOMAL CHANGES IN MAMMALIAN SPECIATION: A LITERATURE REVIEW

    Species are defined as groups of individuals which do not interbreed with other groups under natural conditions. In order for a new species to appear, it is necessary that some change occur which prevents natural interbreeding between two groups which formerly could interbreed. Several mechanisms by which this could be accomplished have been proposed. One proposal which has been widely discussed is based on structural changes in chromosomes. Various kinds of structural changes in chromosomes and how they affect fertility are discussed in this article.
    Fusion of two chromosomes and reversal (inversion) of a portion of a chromosome are the most commonly observed structural changes in mammalian chromosomes. Some populations which differ by such chromosomal rearrangements can interbreed, while other populations with similar chromosomal differences cannot. This suggests that the reasons for sterility are somewhat complex and may often be caused by factors other than differences in chromosomal structure. The fact that most species show chromosomal differences may be due to changes which have occurred after the species became reproductively isolated.

RAPID EROSION AND MOUNT ST. HELENS

    How long does it take for a canyon to be eroded? Is erosion accomplished primarily by the cumulative effects of slowly operating agents acting more or less continuously? Or, is erosion accomplished primarily by the singular effects of catastrophic agents acting intermittently?
    Research at Mount St. Helens in Washington State shows that gorges and canyons can form rapidly. This article contains several photographs which depict the rapid changes which have been caused by erosion (gravitational slumping, jetting stream, flowing mud, sheetflooding of water, and channelized flow of water) forming a branching pattern of canyons, some over 100 feet deep.
    The data reveal that tremendous changes may be accomplished very rapidly by an erosional agent having power which exceeds a certain threshold. The erosional features at Mount St. Helens are compared to erosional forms at Lituya Bay (Alaska), Surtsey Island (Iceland), Lake Peigneur (Louisiana), Sao Nicolau (Brazil), Waiho River (New Zealand), Providence Canyon State Park (Georgia), and the Imperial Valley (California).
    The processes which conventional geomorphic theory has suggested takes thousands of years may, instead, be accomplished within a few years.