New species may be formed through hybridization and without an increase in ploidy. The challenge is for hybrid derivatives to escape the homogenizing effects of gene flow from parental species. The mechanisms hypothesized to underlie this process were modelled using a computer simulation. The model is of recombinational speciation, in which chromosomal rearrangements between parental species result in poor fertility of F1 hybrids, but through recombination, novel homozygous types are formed that have restored fertility. In simulations, stable populations bearing the recombinant karyotypes originated frequently and were maintained when the fertility of F1 hybrids was high. However, this high rate of origination was offset by low genetic isolation, and lower F1 hybrid fertility increased the evolutionary independence of derived populations. In addition, simulations showed that ecological and spatial isolation were required to achieve substantial reproductive isolation of incipient species. In the model, the opportunity for ecological isolation arose as a result of adaptation to extreme habitats not occupied by parental species, and any form of spatial isolation (e.g. founder events) contributed to genetic isolation. Our results confirmed the importance of the combination of factors that had been emphasized in verbal models and illustrate the trade-off between the frequency at which hybrid species arise and the genetic integrity of incipient species.

Using a computer simulation, we have examined the dynamics of recombinational speciation, a potentially rapid mode of evolution dependent on chromosomal reassortment in populations of partially sterile interspecific hybrids. We describe how various parameters affect the time required for a new recombinant species to become established within the setting of a spatially structured hybrid zone. Our results indicate that recombinational speciation is most likely to occur where (1) the hybrid zone interface is long, (2) the organisms involved are predominantly selfing, (3) the hybrids are relatively fertile, and (4) the number of differences in chromosomal structure between the parental species is small. The speciation dynamics are characterized by long-term stasis followed by an abrupt transition to a new reproductively isolated type. The results are largely the same whether the nascent recombinant species is favoured by a fertility or a viability advantage. Recombinational speciation, like polyploidy, appears to be a feasible mechanism for sympatric speciation in plants.

Species-specific RFLP markers from mitochondrial DNA (mtDNA) were identified and employed in conjunction with previously reported data for nuclear allozyme markers to examine the genetic consequences of an artificial introduction of spotted bass {Mlcroptenis punctulatus) into a north Georgia reservoir originally occupied by native smallmouth bass (Af. dolomleut). The cytonuclear genetic data indicate that within 10-15 years following the unauthorized Introduction, a reversal in these species' abundances has occurred and that more than 99% of the population sample analyzed here consists of spotted bass or products of interspecific hybridization. This demographic shift, perhaps ecologically or environmentally mediated, has been accompanied by introgressive swamping; more than 95% of the remaining smallmouth bass nuclear and cytoplasmlc alleles are present in Individuals of hybrid ancestry. Dilocus cytonuclear disequillbria were significantly different from zero, with patterns Indicative of an excess of homospecific genetic combinations (relative to expectations from single-locus allelic frequencies) and a disproportionate contribution of smallmouth bass mothers to the hybrid gene pool. Results document dramatic genetic and demographic changes following the human-mediated introduction of a nonnative species.

The reproductive capacity of a species is one of its most important attributes, intimately related to its ability to persist in a sometimes harsh environment.Reproductive capacity is a particularly good index of fitness in organisms such as many insects that go through repeated cycles of rapid population growth. In such organisms any feature of the reproductive biology that increases reproductive rate will be favored by natural selection. Repeated mating and sperm storage are specific features which can play important roles in determining female fecundity and male mating success, and hence fitness.Both females and males of many animal species mate repeatedly, and in a sizable fraction of these species the females store sperm. Insects in particular may store sperm for periods of time approaching the lifetime of an adult female. Parker (1970) reviewed the extensive literature on repeated mating and sperm storage in insects. He concluded that these processes are adaptive and the products of a kind of selection he termed sperm competition. The existence of these phenomena in a variety of species, including dragonflies, beetles, bugs, and dipteran flies, makes them of general interest to population biologists. The same processes of repeated mating, sperm storage, and the resultant selection undoubtedly operate in other arthropods and in other phyla. Our own attention has been focused on these processes in Drosophila flies.That female Drosophila store sperm from a mating has been known a long time.Likewise, it has been common knowledge that females in the laboratory will accept additional mates, sometimes long before the sperm from the first mating are exhausted (Lefevre and Jonsson 1962). An up-to-date review of sperm transfer, sperm storage, and sperm utilization in Drosophila may be found in Fowler (1973).Multiple insemination was shown to be rather common in laboratory populations of D. pseudoobscura (Dobzhansky and Pavlovsky 1967) and D. melanogaster

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