A tremendous wealth of data is accumulating on the variety and distribution of transposable elements (TEs) in natural populations. There is little doubt that TEs provide new genetic variation on a scale, and with a degree of sophistication, previously unimagined. There are many examples of mutations and other types of genetic variation associated with the activity of mobile elements. Mutant phenotypes range from subtle changes in tissue specificity to dramatic alterations in the development and organization of tissues and organs. Such changes can occur because of insertions in coding regions, but the more sophisticated TE-mediated changes are more often the result of insertions into 5 flanking regions and introns. Here, TE-induced variation is viewed from three evolutionary perspectives that are not mutually exclusive. First, variation resulting from the intrinsic parasitic nature of TE activity is examined. Second, we describe possible coadaptations between elements and their hosts that appear to have evolved because of selection to reduce the deleterious effects of new insertions on host fitness. Finally, some possible cases are explored in which the capacity of TEs to generate variation has been exploited by their hosts. The number of well documented cases in which element sequences appear to confer useful traits on the host, although small, is growing rapidly.The book whose publication we are celebrating in this colloquium indicates that Theodosius Dobzhansky had a very special interest in gene mutation and its causes. Dobzhansky recognized mutation as the ''raw material'' on which natural selection acts and as the first of three steps necessary for evolution to take place. However, the discovery of transposable elements (TEs) in the 1940s by Barbara McClintock occurred a decade later, and it was a further 30 years before the significance of her findings started to be fully appreciated. Sixty years ago, Dobzhansky was well aware of the mutagenic properties of ionizing radiation discovered in 1927 by H. J. Muller but acknowledged that much less than 1% of spontaneous mutations were attributable to this cause. He distinguished between spontaneous and induced mutations: ''The former are those which arise in strains not consciously exposed to known or suspected mutation-producing agents.'' He also pointed out that ''since the name spontaneous constitutes only a thinlyveiled [sic] admission of the ignorance of the phenomenon to which it is applied, the quest for the causes of mutation has always occupied the attention of geneticists.'' Although at that time no clues to its nature were yet available, Dobzhansky realized that a major piece of the mutation puzzle was still missing. We believe he would have been intrigued with the discoveries of TEs in natural populations that have taken place during the last 20 years and that he would have been an active participant in the continuing debate about their role in evolution.

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Hybrid dysgenesis is manifested as a group of correlated aberrant genetic traits such as sterility, increased mutation rate, and male recombination. Previous work has shown that it appears when males of strains carrying either of two independent families of transposable elements called I and P factors are hybridized with females of susceptible strains called R and M, respectively. Here the results of an extensive survey for dysgenic potential in Drosophila melanogaster strains are reported. Striking temporal trends in the distribution of strains were observed with respect to the two transposable element systems; in particular, the frequency of R and M strains is positively correlated with laboratory age. In recent tests of strain samples, those collected from nature about 50 years ago were the earliest observed to possess I characteristics. The I type was increasingly frequent in samples from strains more recently originating in the wild. This type is apparently ubiquitous in present day natural populations. The P type was not found in strain samples collected before 1950, and collections made subsequently showed increasing frequencies of P-factor activity with decreasing laboratory age. Marked geographical patterns are documented in the contemporary worldwide distribution of variant strains within the P-M system. M strains are currently fairly common in natural populations from various parts of the world, except on the American continent where they are rare. The degree and distribution of quantitative variation within M and P strain categories is related to their time of origin in the wild. The implications of these results are discussed in relation to the hypothesis that hybrid dysgenesis determinants have evolved recently in natural populations and to an alternative hypothesis of laboratory evolution.Approximately 9% of the Drosophila melanogaster genome exists as dispersed moderately repetitive sequences (1). These sequences have been grouped into classes based primarily on their internal structure. Two clearly distinct structural classes of middle repetitive sequences are the copia-like elements and the foldback elements. A third class is that constituting the P elements (2, 3). This is a family of mobile dispersed genetic elements implicated in the P-M system, one of two systems associated with the phenomenon of hybrid dysgenesis (4). An additional class of transposable elements called I factors (5) are associated with a second system of hybrid dysgenesis, the inducer-reactive (I-R) system. Both genetic and molecular evidence indicates that the two hybrid dysgenesis systems are essentially independent of one another (3, 6).Although the emphasis in much of previous research on transposable element families has been on their molecular properties, we are now beginning to consider the population dynamics and possible evolutionary significance of such dispersed sequences. The phenomenon of hybrid dysgenesis, which appears in hybrids between certain mutually interacting strains, offers particular promis...