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doi: 10.1111/j.1095-8312.1987.tb01977.xpmid: N/A
Mark recapture and morph frequency data, gathered during a population irruption of Hypolimnas misippus in southern Ghana, provide evidence for apostatic and mimetic selection. During a period of low adult survival, both the recapture rate and the frequency of the commonest morph (misippus) were significantly reduced. Selection against this form increased phenotypic diversity and generated significant disequilibrium in the combinations of unlinked fore‐ and hindwing phenotypes. There was also evidence for selection against those forms (weak alcippoides) which most closely resemble misippus. Other morphs, including both good mimics of Danaus chrysippus and rare non‐mimics, showed no reductions in recapture rate during the period of low survival, but only the good mimics increased significantly in frequency. The results provide a predictive ecological model for density‐dependent selection by predators which is consistent with field data from previous studies of H. misippus in Ghana and Tanzania. Their evolutionary implications are discussed, and it is suggested that anomalies in the mimicry of this species may be partly due to lack of predation when it is scarce.
HEWITT, G. M.; BUTLIN, R. K.; EAST, T. M.
doi: 10.1111/j.1095-8312.1987.tb01978.xpmid: N/A
Two subspecies of Chorthippus parallelus meet in a narrow hybrid zone in the Pyrenees. Laboratory Fl hybrids between the two races had atrophied testes and were virtually sterile. Backcross males had variable levels of dysfunction but were generally intermediate between Parental and Fl males; there were some reciprocal differences. No other effects on fertility and viability of males and females were apparent, and Fl males had normal accessory glands. These data provide another clear example of Haldane's rule and have implications for the structure of hybrid zones, the evolution of mating isolation and mating strategies.
doi: 10.1111/j.1095-8312.1987.tb01979.xpmid: N/A
Work on Cepaea land‐snails since 1950 is surveyed, and various explanations for their visible polymorphism, including predator selection, the influence of sunlight and temperature, co‐adaptation and linkage disequilibrium, and “area effects”, are discussed. All of these can modify the genetic make‐up of natural populations in particular circumstances, but none provide a satisfactory answer to the question of why some species are visibly polymorphic whereas others present a uniform external appearance. The likely explanation is that probably all species are genetically heterogeneous at numerous loci, as a result of heterozygote advantage, co‐adaptation and other selective factors maintaining the different alleles in equilibrium, which may sometimes have visible effects on the phenotype. If these are positively disadvantageous, selection for epistatic genes will suppress the visible polymorphism, without affecting the underlying genetical heterogeneity, preserved by selection for other non‐visible pleiotropic effects of the alleles involved. But this will not happen if the visibly distinct effects of these different allelomorphs are selectively more or less neutral. Many examples of polymorphism, including the so‐called ‘pseudo’‐polymorphism, are therefore essentially non‐adaptive so far as their visible manifestation is concerned, being maintained as balanced polymorphism by selection for non‐visible pleiotropic effects of the genes involved.
BROWN, VALERIE K.; HENDRIX, STEPHEN D.; DINGLE, HUGH
doi: 10.1111/j.1095-8312.1987.tb01980.xpmid: N/A
The plant and insect communities of early, secondary succession beginning with bare ground in an Old World site (southern Britain) and a New World site (Iowa, U.S.A.) shared a number of characteristics. Both sites showed similar temporal patterns of plant species cover and species richness, although overall richness was greater at the Old World site. Annuals dominated at both sites during the first year of succession and were largely replaced by perennials in the second year. Monocotyledons were more abundant at the Old World site, especially in the second year. The two sites differed markedly in the contribution of native and introduced plant species, with the Old World site dominated by natives and the New World site by alien plant species. Insect herbivore load was greater at the Old World site, when expressed in terms of structural complexity of the vegetation, suggesting that there may be major differences in the influence of herbivores on the direction and rate of succession at the two sites.
doi: 10.1111/j.1095-8312.1987.tb01981.xpmid: N/A
The stability of complex patterns of geographic variation was investigated by assessing the congruence between multivariate ordinations derived from randomly chosen real characters. Two series of populations were analysed representing two situations with complex patterns of geographic variation. The first, a ‘Eurasian’ series of populations, showed a strongly structured hierarchical pattern, the second, an ‘eastern’ series of populations, showed a more subtle complex pattern of smooth clines and steps. The characters were selected from a total of 81 (Eurasian) or 61 (eastern) within‐population independent characters from six different systems. The congruence between ordinations of the geographical populations was measured by the rotational fit statistic, R2. Three procedures were used to compare ordinations based on from two to up to 80 characters randomly chosen to give: A, completely independent character sets; B, subsets compared to the total set; and C, potentially overlapping sets. All three procedures showed that congruence between the ordinations was asymptotic in relation to character number. This relationship was described by one of two mathematical models (procedure B did not result in a hyperbolic model as found with simple patterns of geographic variation). Generally speaking, once a sufficient number of characters are used, the complex patterns of geographic variation are stable, reliable and predictive and not substantially influenced by character choice. The strongly structured hierarchical pattern required 15 or so characters to achieve reliability whilst the more subtle patterns required 20 or so characters. However, the addition of further characters does improve reliability in both cases. The greater percentage of variance portrayed by three‐dimensional ordinations compared to two‐dimensional ordinations is achieved at the cost of lower congruence when a sufficient number of characters are used. If case studies of geographic variation were to adopt these procedures (preferably using completely independent character sets; procedure A) the reliability of their results would be indicated).
doi: 10.1111/j.1095-8312.1987.tb01982.xpmid: N/A
Book reviewed in this article: The OU Project Guide: Fieldwork and Statistics for Ecological Projects, by Neil Chalmers and Phil Parker Ecology. Individuals, Populations and Communities, by M. Begon, J. L. Harper and C. R. Townsend
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