Showing papers in "Biological Journal of The Linnean Society in 1987"

Interspecific allometry of population density in mammals and other animals: the independence of body mass and population energy‐use

Abstract: Global regressions of ecological population densities on body mass for mammals and for terrestrial animals as a whole show that local population energy-use is approximately independent of adult body mass—over a body mass range spanning more than 11 orders of magnitude. This independence is represented by the slope of the regressions approximating –0.75, the reciprocal of the way that individual metabolic requirements scale with body mass. The pattern still holds for mammalian primary consumers when the data are broken down by geographic area, by broad habitat-type and by individual community. Slopes for mammalian secondary consumers are also not statistically distinguishable from –0.75. For any given body mass temperate herbivores maintain on average population densities of 1.5 to 2.0 times those of tropical ones, though slopes do not differ. Terrestrial animals of all sizes exhibit approximately the same range of population energy-use values. These results agree with those reported for population energy-budgets. It is suggested that rough independence of body mass and the energy-use of local populations is a widespread rule of animal ecology and community structure.

Altitude, habitat and tropical insect diversity

Abstract: Sample size and species richness of a series of light-trap samples taken in a number of localities in the Republic of Panama are given. Species richness as well as sample size decreased gradually with increasing altitude over a 100–2200 m range, in contrast to data from the literature which demonstrated a maximum at intermediate elevations. It is suggested that differences in technique, especially the continuous nature of the sampling in the present case are responsible for this discrepancy. Moderate human disturbance, which leaves a relatively high tree species diversity in the area has a surprisingly little, if any, effect on insect diversity. A large perturbation of the environment does, however, decrease species richness of the insect fauna.

Individual selection, kin selection, and the shifting balance in the evolution of warning colours: the evidence from butterflies

Abstract: It is difficult to imagine how warning colours evolve in unpalatable prey. Firstly, novel warningly coloured variants gain no protection from their colours, since predators have not previously encountered and learnt their colour patterns. This leads to a frequency-dependent disadvantage of a rare variant within a species. Secondly, novel warningly coloured variants may be more conspicuous than non-aposematic prey. Nevertheless, it is obvious that many palatable butterflies have bright colours used in intraspecific communication and in duping predators. Other palatable butterflies are already warningly coloured. Should such butterflies evolve unpalatability, perhaps because of a host-plant shift, these bright colours would be preadapted to a warning role. Warning colours could then continue to evolve by enhancement of memorable characteristics of these patterns, or by mimicry. Even within lineages of warningly coloured, unpalatable butterflies, colour patterns have continued to evolve rapidly. This diversity of warning colour patterns could have evolved in a number of ways, including individual and kin selection, and by the shifting balance. Evidence for these mechanisms is discussed, as are the similarities between the evolution of warning colours and more general evolutionary processes, including sexual selection and speciation.

The ecological and evolutionary interdependence between web architecture and web silk spun by orb web weaving spiders

Abstract: Spider orb webs are dynamic, energy absorbing nets whose ability to intercept prey is dependent on both the mechnical properties of web design and the material properties of web silks. Variation in web designs reflects variation in spider web spinning behaviours and variation in web silks reflects variation in spider metabolic processes. Therefore, natural selection may affect web function (or prey capture) through two independent and alternative pathways. In this paper, I examine the ways in which architectural and material properties, singly and in concert, influence the ability of webs to absorb insect impact energy. These findings are evaluated in the context of the evolution of diverse aerial webs. Orb webs range along a continuum from high to low energy absorbing. No single feature of web architecture characterizes the amount of energy webs can absorb, but suites of characters indicate web function. In general, webs that intercept heavy and fast flying prey (high energy absorbing webs) are large, built by large spiders, suspended under high tension and characterized by a ratio of radii to spiral turns per web greater than one. In contrast, webs that intercept light and slow flying prey (low energy absorbing webs) are suspended under low tension, are small and are characterized by radial to spiral turn ratios that are less than one. The data suggest that for spiders building high energy absorbing webs, the orb architecture contributes much to web energy absorption. In contrast, for spiders that build low energy absorbing webs, orb architecture contributes little to enhance web energy absorption. Small or slow flying insects can be intercepted by web silks regardless of web design. Although there exists variation in the material properties of silk collected from high and low energy absorbing webs, only the diameter of web fibres varies predictably with silk energy absorption. Web fibre diameter and hence the amount of energy absorbed by web silks is an isometric function of spider size. The significance of these results lies in the apparent absence of selective advantage of orb architecture to low energy absorbing webs and the evolutionary trend to small spiders that build them. Where high energy absorption is not an exacting feature of web design, web architecture should not be tightly constrained to the orb. Assuming the primitive araneoid web design is the orb web, I propose that the evolution of alternative web building behaviours is a consequence of the general, phyletic trend to small size among araneoids. Araneoids that build webs of other than orb designs are able to use new habitats and resources not available to their ancestors.

Multivariate analysis of genetic exchanges between Solea aegyptiaca and Solea senegalensis (Teleosts, Soleidae)

Abstract: Electrophoretic data from different populations of the teleosts Solea senegalensis, S. aegyptiaca and S. vulgaris were analysed using multivariate analysis. This confirmed that Solea vulgaris is very different genetically and isolated from both senegalensis and aegyptiaca, and that the latter two forms are also well differentiated. The presence of senegalensis alleles in the aegyptiaca population of the Golfe-du-Lion, and reciprocally, aegyptiaca alleles in the senegalensis population of the Gulf of Tunis, suggests that genetic exchange occurs between them. The association of these alleles (gametic disequilibria) and the existence of hybrid individuals show that these exchanges are an actual process rather than a consequence of ‘fossil’ gene flow. Furthermore, the genetic introgression seems to be asymmetrical in the two regions studied where they come into contact.

Insect diversity of a Costa Rican dry forest: why keep it, and how?

Abstract: The dry forest of the 108 km2 Santa Rosa National Park in northwestern Costa Rica contains as many as 13 000 species of insects (including 3140 species of moths and butterflies) sustained by and sustaining about 700 species of plants and 400 species of vertebrates. These insects require explicit conservation attention. They are more than just decorations on the plants; rather, they are the building blocks and glue for much of the habitat. They are the food for much of the carnivore community, and the insect species are not merely interchangeable bits of nutrients. They are major killers of seeds, and thereby influence both the plant species composition of a habitat and prevent the better competitors from taking it over. They are the primary pollinators and are specific enough to be neither interchangeable nor replaceable with other animals; the seeds resulting from their pollination activities are major animal foods in the habitat. They are a diverse, puzzling, complex, intrinsically attractive, and major part of the intellectual display offered by tropical wildlands, the display that will be the eventual foundation for most of the reason why tropical wildlands will be retained as such in the future. The retention and maintenance of insect species richness in a tropical wildland is strongly rooted in the preservation of plant species richness, in maintaining habitat mosaics (different members of which are used by a given insect in different seasons), in preserving a large diversity of habitats, and in recognizing the threat posed by insect crop associates in nearby agricultural lands.

The biological roles of pyrazines: evidence for a warning odour function

Abstract: Recently it has been shown that pyrazines are associated with many aposematic, chemically defended insects. We have demonstrated that naive hatchling chicks, when offered drinking water to which 2-methoxy-3-isobutylpyrazine is added, develop a neophobic ‘alerting’ reaction. The birds are not only capable of forming a conditioned aversion to the pyrazine when it is paired with quinine sulphate but they can also detect it from a distance probably by olfaction. This suggests that birds, a major group of insect predators, can interpret pyrazines as alerting or warning signals. The relationship of pyrazines with other alerting signals, and their evocative qualities are discussed.

Allozyme and shell variation in two marine snails (Littorina, Prosobranchia) with different dispersal abilities

Abstract: As adults Littorina littorea and L. saxatilis have overlapping distributions, both on the shore and on a geographical scale, this implies that they are affected by similar selective forces to a large extent. The two species have, however, different reproductive strategies. Littorina littorea has a planktonic larval stage lasting 4–6 weeks, whereas L. saxatilis has direct development. This influences dispersal rates of the two species, and gene flow and effective population size are both assumed to be much smaller in L. saxatilis than in L. littorea. Intraspeciflc variation in morphological and allozyme characters differs between the two species. Littorina littorea is homogeneous for both types of characters, both within and between populations, while populations of L. saxatilis may show pronounced differences in shell characters with micro-environment changes, and allozyme variation is markedly higher in this species too. The polymorphism in shell characters of L. saxatilis and the monomorphism of L. littorea, may be expected from theory as a consequence of the different dispersal strategies. Also the lack of divergence between geographically separate populations of L. littorea, but the high degree of between population differentiation in L. saxatilis, could be attributed to the high and low dispersal rates, respectively. However, the larger allozyme variation within subpopulations of L. saxatilis compared to that of L. littorea, is surprising as the former species has a smaller effective population size than the latter. Furthermore, there is no correlation between dispersal ability and overall allozyme heterozygosity, when 10 additional species of Littorina are included in the comparison. It is suggested that different evolutionary backgrounds of different species will possibly explain this, as neither selectionist nor neutralist models alone account for the observation. That is, an earlier passage through a population bottleneck may, for example, influence the genetic variation for a long period of time.

The origin of eutherian mammals

Abstract: Palaeontologically recognizable eutherians originated no later than the Early Cretaceous in warm, probably moderately seasonal climates. Immediate ancestors were small, sharing many anatomical, physiological and reproductive features with small modern marsupials. Development of characteristically eutherian features involved interactions of body size, rates of metabolism, energetic costs of reproduction, anatomical/physiological processes of development and effects of each upon rates of population growth. In contrast to eutherians, marsupials have a narrow range of basal metabolic rates (lacking high rates), and show no direct links between rate of energy expenditure and gestation period, postnatal growth rate, fecundity or reproductive potential. Biological implications of this contrast are most pronounced at small body sizes. When resources are abundant, the relatively higher growth rates and earlier maturation of small eutherians (particularly those with high rates of metabolism) can lead to rapid population growth; among most marsupials, however, both pre- and postnatal constraints apparently preclude attainment of such high rates of reproduction. Also, only eutherians among the amniotes combine intimacy of placentation with prolonged active intra-uterine morphogenesis. Once established, that combination permitted (and even favoured) increases in diversity of adaptation in such disparate aspects as elevated metabolic rate, increased pre- and postnatal growth rates, increased encephalization, greater longevity, increased gregariousness, greater karyotypic flexibility, and augmented variability in adult morphology. However, all such boosts in diversity were probably secondary and dependent upon prior innovation of trophoblastic/uterine wall immunological protection of foetal tissues during prolonged intra-uterine development. Increased metabolic rates followed thereafter, with synergisms that may have speeded evolution among early eutherians. Eutherian-style trophoblast probably originated in the Mesozoic. Dependent adaptations, variably expressed, evolved later in sundry descendant lineages. Reproductive differences between marsupials and eutherians are not biologically trivial; to the contrary, breakthroughs among eutherians assured their dominance: (1) in high intensity food habits; (2) at small body masses; and (3) in very cold climates.

The polyphyletic origin of laboratory inbred mice and their rate of evolution

Abstract: The analysis of various genetic components in the standard inbred laboratory strains of mice shows that several taxonomic units of the complex species Mus musculus have contributed to their genetic background. These laboratory lines are often taken as archetypes of the mammalian genome and since their genealogy is known for around 80 years they have been used to estimate mutation rates and various other evolutionary parameters. The knowledge of their origins is important if one is to avoid erroneous interpretations. Do they possess haplotypes that could have existed in natural populations of M. musculus domesticus or are they the fruit of artificial recombinations between divergent genomes? We discuss this in the light of various genetic systems.

Interacting evolutionary constraints in pelvic reduction of threespine sticklebacks, Gasterosteus aculeatus (Pisces, Gasterosteidae)

Abstract: The pelvic girdle of Gasterosteus aculeatus is normally a complex and robust structure that varies moderately among and within populations, but in several populations it occurs as a vestigial series. Based on 14 Recent and fossil populations from North America and Scotland, three general patterns of intrapopulation variation of vestigial pelvic structures can be recognized. In most cases of pelvic reduction, the size and complexity of the pelvic girdle are reduced mostly by progressive truncation of distal structures, but two patterns of distal truncation occur. Paedomorphosis appears to play a role in all cases of pelvic reduction, but in a fossil assemblage and one extant population it accounts for most of the structural simplification. Pelvic reduction in populations of Gasterosteus depends on an interaction between natural selection, which appears to determine whether or not pelvic reduction occurs, and phylogenetic constraint (especially developmental constraint), which strongly influences the form of pelvic vestiges. The empirical difficulty of discriminating between phylogenetic constraint and effects of natural selection, and the potential for phylogenetic constraint to produce homoplasy are discussed. Pelvic reduction apparently has been a recurrent phenomenon in Gasterosteus since at least the early Pliocene, but it has not resulted in evolution of a separate lineage of ‘pelvicless’ sticklebacks because such populations are restricted to lakes, which are ephemeral over evolutionary time. Rapid phyletic pelvic reduction coupled with selective extinction of populations with vestigial pelvic structures has resulted in phylogenetic stasis.

Macrolepidoptera diversity in the Indo-Australian tropics: geographic, biotopic and taxonomic variations

Abstract: Data are presented on the richness of Macrolepidoptera faunas in the Indo-Australian tropics, and on the proportional representation of the various families. Both overall diversity and diversity of families decline with latitude and also with isolation from major land areas. Exceptions are noted. Measures of alpha-diversity from quantitative light-trap samples indicate that diversity within the tropics is often greatest at altitudes around 1000 m, declining above that level. Diversity is much lower in Pacific archipelagos. Loss of diversity through disturbance and conversion to agriculture is evident and marked. Problems in the measurement of diversity with light-traps are discussed, exemplified by results obtained using different trap types. Changes in family proportions with habitat type are examined. Family diversity generally decreased with altitude and with disturbance and conversion to agriculture, though there are exceptions to this decline.

Sexual size dimorphism in parasitoid wasps

Abstract: Sexual dimorphism in body length and proportion of overlap between the ranges of body length for males and females were estimated for 361 species of parasitoid wasps from 21 families. In most species, females are generally larger than males, though the range of male and female sizes overlap. Species in the family Ichneumonidae differ significantly from species in other families in three ways: (1) ichneumonids on average are larger, (2) in most species, females are generally smaller than males, and (3) on average, proportion overlap between the ranges of body length for males and females is greater. At present, there is a paucity of life history data on parasitoid wasp species for which size dimorphism is known. Thus it is not clear why ichneumonids differ from species in other families. Possible evolutionary explanations for variation in dimorphism among parasitoid wasp species are discussed.

Zebra stripes and tiger stripes: the spatial frequency distribution of the pattern compared to that of the background is significant in display and crypsis

Abstract: Some striped animals are camouflaged in their natural environment, whereas others are conspicuous. Mammals are known to have spatial frequency analysers in their visual mechanism, and it is suggested that the spatial characteristics of a striped pattern are different in camouflaged and conspicuous animals. Fourier analysis of the stripes of the zebra shows spatial frequencies in the pattern that are unlikely to be present so strongly in their natural background scene. A similar analysis of the camouflaging stripes of a tiger show that the distribution of spatial frequencies are similar to that in the background scene.

The implication of survival rates in natural populations of Drosophila: capture‐recapture experiments on domestic species

Abstract: Adult survival rates were estimated in field populations of domestic Drosophila species by means of multiple capture-recapture experiments. Micronized fluorescent dusts were used as marking agents and analysis used the Fisher-Ford model: the assumptions of this model are justified using evidence from this study and the literature. Survival rates were commonly 0.45-0.85 per day, suggesting that mean life expectancy for adult flies is from 1.3 to 6.2 days in natural populations. No consistent differences in survival between sexes or seasons were demonstrated. The implications of low natural survival rates are discussed.

Mate choice in Darwin's Finches

Abstract: Female Geospiza conirostris on Isla Genovesa, Galapagos, pair preferentially with males who have had previous breeding experience. They choose mates on the basis of courtship behaviour and black adult plumage. By mating with experienced black males, they gain a fitness advantage in terms of fledgling production and recruitment of young into the breeding population. Behavioural signs of past breeding experience and black plumage are reliable age- and condition-dependent traits. We suggest that females use conspicuous black plumage to identify old males at a distance, then interactions through courtship to modify initial assessments. Females paired with inferior males may increase the genetic quality of their offspring by extra-pair copulations; results of heritability analysis of morphology are consistent with this suggestion. Females change mates at a frequency of 12–27% per breeding season. They re-pair with males who are generally old, experienced, and hold territories adjacent to the deserted male. Females that re-pair gain a benefit, whereas males who are deserted within a breeding season incur a cost of more than 50% of their future potential production for that season. We conclude that females in choosing males seek reliable indicators of potential parental care, and in addition they may seek indicators of genetic quality.

Some factors affecting the composition of tropical ichneumonid faunas

Abstract: An increasing accumulation of data shows that tropical ichneumonid faunas are no more species-rich than extra-tropical ones, despite the fact that most of their host groups show increased tropical species-richness. This lack of increase in ichneumonid species-richness can be attributed to the absence of groups whose hosts are not present (e.g. Ctenopelmatinae) and poor tropical representation by many groups of diurnal koinobionts (e.g. Campopleginae). Low host density has been postulated as a barrier to tropical koinobiont species-richness, but it is here suggested that this is not the only limiting factor as groups of nocturnal koinobionts, such as the Ophioninae, show increased tropical species-richness. It is postulated that koinobionts have the capability of being able to locate sparse hosts, but as they host-search in flight, a prolonged daytime host-searching period in the tropics would expose them to a high level of predation pressure. By being active at night koinobionts can avoid diurnally active predators. It is also postulated that sparse hosts may be located more easily at night and more habitats may be climatically suitable for ichneumonid activity when they are not subject to direct sunlight. Idiobionts, such as the Mesostenini and Pimplini, are more species-rich and morphologically diverse in the tropics than they are in extra-tropical regions. It is suggested that this results from the fact that tropical idiobionts can be active during the whole of the diapausing period, when their hosts are available, whereas activity by temperate idiobionts is prevented by inclement weather. Although many idiobionts are probably less exposed to predators than koinobionts, many have evolved obvious protective devices.

Sexual competition during colony reproduction in army ants

Abstract: We review the unusual processes of sexual reproduction and colony fission in army ants and briefly compare this to reproduction in other ants. Army ants are a polyphyletic group and are characterized by a syndrome of convergently evolved traits including large colony size, group foraging for large prey, nomadism, cyclical brood production and queens who are large and wingless. Because queens are flightless and never leave their colony, workers are in a position to choose which queen will take over each new colony. Males fly between colonies and must run the gauntlet of the workers in alien ones before they can approach the queen. For this reason, workers can also choose which males will inseminate their queen. Army ant workers may therefore be involved in choosing both the matriarch and patriarch of new colonies. We suggest that this unusual form of sexual selection has led to the close resemblance of conspecific males and females in all the separate lineages of army ants. Males are queen-like in that they are large and robust, have long cylindrical abdomens, with exocrine glands of similar form and location to those of females and shed their wings when they enter new colonies. Furthermore, when males enter new colonies they are followed by an entourage of workers which resemble those that accompany queens. We suggest that males resemble queens not as a form of deceitful mimicry but because under the influence of sexual selection they have come to use the same channels of communication to demonstrate their potential fitness to the workforce as those used by queens.

Natural selection for apostasy and crypsis acting on the shell colour polymorphism of a mangrove snail, Littoraria filosa (Sowerby) (Gastropoda: Littorinidae)

Abstract: Littoraria filosa (Sowerby) is a member of the L. scabra group, found amongst the foliage of mangrove trees in northern Australia. The colour of the shell is polymorphic, showing two discrete ground colours, either yellow or orange-pink, with a variable degree of superimposed brown patterning. At a site on Magnetic Island, northern Queensland, colour frequencies of small snails were similar on different backgrounds. Amongst larger shells yellows were more frequent on Avicennia trees with abundant foliage, and browns on relatively bare trees, suggesting that visual selection for crypsis occurred. There was no evidence of substrate selection by the morphs. Yellow shells were cooler than brown shells, but differences in colour frequencies on sunny and shaded trees, and at different seasons, did not suggest climatic selection. By manipulating the colour frequencies of subpopulations of small snails isolated on individual trees, it was shown that the disappearance of yellow and brown shells was frequency-dependent. This result is consistent with hypotheses of mimicry of background elements by the morphs and of apostatic selection by unknown predators. Only the latter can account for the persistence of the highly conspicuous pink morph at a low frequency.

On the reproductive biology of the sand smelt atherina boyeri risso pisces atherinidae and its evolutionary potential

Abstract: The small, typically estuarine fish Atherina boyeri Risso forms local populations which can differ greatly in their population biology and morphology; this species is viewed as being on the brink of speciation. This analysis of the reproductive strategy shows that A. boyeri can rapidly adapt its life history and morphology to environments ranging from freshwater to polyhaline coastal lagoons and oceanic waters. The optimum strategy is shown to be tied to temperate seasonality: the cold winters act to lengthen the life cycle, and result in cyclic switching of energy between reproduction, somatic growth and fat storage. The adaptability of the fish to a range of environmental characters is viewed as being due to natural selection acting to produce a genotype capable of a flexible phenotypic response. This flexibility is essential for survival in the highly variable inshore/brackish habitat. It is argued that such preadaptive plasticity, coupled with the potential for populations to become isolated, produces ideal conditions for speciation. The importance of the coastal, estuarine and lagoonal environment as a springboard for teleost evolution is discussed.

Testicular dysfunction in hybrids between parapatric subspecies of the grasshopper Chorthippus parallelus

Abstract: 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.

Tropical dry and wet season polyphenism in the butterfly Melanitis leda (Satyrinae): Phenotypic plasticity and climatic correlates

Abstract: Changes in wing pattern, colour, shape and size associated with seasonal polyphenism in Melanitis leda were quantified using a series of 155 butterflies collected by N. Manders on Mauritius in 1905. Butterflies of the wetter period were predominantly of the wet season form with large, well differentiated eyespots, short tails, smaller wings and a characteristic background colour. The dry season form occurred only in the drier period and has much smaller eyespots, longer tails, larger wings and a variable background. Many intermediates occurred, mainly in the drier period. These are associated with an absence of extreme seasonal change in Mauritius. The first principal component (PCI) describing the morphometric and colour data is closely related to the wing form (r = 0.80). Regression analyses using temperature and rainfall data for the 8 weeks before each capture showed that about 40% of variation in PCI could be accounted for by temperature in weeks 2–3 before capture. Many of the characters measured are redundant; a subset of seven morphometric characters yields a closely similar PCI. Analysis of is subset in an additional sample of 70 M. leda from Kenya showed that the seasonal polyphenism overrides a small degree of sexual dimorphism. The results are discussed with regard to seasonal changes in adult activity, resting backgrounds and visual predation. Wing phenotype characters are part of an array of coordinated morphological and life history traits which include ovarian dormancy and fat body development in dry season adults. A partial independence occurs in the proximal control of these traits as indicated, for example, by the larger wing and tail size, and smaller eyespots of the small number of the wet season form captured in the drier period in comparison to those of the wetter months.

Symbiont-induced speciation

Abstract: Speciation induced by parasitic or mutualistic symbionts has been suggested for taxa ranging from plants to insects to monkeys. Previous models for symbiont-induced speciation have been based upon hybrid inferiority and selection for reinforcement genes. Taken on their own, however, such models have severe theoretical limitations and little empirical support. Two conditions that may favour symbiont-induced speciation are presented here: (1) interaction norms in which the outcomes of host/symbiont interactions differ between environments and (2) differential coadaptation of host and symbiont populations between environments or along an environmental gradient. Symbiont-induced speciation can be considered as one form of ‘mixed-process coevolution’: reciprocal evolution in which adaptation of a population of one species to a population of a second species (or coadaptation of the populations) causes the population of the second species to become reproductively isolated from other populations.

A striking example of the founder effect in the mollusc Littorina saxatilis

Abstract: Two South African populations of Littorina saxatilis were examined by starch-gel electrophoresis at 16 enzyme loci and compared with 13 populations of North Atlantic saxatilis from both American and European coasts, and with six British populations of the closely related species Littorina arcana. The South African animals showed a severely reduced heterozygosity (H– 0.052) compared with Atlantic populations of saxatilis (H= 0.181), and the mean genetic distance between the two areas was high (F= 0.203) compared with distances within the North Atlantic saxatilis populations (D = 0.034). In fact, the saxatilis from South Africa were genetically more distant from the North Atlantic samples of L. saxatilis than were the arcana from British shores. The reduced genetic heterozygosity and genetic divergence of the South African populations is attributed to founder effects following a postulated recent introduction by man.

The understanding of industrial melanism in the peppered moth (Biston betularia) (Lepidoptera: Geometridae)

Abstract: Melanic polymorphism in B. betularia has been extensively studied. Correlations between high melanic frequency and high levels of air pollution have been demonstrated. Kettlewell and others have shown that differential bird predation has an important effect on the maintenance of the polymorphism, and coefficients of visual selection have been obtained on the assumption that the moth habitually rests on tree trunks. Computer models based on these selective coefficients show that they are not sufficient accurately to explain observed melanic frequencies. Other non-visual selective factors and weak frequency-dependent selection have been invoked to improve fits. Analysis of the resting positions of moths recorded in the wild demonstrates that B. betularia does not usually rest in exposed positions on tree trunks, but rather rests on the underside of branches, on trunks in shaded positions just below major branch joints or on foliate twigs. The results of a pilot selection experiment, while agreeing qualitatively with Kettlewell's results, suggest that fitness estimates that assume trunk-resting are quantitively incorrect. The error is greatest for melanic moths in rural areas. It is suggested that visual selective coefficients based on a true assessment of the resting behaviour of the moths may considerably improve the fit between computer predictions and observed phenotype frequency distributions.

Why are some snails visibly polymorphic, and others not?

Abstract: 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.

Multiple sex expressions in Cimicifuga simplex: dichogamy destabilizes hermaphroditism

Abstract: Cimicifuga simplex (Ranunculaceae) has four types of individuals, differing in their sex expression: there are individuals with hermaphroditic, hermaphroditic and male (andromonoecious), only male, or only female flowers. Purely female individuals appear during the early part of the flowering period of the population, and males during the later part. Strong protandry in hermaphroditic flowers causes a shift in functional sex ratio in the population over time. This facilitates the evolution and maintenance of small proportions of unisexual individuals in the population. A hypothesis developed from this case of multiple sex expression predicts dichogamy as a near-universal feature of andro-and gynomonoecious, androdioecious, and gynodioecious species, and also specific temporal distributions for each sex form depending upon the form of dichogamy. Results of tests on large sets of species strongly support the relevance of this hypothesis.

Behavioural studies on the peppered moth Biston betularia and a discussion of the role of pollution and lichens in industrial melanism

Abstract: We examined the resting behaviour during pairing and egg-laying of bred female Biston betularia (L) after they were set free at wooded sites in Somerset and Cardiff Moths were not released immediately on emergence but were held for three nights to overcome an initial period of flight activity Females (N= 257) were placed on trunks, and on large and small branches of trees, especially oaks, over periods of several weeks in 1984, 1985 and 1986 Releases were not made within 50 m of any known survivor One hundred and thirty-one pairings were observed We also examined oviposition behaviour in a cage experiment using different types of branches Our observations support Mikkola's earlier conclusion from cage experiments with male moths that the species rests predominantly on branches and shows an appropriately specialized resting attitude, demonstrated here in a series of photographs Many moths will rest underneath, or on the side of, narrow branches in the canopy Once released females had settled and paired they only moved quite short distances, but positioning and egg-laying were profoundly influenced by the presence of foliose lichens Females of all phenotypes show a strong preference to oviposit beneath a thallus of foliose lichens Their tendency to rest against or close to the lichen during the day could markedly influence their crypsis In the absence of foliose lichens, eggs are laid in cracks in the bark The survivorship of released females both in different pairing combinations and during their whole reproductive life-span is analysed These preliminary data suggest that bird predation can give rise to differential mortality of different pairing combinations and of the phenotypes This technique provides a powerful method of measuring the relative crypsis and differential mortality of phenotypes with living females in natural resting sites The significance of our observations on resting behaviour is discussed with regard to the role of air pollution and epiphytes in industrial melanism Particular attention is given to the causal mechanisms underlying recent declines in the frequency of carbonaria melanics in parts of Britain One important factor is the new growth made by trees since the marked reduction in particulate air pollution in the 1960s It is emphasized that we need to know much more about the interactions between pollution, epiphytes and resting backgrounds, especially in the canopy, before we can be confident of our understanding of the evolution of industrial melanism

Adaptation and speciation of Dolichopoda cave crickets (Orthoptera, Rhaphidophoridae): geographic variation of morphometric indices and allozyme frequencies

Abstract: Morphometric analyses were carried out for 14 cave dwelling populations of Dolichopoda (Orthoptera, Rhaphidophoridae) from peninsular Italy belonging to the laetitiae-geniculata complex. The selected set of variables includes linear measures of body parts and appendages whose variation is expected to be adaptive and to reflect the degree of specialization to the cave environment. Inter-population morphometric variation was compared with the patterns of allozyme variation, investigated electrophoretically in a previous paper, by means of ordination and classification multivariate analyses. Results from this comparison reject the hypothesis of parallel geographic variation of morphometric indices and allele frequencies, revealing independent trends for the two sets of characters.

Shell shape and land‐snail habitat in a Mediterranean and desert fauna

Abstract: The bimodal distribution of shell shape (height: diameter), that is found in various geographically widely separate and taxonomically distinct land snail faunas of many different regions of the world, occurs also in a Mediterranean fauna and in a desert fauna that is derived from it. The desert fauna is, however, closer to the bisector than the Mediterranean one. High-spired snails are mainly rock-dwellers, and equidimensional to low-spired snails are bush-dwellers or soil-diggers, with a few rock-dwellers; litter-dwellers are small-sized species that may have either high- or low-spired shells. These results are discussed in adaptive terms. Litter is probably the more primitive of these micro-environments. Many of the small, litter-dwelling snails are ovo-viviparous rather than oviparous, perhaps so as to avoid attacks on the eggs by saprophytic fungi. The shift away from the litter environment is accompanied by a trend to abandon the ovo-viviparous strategy, in favour of oviparity, the snail using its foot to dig into the soil and lay eggs. The conchometric differences between bush-, ground- and rock-dwelling snails may perhaps reflect selective pressure to increase the size of the foot; and constraints of a habitat that consists of narrow interspaces between rocky boulders. Snails that habitually dig into the ground during periods of inactivity, and roam over the ground when active, requires a very large foot and, consequently, a very large-mouthed shell to accommodate it; the result is an equidimensional shell, globose or turbiniform in shape. Snails that climb up vertical vegetation would also require a large foot, and consequently a large-mouthed shell to contain it. A fully globose shell would however be disadvantageous, since it might cause undesired torque. Hence, bush-dwellers tend to be flatter than soil-diggers. Snails that habitually live in rock crevices, and on hard substrata, would not require a very large foot; they would need a narrow shell, both to enable easy manoeuvring through crevices and to reduce torque, the result being a small-mouthed, usually high-spired shell. The classification of land snails into bush-, soil- or rock-dwellers closely follows the taxonomic classification. In those species that depart from the habitat that is typical of their taxonomic group towards another habitat, the shell alters its shape accordingly.