ADAPTIVE RADIATION IN ALCIDAE

Because of its homogeneity, both taxonomical and ecological, the sea-bird family Alcidae constitutes an appropriate group for the study of adaptive radiation. This radiation involves mainly the acquisition of specialized feeding habits and the consequent specialization of the various species of the family at different trophic levels.
The plankton-feeders, exemplified by the Least Auklet Aethia pusilla, have a relatively wide beak with a fleshy tongue and a broad palate with numerous denticles. The fish-feeders, exemplified by the Razorbill Alca torda, have a narrow bill, a certain degree of tongue cornification and few, but sharp and regularly arranged palatal denticles. A few species, including the puffins and one auklet (Cyclorrhynchus), have characteristics intermediate between these extremes and feed partly on fish and partly on plankton.
Body-size in the predator is related to the size of the prey and these relations are examined within the family. Within a group with similar adaptations, species which are alone in their niche in their area tend to be of intermediate body size for that group. Finally, though the family may exhibit a series of parallel trends in bill shape when the bill is considered as a social releaser, it exhibits a gradation in shape and structure (adaptive radiation) when considered as a food-getting tool.     

EVOLUTIONARY ASPECTS OF FRUIT-EATING BY BIRDS

In spite of a considerable literature on fruit-eating, the general evolutionary implications of fruit as a source of food for birds have been neglected. A preliminary attempt is made to explore the evolutionary and ecological consequences of fruit-eating, considered as a mutual interaction between parent plant and dispersal agent. The relationship considered is that obtaining between fleshy fruits and the “legitimate” fruit-eating birds which digest the fleshy part of the fruit and void the seed intact. Evolutionary aspects of seed-eating are also briefly discussed. The “strategies” adopted by fruits for dispersal by birds result in the production of abundant food supplies which are easy of access and exploitable by many species of birds. By contrast, the predation of birds on insects leads to a heterogeneous, sparse and cryptic food supply, to exploit which many different hunting techniques are necessary. Two important evolutionary developments in birds are attributed to these differences in food supply: there tend to be more species in families of insectivorous than of frugivorous birds, and lek behaviour in tropical forest has evolved in predominantly frugivorous birds. The seasonal succession of fruits in temperate latitudes is discussed, and contrasted with the situation in the tropics, using examples from Europe and Trinidad. In general, the succession of ripe fruits in Europe seems to be adapted to the seasonal shifts of the bird populations, and the more nutritious fruits tend to have a more southerly distribution and to ripen later than the more succulent fruits. In the tropics the distinction between nutritious and succulent fruits seems to be largely one of habitat. The constant succession of ripe fruits throughout the year in the tropics probably depends on competition for dispersal by frugivorous birds, which thus ensure the maintenance of their own food supply. This may be regarded as a symbiosis at the level of the ecosystem.     

ADAPTIVE LIMITATION OF THE REPRODUCTIVE RATE OF BIRDS

Some biologists have held that the rate of reproduction in birds tends to be adjusted to the average annual mortality; others, that it is limited only by the parents' ability to raise sturdy young. The latter theory, that of maximum reproduction, is likely to be true only if three related propositions are true:(1) that an increase of clutch size is more likely to occur than some other mutation affecting the rate of reproduction; (2) that a genotype with a wastefully high rate of reproduction can supplant a genotype with a more conservative but adequate rate; and (3) that an excessive rate of reproduction is not harmful to the species. None of these assumptions has been proved. In a population not obliged to employ its full reproductive potential to maintain itself at a favourable level, mutations which limit this potential may arise and persist. They may effect this limitation by means of:(i) reduction in clutch size; (ii) reduction in the number of broods; (iii) failure of the male to attend the nest, often followed by failure to form pairs; (iv) deferment of reproductive maturity; (v) developments in territorialism that limit the number of nesting birds or the number of progeny they can rear; (vi) restriction of nesting to traditional sites; and (vii) the time-consuming construction of elaborate nests. These limitations of the rate of reproduction must be regarded as adaptive because, like other adaptations, they adjust the birds more perfectly to the conditions in which they live and reduce the stress to which they are subjected. In both tropical and temperate regions, species in which only the female feeds the nestlings have broods as large as species in which both parents feed them. It follows that the two parents are not rearing as many young as they could nourish. The view that hole-nesting birds can rear larger broods than open-nesters because their young develop more slowly, and require less food per capita per day, is untenable. Nestlings raised in holes and burrows gain weight about as rapidly as those in more exposed nests, but for safety they remain longer in their protected abodes. The larger broods of hole-nesters evidently compensate for the difficulty of obtaining nest sites, which delays the breeding of some pairs and prevents that of others. Clutch size is by no means closely adjusted to the number of young the parents can raise. If given additional nestlings, some birds attend them adequately. In other species, young are rarely fledged from all the eggs. In many cases, asynchronous hatching is not, as has been claimed, an arrangement which permits the parents to adjust to a varying food supply the number of young that they rear. In many raptors, fratricide and cannibalism reduce the size of the brood, sometimes to a single nestling, regardless of the abundance of food. The more stable the environment, the more closely the reproductive rate tends to be adjusted to the mortality; the more a population is subject to catastrophic reductions, the more the rate will approach the maximum. Primarily, the reproductive rate is controlled by heritable characters, which can adjust the rate to a stable environment but rarely respond to short-term fluctuations in external conditions or population density. The last fine adjustment of a population to its habitat is effected by processes that are density-dependent:either density-dependent regulation of the reproductive effort, or density-dependent mortality of adults or young, or a combination of the two. The general evolutionary trend in the Metazoa is toward producing fewer offspring and taking better care of them. This would hardly be possible if the more prolific genotype always prevails over those which raise smaller families and in consequence can attend their young somewhat better. The regulation of the rate of reproduction is a unique evolutionary problem, because a mutation conferring greater fertility, although often detrimental to the species, tends to diffuse through it as no other harmful mutation can. Yet it is counteracted by many factors, chiefly ecological, which operate subtly and are more difficult to appreciate than the force of numbers.     

ADAPTIVE RADIATION AND GENETIC DIFFERENTIATION IN HAWAIIAN BIDENS

The genus Bidens (Asteraceae) has undergone extensive adaptive radiation on the Hawaiian Islands. The 19 species and eight subspecies endemic to Hawaii exhibit much more morphological and ecological differentiation than the continental members of the genus. However, the Hawaiian taxa have the same chromosome number and retain the capacity to interbreed in all possible combinations. Twenty-two populations of 15 Hawaiian taxa and four populations of American taxa were compared at 21 loci controlling eight enzyme systems. Populations of Hawaiian taxa are highly polymorphic. However, little genetic differentiation has occurred among taxa in spite of the high levels of genetic variability. Genetic identities calculated for pairs of populations show that populations of the same taxon are genetically more similar than are populations belonging to different taxa, but all values are high. The level of genetic differentiation that has occurred among the species of Hawaiian Bidens is comparable to the level of genetic differences found among populations within single continental plant species. Moreover, there is no correlation between the isozyme data and morphological data. No groups of taxa are evident in the genetic data, although morphological groups exist. Genetic differentiation at isozyme loci has not occurred at the same rate as the acquisition of presumably adaptive morphological and ecological characters in Hawaiian Bidens. Adaptive radiation may be limited to a few genes controlling morphological and ecological characters.     

ECOLOGICAL ASPECTS OF SEX EXPRESSION IN SUBDIOECIOUS SCHIEDEA GLOBOSA (CARYOPHYLLACEAE)

The importance of ecological factors such as sex lability, spatial segregation, and resource allocation in the evolution of dioecy were examined in Schiedea globosa. S. globosa is a subdioecious species with equal numbers of plants possessing strictly male or female function and a small proportion of hermaphrodites. The propensity for labile sex expression was under both environmental and genetic control; some plants with male function became hermaphroditic (by producing female flowers) under better growing conditions in the field and in the greenhouse. There was some spatial segregation of the sexes. Because of sex lability, more hermaphrodites than males occurred on moister slopes. Although there were not measurable sex-related differences in mortality within or between two flowering seasons, more females than males and hermaphrodites occurred at the bottom of slopes. Males and females produced the same number of ramets and inflorescences, but females had a greater number of flowers per inflorescence. Males and females had the same number of ovules (vestigial in males), but females had larger ovules and longer stigmas. Hermaphrodites and males had the same amount of pollen per flower despite the production of fruit by the hermaphrodites. In hermaphrodites, there was no apparent tradeoff within flowers between pollen production and ovule production. These results indicate that spatial segregation, sex lability, and environmental conditions influence allocation patterns of S. globosa, and in combination with high inbreeding depression and selling rates, may promote the further evolution of dioecy in S. globosa.     

DISSECTING CORRELATED CHARACTERS: ADAPTIVE ASPECTS OF PHENOTYPIC COVARIATION IN MELANIZATION PATTERN OF PIERIS BUTTERFLIES

In this study we address the question of how much of the covariation among phenotypic characters observed in natural populations is adaptive. We examine covariation among a set of phenotypic characters that describe the wing-melanization pattern of Pieris butterflies. Previous functional analyses of thermoregulatory performance allow us to predict a priori whether and how different wing melanic characters should be correlated. We quantify and analyze the variation in the wing-melanization pattern within species for a series of Pieris populations from relatively cool environments in North America and compare these results with the predictions based on our adaptive hypothesis. We consider adaptive covariation both for biogeographic variation among populations and for seasonal polyphenism (phenotypic plasticity) within populations. Our hypothesis correctly predicts many of the qualitative features of covariation in melanization among major regions of the wings, at the level of biogeographic variation among populations, for both males and females of Pieris occidentalis. When within-population variation is considered, agreement with the adaptive predictions varies considerably in different populations for both P. occidentalis and P. napi males and females. Agreement for P. napi, particularly the females, is generally poorer than for P. occidentalis. In both species, there is a consistent difference in melanization pattern between alpine and arctic sites; this difference is discussed in relation to the differences in the radiative environment between these two types of “cold” habitats. Our results suggest that some important aspects of phenotypic correlation among wing melanic characters in Pieris are adaptive. We emphasize the important distinction between covariation and co-occurrence of characters, and we discuss these results in relation to the extensive biogeographic variation and phenotypic plasticity (seasonal polyphenism) in Pieris wing-melanization patterns.     

ADAPTIVE RADIATION AND THE TOPOLOGY OF LARGE PHYLOGENIES

The idea that some organisms possess adaptive features that make them more likely to speciate and/or less likely to go extinct than closely related groups, suggests that large phylogenetic trees should be unbalanced (more species should occur in the group possessing the adaptive features than in the sister group lacking such features). Several methods have been used to document this type of adaptive radiation. One problem with these attempts is that evolutionary biologists may overlook balanced phylogenies while focusing on a few impressively unbalanced ones. To overcome this potential bias, we sampled published large phylogenies without regard to tree shape. These were used to test whether or not such trees are consistently unbalanced. We used recently developed null models to demonstrate that the shapes of large phylogenetic trees: 1) are similar among angiosperms, insects, and tetrapods; 2) differ from those expected due to random selection of a phylogeny from the pool of all trees of similar size; and 3) are significantly more unbalanced than expected if species diverge at random, therefore, conforming to one prediction of adaptive radiation. This represents an important first step in documenting whether adaptive radiation has been a general feature of evolution.     

PATTERNS OF GROWTH IN BIRDS

Parameters used to characterize the course of growth are described, and calculated growth parameters are presented for 105 species of birds of many taxonomic groups from a wide range of geographical localities. Growth parameters are found to exhibit as much as 20% variation within a species with respect to geographic locality and time of the nesting season. There is also considerable local variation, irrespective of season and locality, which is related to nutrition and perhaps to an inherited variability. The application of curve-fitting as a method of analysing intraspecific variation is discussed briefly, and the importance of comparative growth studies is emphasized. Growth patterns are correlated with other parameters of the life-history to evaluate the extent of diversity in the course of growth. Low rates of growth and prolonged growth periods occur primarily in species large for their families and in oceanic species. In most others, high rates of growth are maintained for longer periods of time. The shape of the growth curve is not related to the mode of development (i.e. whether precocial or altricial). Overall relative, or weight-specific growth rates, as measured by the constants of fitted growth equations, are most highly correlated with the adult body size of the species, changing as the -0–278 power of adult body weight. Smaller variations in the rate of growth appear to be correlated with differences in nesting success; open-nesting passerines grow faster than hole-nesting species of a similar size. Growth rate is further correlated with brood size. Oceanic species with single egg clutches and tropical land-birds with small clutches have low growth rates. The asymptote of the growth curve of the young (in relation to the adult weight) is related to the foraging behaviour of the adults. Aerial feeders generally have high asymptotes while those of ground feeding species are usually below adult weight. These differences are related to the need in the former for well-developed flight at the time of fledging. The diversity of growth patterns is related to evolutionary trends which are the result of (1) selective forces acting at stages of the life-history cycle other than development, (2) factors which affect the survival of offspring during the growth period, and (3) adjustments made to balance the energy budget of the family group. The last trend is discussed in detail in relation to the correlations found in the analysis. Two hypotheses are presented. Firstly, in species which cannot gather enough food to support even one young at a normal growth rate, the pace of development is reduced to decrease the energetic requirements of the young. Secondly, in species with small clutches, where adjustments to feeding capacities are not readily made by changing brood size, growth rate may be adjusted to accomplish this. The lack of critical energetic data to test these hypotheses is emphasized as a major deficiency in our understanding of the breeding biology of birds.     

OBSERVATIONAL LEARNING IN THREE SPECIES OF BIRDS

White-throated Sparrows Zonotrickia albicollis, Fork-tailed Flycatchers Muscivora tyrannus and Black-capped Chickadees Porta atricapillus were placed in experimental situations where they could watch a trained companion uncover a hidden mealworm. After a period of watching its companion find the concealed mealworm, the observer bird was isolated visually from the other bird. While in isolation the observer birds approached, touched and pecked at a hidden food tray significantly more often than control birds which had not had an opportunity to observe a trained companion. However, the experimental birds failed to learn from observation alone how to uncover the concealed mealworm. But if rewarded in the presence of the companion bird (by taking a mealworm from it or by uncovering a mealworm as a result of local enhancement), then the observer bird frequently did come to find the prey in isolation. It is suggested that the interaction between observational learning and a bird's reinforced socially facilitated experience could assist in the development and expansion of a bird's behavioural repertoire.     

ADAPTIVE RADIATION IN THE HAWAIIAN SILVERSWORD ALLIANCE (COMPOSITAE-MADIINAE). II. CYTOGENETICS OF ARTIFICIAL AND NATURAL HYBRIDS

The Hawaiian silversword alliance of Argyroxiphium, Dubautia, and Wilkesia, in spite of exhibiting spectacular morphological, ecological, physiological, and chromosomal diversity, is remarkably cohesive, genetically. This is attested to by the ease of production of artificial hybrids and by the high frequency of spontaneous hybridization among such life forms as mat-forming subshrub, monocarpic rosette shrub, polycarpic shrub, cushion plant, tree, and vine. Even the least fertile of these hybrids is capable of producing backcross progeny. Moreover, first generation interspecific and intergeneric hybrids have been successfully used to produce trispecific hybrids in a number of instances. In general, the widest hybrid combinations have been as readily produced as crosses within a species. At present eight genomes or chromosome races distinguished by reciprocal translocations are recognized on the basis of meiotic analysis of artificial and spontaneous hybrids. Seven of these races are found among those species with 14 pairs of chromosomes. The eighth genome very likely characterizes all nine species of this alliance that have 13 pairs of chromosomes. The cytogenetic data indicate that redundancy of translocations involving the same chromosomes has been a recurrent theme in the chromosomal differentiation of these taxa. There appears to be little, if any, correlation between chromosomal evolution and adaptive radiation as assessed by gross habital differentiation in this group. However, within Dubautia, a novel ecophysiological trait associated with colonization of xeric habitats is restricted to species with n = 13. In contrast to the bulk of the Hawaiian flora, which is characterized by self-compatibility and chromosomal stability, it is suggested that the occurrence of self-incompatibility in the Hawaiian Madiinae may have favored selection of supergenes via chromosomal repatterning, and this may account for the diversity of chromosome structure seen in this group.     

BIRDS AS SCAVENGERS OF REFUSE IN UGANDA

Refuse tips, abattoirs and fishing villages provide an important source of food for several species of large scavenging birds in Uganda. The most numerous are Marabou Storks, Hooded Vultures, Black Kites and Pied Crows. Estimates of the populations of these species are given for Kampala, several smaller towns in Uganda and Rwenzori National Park (where there is an important fishing industry). Crows and kites are spread widely through Kampala and the population densities of both species exceed 12 km^-2 (the latter seasonally). On average there were about 200 Hooded Vultures at the Kampala abattoir and nearly as many Marabous at the refuse tips. Abattoirs and refuse tips in smaller towns often had proportionately higher populations of scavengers than Kampala. The food consumption of these birds is calculated to be nearly 100 t per year in Kampala and 800 t per year in Rwenzori National Park. It is suggested that although man, through his various activities, provides most of the food for these scavengers, they are beneficial to him in the sense that the food which they eat might otherwise become a health hazard.