Showing papers in "Evolution in 1966"

A general theory of clutch size.

Abstract: It is possible to think of organisms as having a certain limited amount of time or energy available for expenditure, and of natural selection as that force which oper- ates in the allocation of this time or energy in a way which maximizes the contribution of a genotype to following generations. This manner of treatment of problems con- cerning the adaptation of phenotypes is called the "Principle of Allocation" (Levins and MacArthur, unpublished), and one of its applications might be the formulation of a general theory to account for clutch size in birds. At this stage we will assume that clutch size is a hereditary phenotypic characteristic which can be affected to a greater or lesser extent by the prevailing environmental conditions and which ex- hibits the normal variability of such char- acteristics. Lack (1954) discusses the validity of several hypotheses which' at- tempt to account for clutch size and its variation under different circumstances and conditions, all of which were rejected in favor of his now widely accepted theory that clutch size is adapted to a limited food supply. This paper is an attempt to show that this and other existing hypotheses when taken singly are inadequate in some respect to account for all the data, that each holds for some particular set of con- ditions, and that each is but a part of the complete explanation. The theories will be dealt with individually and it will be shown that as environment varies so will the fac- tors which determine clutch size. PRESENTATION OF THE THEORY

Coevolution of mutualism between ants and acacias in central america

Abstract: The purpose of this paper is to discuss the coevolution of one of the more thoroughly studied mutualistic systems in the New World tropics: the interdependency between the swollen-thorn acacias and their ant inhabitants. This system has recently been described in detail in respect to one species of plant, Acacia cornigera L. (Mimosoideae; Leguminosae), and one species of ant, Pseudomyrmex ferruginea F. Smith (Pseudomyrmecinae; Formicidae), and shown experimentally to be a case of mutualism (Janzen, 1966a). In this species pair, the ant is dependent upon the acacia for food and domicile, and the acacia is dependent upon the ant for protection from phytophagous insects and neighboring plants. The literature dealing only with the New World tropical acacias (Acacia spp.) and their ants (Pseudomyrmex spp.) has been re-evaluated by Janzen (1966a) and will not be discussed further in a review sense. The higher plants that commonly have ant colonies living in them have long been termed myrmecophytes. Ants living in plants range in habit from fortuitous usage of a plant cavity to highly complex interaction systems between the ant and the higher plant. The ant-acacia system represents this latter extreme, and the acacia is by any definition a myrmecophyte. A review paper on the subject of myrmecophytes is in preparation. In the present paper, plants with ants living in them will be called "ant-plants"; the ants will be called "plant-ants." The "swollen-thorn acacias" are those with 1) enlarged stipular thorns normally tenanted by ants, 2) enlarged foliar nectaries, 3) modified leaflet tips called Beltian bodies (eaten by the ants), and 4) nearly year-round leaf production and maintenance even in areas with a distinct dry season (Fig. 1). Swollen-thorn acacias have been shown experimentally to have a virtually obligate dependency on 1 Contribution No. 1308 from the Department of Entomology, The University of Kansas, Lawrence. This study was supported by National Science Foundation Grant GB-1428 to the University of California, Berkeley (Dr. R. F. Smith), University of California Associates in Tropical Biogeography Research Grants 66 and 81, and The University of Kansas General Research Grant 3173-5038, and also is a by-product of National Science Foundation Grant GB-91 (Dr. C. D. Michener).

Hybridization as a source of variation for adaptation to new environments

Abstract: We generally tend to think of a species as being characterized by particular geographical and ecological ranges, ranges which, except for minimum fluctuations at the margins, are as much characteristic of the species as morphology. Of course, we have many examples of radical expansion of the geographic range of a species. By and large such expansions are the result of the introduction, by man, of the species into an area that was previously too remote for propagules to arrive in the normal course of events or else a radical remaking of the environment, again by man usually, so that it is now hospitable to a new species. But both of these kinds of events are geographical expansions without being ecological expansions. Many such cases are documented in Elton (1958). A little reflection makes it obvious that real ecological expansions involving adaptation to new environments must be the usual occurrence in speciation and that the ecological range of any species is the result of an evolutionary process involving more or less profound genetic changes. The rarity with which such a change has been observed is undoubtedly a reflection of the fact that this process occurs only occasionally in the evolution of a given species, perhaps only once. Moreover, when it does occur, it may be a very slow process, thus difficult to observe. It is, nevertheless, a process of supreme importance both to the species and the community, and if a case could be found of a species rapidly expanding its ecological range, caught in flagrante delictu, it might be possible to study the genetic basis of such a change. It is necessary to distinguish cases of new adaptation from cases of preadaptation since both will appear as ecological expansions, but the latter involves little or no genetic reorganization. Many cases of insect pests attacking and decimating cultivated plants are cases of preadaptation as are instances of diseases spreading to new hosts. If a phytophagous insect is placed suddenly in contact with a new host and is not excluded by other phytophagous animals or predators, we might expect quite often a successful invasion of the new host and even a shift from the old host without major genetic change. How are we to distinguish such preadapted ecological shifts from cases of real change in the adaptive norm of a species? We might suspect the latter was happening if long established boundaries of its distribution began to move despite lack of change in relevant components of of the environment in the invaded area. Of course, we may never be certain that some subtle but relevant change has not occurred. However, this is a limitation that all studies of ecology and genetics of natural populations suffer from. One can never be certain he has exhausted the relevant aspects of the environment. One can only be critical, and hope. It is the purpose of this paper to describe a case of a species rapidly expanding its ecological tolerance and geographical range, and to give the results of some experiments that show how hybridization between species or near-species can provide the genetic variation necessary for natural selection to produce a new adaptive norm.

Body sizes of poikilotherm vertebrates at different latitudes.

Abstract: In the course of collecting freshwater fish both in a tropical region (Malaysia) and in a temperate region (Canada), I was struck by the seemingly higher proportion of small species in the hot than in the cold climate. An examination of recorded fish sizes from the two areas confirmed this impression. The same general trend was revealed when freshwater fish in many tropical regions were compared with those in many cold regions. The search' for global trends in body size with latitude was then extended to other poikilotherm vertebrate groups. Most previous discussions of variation in body size with latitude have centered around Bergmann's rule, which concerns a somewhat different phenomenon from that to be considered here. Bergmann's rule states that geographic races of a species are usually larger in cooler than in warmer climates. The rule concerns variation only within a species, or at most between series of closely related species. It is empirical and implies no physiological theory (Mayr, 1956), although arguments concerning its possibly physiological basis, particularly with respect to temperature control, have not been lacking (Scholander, 1956). Initially the rule was applied only to homeotherms, but recently Ray (1960) concluded from both experimental and field observations that the rule (and also Allen's rule) applied equally to many poikilotherms. In contrast, the present study examines whether a trend towards larger sizes in cool regions may be found when comparisons are made between whole faunas of poikilotherm vertebrates, including quite un-

The evolution, functioning and breakdown of heteromorphic incompatibility systems. i. the plumbaginaceae

Abstract: It is well established that, in certain circumstances, outbreeding and the consequent promotion of genetical variation are adaptively advantageous while, in other circumstances, the genetical fixity and reliability of seed-formation made possible by inbreeding fit the needs of a population more appropriately. In angiosperms, the degree of crossor self-pollination (not to mention the replacement of sexual reproduction by apomictic seedformation) is largely determined by the structure and physiology of the flowers and it is to comparative studies of these organs that we may look for evidence of the building-up, maintenance and, in appropriate circumstances, the breakdown of outbreeding mechanisms. One type of mechanism which is relatively easily studied, because its presence and condition may be ascertained visually even when experimentation is not possible, is that in which the physiological incompatibility system is linked to floral heteromorphy. Usually one of the morphological features of such a system is variation in style-length, hence the name heterostyly bestowed on it by Darwin (1877). Another advantage of heteromorphic incompatibility systems for this kind of study is their relatively simple genetical basis with only one or two incompatibility loci and usually only two alleles at such loci. Heteromorphic incompatibility systems are widely distributed amongst the families of angiosperms, suggesting that they have evolved separately many times (Baker, 1964). They appear to have broken down again even more often (Baker, 1961a, 1961b) and it is the purpose of the present paper and its companions in a series to indicate general patterns in the evolution of heteromorphic incompatibility systems (including those which are clearly heterostylous) , their functioning, and their breakdown to give monomorphic, self-compatible (or even apomictic) derivative populations. Within the family Plumbaginaceae, heteromorphic incompatibility systems are to be found controlling the breeding behavior of the plants in both of the recognized tribes (the Plumbagineae and the Staticeae). In the Staticeae, the heteromorphism includes dimorphism of the pollen and often of the stigmas as well (Macleod, 1887; Baker, 1948a; etc.). Such heteromorphism has been studied in fossil material by Kulczynski (1932) and Baker ( 1948b) as well as in living material by Erdtman (1940, 1942), Iversen (1940), Szafer (1945), and Baker (1948a, 1953a, 1953b, 1954b, 1954c). In Armeria maritima Willd., the two kinds of plant involved are self-incompatible but are thoroughly cross-compatible (Iversen, 1940; Baker, 1954b, 1954c).

The periodical cicada problem. ii. evolution.

Abstract: \"P.S. I really must bore you with the additional remark, that the tredecim seems to have been derived from the septendecim and not the reverse, for the following reason:-Where tredecim prevails, it is very often the case that a small brood appears in the preceding or succeeding year to the regular periodic time. (We can trace back the regular brood ... by the most indisputable evidence to A.D. 1829, 1842, 1855, and 1869.) Now, this does not seem to be the case with septendecim, or at all events but rarely. Hence I infer that tredecim is a sort of incipient species, which has not yet settled down into a perfectly regular periodic time.\" ... unpublished letter from Benjamin Walsh to Charles Darwin.

The periodical cicada problem. i. population ecology.

Abstract: Periodical cicadas, Magicicada spp., have the longest life cycles known for insects. In anyone population, all but a tiny fraction are the same age. The nymphs suck juices from the roots of deciduous forest trees in eastern United States (see Marlatt, 1907, for a complete account of the life cycle). Mature nymphs finally emerge from the ground, become adults, mate, lay their eggs, and die within the same few weeks of every seventeenth (or, in the South, every thirteenth) year. Not one species does this, but three. There are three separate and distinct species that occur together over most of the range of periodical cicadas (Alexander and Moore, 1962) and, wherever the species coexist, they are invariably synchronized. In different re-

Origin of deciduous and evergreen habits in temperate forests.

Abstract: Deciduous hardwood forests in temperate parts of the Northern Hemisphere, brilliantly colored in autumn and leafless in winter, provide a striking contrast with the somber broadleafed forests that extend to tree line in the temperate Southern Hemisphere and remain green through the year. These northern and southern temperate forests are also very dissimilar in composition, for the genera differ in each area and most of them belong to divergent families. The fossil record suggests that the morphologic differences between the plants that contribute to these forests are the result of their independent evolution on opposite sides of the tropical rainforest belt during Cretaceous and later times. Presumably the deciduous and evergreen habits displayed by woody plants in boreal and austral regions reflect fundamental differences between the environments in which they evolved. However, the factors that may account for the presence of a predominantly deciduous habit for woody angiosperms in the temperate forests of one region as opposed to typically evergreen in the other, are obscure, as is the evolution of the deciduous habit itself in northern temperate hardwood forests. In searching for clues to the factors that may have been responsible for the origin of these contrasting habits, it is desirable to turn first to evidence provided by plants living in environments where gradual gradients can now be observed between the evergreen and the deciduous habit.

The biota of long-distance dispersal. ii. loss of dispersibility in pacific compositae.

Abstract: Comparison of fruits and seeds in the endemic floras of oceanic islands with those of related species of mainland floras reveals that the insular species often have fruits or seeds notable for diminished dispersibility. This observation, which has been offered elsewhere (Carlquist, 1965, 1966) needs further explanation, documentation, and qualification. One way of demonstrating this phenomenon is a survey of the floras of oceanic islands of sufficient age and ecological opportunity to have fostered appreciable evolutionary change. The writer is currently preparing such a survey for the Hawaiian flora. Another means of demonstrating the phenomenon is to examine fruits or seeds of a large family, well represented on islands, notable for rapid evolution, and sensitive adjustment to new environments. This latter method, suggested as a means of analyzing various tendencies on islands (Carlquist, 1966), is particularly suitable for study of loss of dispersibility. Among plants, the family Compositae is an outstanding example of evolutionary plasticity; it is also the largest family of flowering plants. Viewed on other grounds, Compositae are also an excellent group for such a survey. Fruits of Compositae are often provided with exceptionally prominent dispersal mechanisms. Any diminution of these devices can be noticed readily. The excellence of dispersal mechanisms of mainland Compositae have permitted them to range far into the Pacific, so that representation of the family is sufficient for study of autochthonous alteration of fruit morphology on islands. The presence of Compositae on numerous islands is advantageous, for each island or archipelago serves as a replication, so multiple confirmations of loss of dispersibility can be afforded. Moreover, Polynesian Compositae represent not one, but a multiplicity of independent systematic groups within the family, also offering replications of any evolutionary phenomenon induced by insular conditions. Interestingly, Pacific Compositae are overwhelmingly American in affinity, thus providing alien notes in Polynesian floras, which are predominantly Indo-Malaysian in character. The fact that Polynesian Compositae are mostly American in origin derives from phytogeographic circumstance. The western Pacific is poor in Compositae, whereas the family is abundantly developed in the western portions of North and South America. Polynesian Compositae with American affinities can mostly be traced to mainland genera and even species with relative clarity. To estimate the degree to which dispersibility has been lost, one must have an approximate idea of the ancestral fruit form. Pacific species in which dispersal mechanisms are still relatively intact are also important, for they show earlier stages in loss of dispersibility. The peculiar morphology of fruits in certain Pacific Compositae, such as Dendroseris litoralis (Fig. 7G) or Bidens macrocarpa (Fig. 2K) first suggested to the writer a phenomenon which could be designated "loss of dispersibility." In addition, the prominence of loss of flight among insular insects and birds (viz., Zimmerman, 1948) suggested that among insular plants, a similar loss of vagility might occur. Ecological considerations have also proved suggestive. The reasons for loss of dispersibility in insular plants prove not unlike the reasons for ecological shift and loss of vagility in insular insects given by Darlington (1943) and Wilson (1959).

The biota of long-distance dispersal. iv. genetic systems in the floras of oceanic islands.

Abstract: Distinctive and characteristic modes of speciation, hybridization, floral biology, pollination relationships, and allied phenomena on oceanic islands have received scattered and limited comment from various authors. Indeed, the lack of studies in these fields on insular areas is noteworthy. These areas are particularly deserving of intensive study, and this paper may be considered more an invitation to further investigations than a summary of what is known. The data and commentary below have been presented as a means of suggesting the significance of such phenomena as the high degree of dioecism on islands, polymorphism of insular species, etc. These suggestions have been so organized as to place them into the context of what may be called the "insular syndrome" of interrelated evolutionary phenomena. Analysis of a single phenomenon on islands has not always led to sound or thoroughgoing interpretations. The floral morphology, breeding systems, ecological preferences, growth forms, dispersal mechanisms, and other features of insular species are closely linked together. When considered together, each feature is more comprehensible. The observations offered below are presented

The origin and significance of intraspecific polyploidy: experimental evidence from solanum chacoense.

Abstract: Where a polyploid series occurs among related species, as in the tuberous solanums, it is of interest to know what mechanisms exist that can initiate such a series and also what effect polyploidy, when established, has in preventing gene-flow between populations. Results obtained in crossing diploid (2n = 2x = 24) and colchicine-induced tetraploid (2n = 4x = 48) clones of the wild Argentinian species Solanum chacoense Bitt. have a bearing on these points and form the basis of this paper.

Feeding specializations and the classification of terrestrial salamanders.

Abstract: The feeding mechanism of the salamanders has never been discussed comparatively. While a detailed review must await a more complete knowledge of the morphology, ecology, and behavior of these animals, certain broad generalizations are now possible. These have direct bearing upon the classification of salamanders, and are attempted here. Salamander families with terrestrial representatives, the Hynobiidae, Ambystomatidae, Plethodontidae, and Salamandridae, each contain species which undergo transformation from an aquatic to a terrestrial existence. Both aquatic and terrestrial life forms have structural modifications correlated with their specific habitat. Thus one must discuss not only the phylogeny of functional shifts in feeding structures, but their ontogeny as well. The present discussion emphasizes the interaction of the tongue and palatal teeth. Minimal mention will be made of paedomorphic evolutionary pathways, notorious in salamanders. This is best handled elsewhere and at appropriate length. The result is a certain amount of taxonomic generalization. THE GENERAL PATTERN OF ADAPTATION Structural Progression of Tongue Types

On the origin of the jumping mechanism in frogs.

Abstract: The literature contains numerous passing remarks concerning the origin of the jumping mechanism in frogs. Among the recent comments on this issue, there have been some arguments for an aquatic origin (Boker, 1935; Griffiths, 1963; Nevo, 1964) while other workers favored a riparian origin (Schmalhausen, 1958; Eaton, 1959; Hecht, 1962). More recently Inger (1962) has argued against these views proposing that the jumping mechanism arose only after the animals became terrestrial. It is obviously dangerous to speculate about the origin of a particular adaptation in a group of essentially unknown ancestry (Parsons and Williams, 1963). Extrapolations of functional correlation or habit pattern from the Recent to the Triassic (or Carboniferous) are furthermore open to question. The lapsed time has given all survivors the opportunity to specialize; it imposes the risk that their specializations may well have been parallel. Yet extrapolations do provide an interesting exercise and the establishment of a step-by-step sequence may force a rethinking of the meaning of the \"functional\" steps ordinarily alluded to en passant. We here present, provisionaIly, such an analysis. We are grateful to the many friends who discussed this manuscript during its several stages and helped us formulate our ideas. Particular thanks are due to Gerd von Wahlert for some very cogent comments. The general functional studies of Gans are supported by N.S.F. grants G-21819 and GB-2460 and the work of Parsons by N.R.C. grant A-1724.

The origin of dioecism from heterostyly in nymphoides (menyanthaceae)

Abstract: Heterostyly refers to a relatively rare combination of morphological, physiological, and genetic mechanisms which promotes outbreeding in certain angiosperms. Under conditions where the selective forces favoring this degree of outbreeding are reduced or removed, one or all of the components of this character complex may break down with the result that inbreeding partially or almost wholly replaces outbreeding. There are numerous examples of such a breakdown of heterostyly in the direction of increased selfpollination in taxa belonging to various plant families. Because the complex combination of floral mechanisms and incompatibility characteristic of heterostylous plants is apparently generally effective in promoting outbreeding, the breakdown of heterostyly in the opposite direction, namely in the direction of increased outbreeding, is uncommon. Nevertheless, there is evidence that in at least two angiosperm families (Rubiaceae and Menyanthaceae) heterostyly has been replaced by an even more effective outbreeding mechanism in the form of dioecism. The purpose of this paper is to discuss the breeding systems of various distylous and dioecious members of the menyanthaceous genus Nymphoides and to present an account of the probable origin of dioecism in this genus. Nymphoides has an almost cosmopolitan distribution, occurring primarily in tropical and subtropical areas but also in certain temperate regions of both the Northern and Southern hemispheres. The plants are aquatic or subaquatic, and are typically limited to the still water of ponds, lakes, and slow-moving streams. In all Nymphoides species examined, maturation of floral buds is underwater. In the morning of anthesis the buds are raised above the water level and flowering occurs. After a few hours the corolla withers, the pedicel deflects, and subsequent fruit development is underwater. DIOECIOUS SPECIES Dimorphic heterostyly is widespread in the genera of Menyanthaceae, but dioecism seems to have been previously unrecorded for this family (Gilg, 1895). The two species of Nymphoides indigenous to the United States are dioecious. These are N. cordata, an inhabitant of ponds and slow streams ranging from eastern Canada to Florida and Louisiana, and N. aquatica, a species of similar habitats ranging from New Jersey to Florida and Texas. The two species are closely related but are distinguishable on foliar, root, and floral characters. The floral morphology of these dioecious species is very similar. The ovary of pistillate flowers is capped by a short style and a large fleshy stigma. The vestigial stamens of such flowers are relatively large and conspicuous, consisting of white anthers and short filaments. Alternating with these non-polliniferous stamens are staminode-like clusters of yellow trichomes (Fig. 1). The yellow corolla bases and additional tufts of yellow trichomes at the basal petal margins provide additional color contrast to the white corollas. Staminate flowers have well-developed stamens that produce copious pollen. The pistil of such flowers essentially lacks a style or stigma, but the ovary is large and is filled with ovules of approximately the size and number of those produced by pistillate flowers (Fig. 1). Nevertheless, despite repeated polli-

The evolutionary significance of mountain sheep horns.

Abstract: In proportion to body size, North American wild sheep (Ovis canadensis and Ovis dalli) carry the largest horns among ruminants. Their horns have been recorded as exceeding 51 inches (130 cm.) in length, and 16 inches (40.6 cm.) in circumference at the base (Anonymous, 1964). Old males may bear eight to twelve per cent of their body weight as horns. The horns of rams grow substantially in length and mass each year. This increase is greatest in the early years of life and smallest in later years. The horns of females remain quite small throughout life and rarely exceed nine inches (23 cm.) in length. The huge horns of rams invite enquiry as to what selection processes brought about and retain such disproportionate organs. Several functions can be proposed for ram horns. Horns act as weapons.-This implies that horns can inflict damage on opponents when used in fighting. This I have found. The tiny horns of mountain goat (Oreamnos), however, also serve as weapons (Geist, 1964). Hence this proposed function alone explains little about the evolution of sheep horns. Horns act as a shield.-During fighting rams attempt to catch the opponents' head blows with their horns (Fig. 1). There is little doubt that sheep horns function in this manner. Other bovids, however, also attempt to catch an opponent's attack with their horns, i.e., Bos and Bison. These cattle carry only small, curved horns. Again, this function explains by itself little about the evolution of mountain sheep horns. Horns act as shock absorbers.-During their frequent social interactions, rams collide head on. They may run on their hindlegs at each other and then jump into the clash. Since the rams may weigh about 220--250 pounds each, the collisions must be frightful. It appears quite reasonable hence, that the heavy horns function in absorbing the heavy impact. This appearance is deceptive. A number of Artiodactyla clash head on during fighting, or deal out blows with their heads. None carry horns to match those of sheep males. Though female sheep fight very similar to rams, they carry only tiny horns. The wart hogs (Phacochoerus) and giant forest hogs (Hylockoerus) in fighting clash head on after a run at each other (Mohr, 1960; Fradrich, 1965). They possess no horns at all. Bison bulls may clash head on with considerable force (McHugh, 1958). They have only tiny horns. Giraffes (Giraffa) deal out severe blows with their heads, but carry only little ossicones on the skull. All these species (including sheep of both sexes) possess however one feature in common. They have a pneumated skull with a double roof of bone over the brain. Since there is no correlation between horn size and the mode of fighting: I reject the "shock-absorption" function. Horns act as display organs.-This is an old, but unverified concept of obscure origin. Beninde (1937) stated it clearly. He proposed that large-antlered deer could intimidate small-antlered opponents with their horn size, and hence gain social advantages without fighting. This hypothesis was never carried through to its various logical conclusions and hence never verified or rejected. If the horns of mountain sheep have been selected for as display or intimidation organs, then the following predictions must be inherent:

The adaptive radiation of the phenacodontid condylarths and the origin of the perissodactyla.

Abstract: The mammalian order Condylarthra includes a heterogeneous assemblage of smallto medium-sized archaic omnivores and herbivores. Most families in the order flourished in the Paleocene and became extinct early in the Eocene. A few lineages, however, developed crucial adaptations which led to their emergence as new orders of mammals, one of which was the Perissodactyla. The origin of the Perissodactyla is better documented than that of any other order of mammals and provides an excellent opportunity to study the emergence of a major taxon. Dental evidence indicates that perissodactyls were derived from the condylarth family Phenacodontidae. To view in proper perspective the evolutionary changes which led to the origin of the Perissodactyla, it will be necessary to survey the adaptive radiation of the Phenacodontidae. The oldest true phenacodontid condylarth, Tetraclaenodon, first appears in faunas of middle Paleocene age, and by the beginning of the late Paleocene appears to have radiated into three main groups, represented respectively by Phenacodus, Ectocion, and an as yet unknown proto-perissodactyl. Forms transitional between Tetraclaenodon and Phenacodus (primitive species of Phenacodus), and between Tetraclaenodon and Ectocion (the genus Gidleyina) are known, but no intermediates between Tetraclaenodon and the most primitive known perissodactyl, the early Eocene genus Hyracotherium, have yet been found. However, Tetraclaenodon is the most advanced form which is still unspecialized enough to have given rise to Hyracotherium. (The occurrence of incipient mesostyles in a small number of Tetraclaenodon specimens does not preclude this possibility; the alternative hypothesis, that proto-perissodactyls and Tetraclaenodon were independently derived from a still more primitive common ancestor, requires an additional complicating factor-an independent acquisition of molar hypocones by perissodactyls and phenacodontids.) Thus, in the absence of evidence to the contrary, Tetraclaenodon may be considered directly ancestral to perissodactyls. The major morphological changes involved in the evolution of the Tetraclaenodon stock into Phenacodus, Ectocion, and Hyracotherium, fall into two functional categories, one concerned with mastication and the other with locomotion. MASTICATION Dentition

Functional morphology and evolution of the ceratopsian dinosaurs.

Abstract: In 1902, H. F. Osborn suggested that the ceratopsian or horned dinosaurs would prove to be "diphyletic" with Torosaurus and Triceratops representing terminal members of two divergent evolutionary trends-one lineage typified by fenestrated parieto-squamosal frills and a second line featuring non-fenestrated frills. Subsequent discoveries have altered some of the details, but these two Lance-age genera are still considered distinct and as yet no other unquestionable contemporaneous genera have been discovered. Consequently, most phylogenies so far proposed (Lull, 1908; Lambe, 1915; Lull, 1933; Schlaikjer, 1935; Colbert, 1948; Sternberg, 1949), although differing in details, have adhered to a "bilineal" evolutionary history for the Ceratopsia. These lineages are now commonly referred to as the "long-frilled" and "short-frilled" lines. If the fossil record has been interpreted correctly and existing ceratopsian specimens do in fact represent two distinct evolutionary lineages, then there must have been a functional reason for these divergent trends. The object of this study was to explore the functional morphology of certain distinctive ceratopsian features in an attempt to assess the adaptive significance of recorded morphologic changes and perhaps thereby the "why" of ceratopsian evolution. In the following discussion, reference is made to specimens in the collections of several institutions, the names of which are abbreviated as follows: American Museum of Natural History (AMNH); National Museum of Canada, Ottawa (NMC); United States National Museum (USNM); Peabody Museum of Natural History, Yale University (YPM). CLASSIFICATION

Estimation of conservatism of characters by constancy within biological populations.

Abstract: The concept of conservatism is basic to much of taxonomy. A conservative character is usually defined as one that has changed more slowly than a non-conservative one during phylogeny. The reliability of a character for placing organisms properly in evolutionary schemes is also included in conservatism. It is argued quite validly that a character which changes slowly is the best kind of character for discerning the evolutionary relationships of organisms. This is true in practice because of the very incompleteness of the historical record of phylogeny. When not all the forms of a line are known, there will be gaps in the record of rapidly-changing characters. But slowly-changing ones may still have a fairly complete series represented, even if only a few forms are known. The patterns of evolutionary relationship are easier to trace in characters that evolve slowly. It is usually assumed that not all characters are equally reliable, and many taxonomic problems can be reduced to the process of recognizing the most reliable ones. It has long been the practice to select conservative characters by the criteria of constancy within known groups and estimated biological importance. This approach has been criticized by numerical taxonomists (Sokal and Sneath, 1963), who point out that biological importance is seldom known, and that calling a character conservative if it is constant within a known group constitutes circular reasoning, if the group was constructed on the assumption that the character was conservative. It is not the concept of conservatism itself that is invalid, but only the means that have been suggested for finding conservative characters. There is little doubt that if a character is constant throughout a group that is natural in an evolutionary sense, then that character is a more reliable indicator of evolutionary relationship than is a character that varies widely in the same group. I shall present an argument to show how it may be inferred which characters are most likely to be conservative. Suppose that a given character is conservative in that it varies little in a given natural group. The variability of that character between the populations of that hypothetical group can only be small if the variability within single biological populations is also small. Otherwise random variation alone would be expected to produce wider variation within the group. Now it has been argued that the fitness of a population is maximized when the population is carrying as much heterogeneity as it can (Mayr, 1963). But then we expect a character to vary little within populations only if the centripetal selective forces acting on it are relatively intense. Thus we would expect that a conservative character would be characterized by a low variability within populations, in turn due to heavy selection on that character. On the other hand, it seems fair to say that a character with high centripetal selection acting on it is of great adaptive importance to the organism, and conversely. Then we expect a character with low variability within populations to be conservative, at least in the sense that it is of great biological importance: it is important to the survival of the organism. It is likely that it would be relatively difficult for such a character to change during the course of evolution. Because of the intense centripetal selection acting on it, a population un-

Morphological variation in natural and experimental populations of Drosophila pseudoobscura and Drosophila persimilis

Abstract: Drosophila pseudoobscura and its sibling D. persimilis are two species of fruit flies which have received considerable attention from the standpoint of population genetics. Examination of the arrangements (inversions) in the third chromosome in salivary gland cells of the larval progeny of flies derived from many geographic localities throughout the distribution of these species reveals that (1) populations within a locality are largely polymorphic; (2) populations from different altitudes and latitudes differ in the frequency of chromosomal arrangements; (3) the frequency of these arrangements varies from season to season; and (4) there are secular changes in the frequency of these inversions (see Dobzhansky, 1951, p. 138, for map showing frequency of the different inversions in D. pseudoobscura through most of its distribution, and Dobzhansky et al., 1964, for the most recent survey of inversion frequencies in this species). Laboratory studies showing that the frequency of these inversions is modified through natural selection and manipulation of environmental conditions are too numerous to mention (review in Strickberger, 1963). A few of these studies (for example, Beardmore et al., 1960; Tantawy, 1961) have combined the use of inversions and quantitative traits to measure the relative fitness of different gene arrangements under different constant and fluctuating temperatures, and one study (Druger,

Birds as agents of long-distance dispersal for disjunct plant groups of the temperate western hemisphere.

Abstract: The existence of disjunct plant distributions has for years prompted plant geographers to offer imaginative accounts of plant dispersal methods. Favorite hypotheses have included dispersal by land bridges, drifting continents, ocean currents, and birds. The last explanation is perhaps the least controversial, although no better understood, as far as long-distance dispersal is concerned. Birds do possess characteristics that appear to qualify them, at least superficially, as agents of long-distance dispersal. It is my intent to explore the plausibility of birds carrying seeds across the tropics in one step from temperate western North America to temperate South America. Admittedly such an event, if it occurs, must be rare; however, the reasons for this rarity are not obvious, especially to the nonornithologist. Long-distance dispersal must also be a repeatable, even if random event; but not fortuitous, as Setchell (1926) has indicated in suggesting that birds blown by storms might be responsible for the movement of seeds. Botanists, in particular, have repeatedly cited the phenomenon of bird migration in attempting to solve problems of long-distance dispersal. However, they have not adequately considered the phenomenon and the processes involved, and have consequently remained unaware of basic weaknesses in their hypotheses. In view of current interest in long-distance dispersal, an examination of various facets of bird migration and birds as dispersal agents is valuable and timely. I will briefly mention the time of origin of bird migrations, discuss the dispersal of seeds by birds, review the salient features of the disjunct plant groups, comment on the various birds which migrate from the United States to Argentina and/or Chile, and, overall, consider birds as long-distance dispersal agents. Finally I will discuss morphological variation and breeding systems with respect to long-distance dispersal and suggest "mountain hopping" as an alternative to one-step, long-distance dispersal. The birds are considered with respect to the plausibility of the hypothesis that they transport seed between California and temperate South America. Although not restricted within these geographical boundaries most of the plants in question are represented in the California flora and most of the birds that migrate through the western United States migrate through California. Furthermore, factors limiting longdistance dispersal between these temperate regions apply to long-distance dispersal in general. I have purposely omitted from the discussion consideration of 16 plant species that live in coastal situations (cf. Raven, 1963, appendix IIB). First, shorebird dispersal of coastal plants is more probable than shorebird dispersal of the "inland" disjuncts considered below. Second, ocean currents might serve as a means of dispersal for these plants. With respect to biology, etc., what is true of the "inland" disjuncts is probably true also of the coastal disjuncts.

The population biology of the butterfly, euphydryas editha. iv. sperm precedence-a preliminary report.

Abstract: The reproductive biology of the checkerspot butterfly, Euphydryas editha, is being investigated in order to integrate the details of the mating system of this nymphalid with other aspects of its population biology which have been elucidated (Ehrlich, 1961; Ehrlich and Mason, 1966; Labine, 1964). In the course of these investigations it was found that some females of E. editha in natural populations mate more than once during their reproductive life. For example, spermatophore counts showed that about a third of the females collected late in the 1964 season from the Silver Creek population, San Jose, California, had mated twice. Females have considerable capacity for sperm storage. In laboratory matings a single insemination is usually sufficient to fertilize all of a female's eggs, which can occasionally number 1200 or more. Clearly, most females mating for the second time in the field have, at the time of mating, sperm from their first mate stored in the spermatheca. Thus, they will subsequently lay eggs which theoretically could have been fertilized by either of the two males. The apportioning of paternity after second matings is obviously of great selective importance to individual males, and its specification seemed critical to an understanding of the mating system of E. editha. Two possible alternatives have been considered: either the two males mating with the same female contribute offspring to the the next generation in numbers directly proportional to the relative amounts of their sperm in the spermatheca, or one of the males fertilizes a disproportionate number of the eggs. In Tribolium (Schlager, 1960) and Drosophila (LeFevre and Jonsson, 1962) the latter alternative has been found. Females mating with several males lay eggs which have been predominantly fertilized by sperm of the last male to mate. This phenomenon has been described as "sperm precedence" or "sperm displacement." The following experiments were undertaken to determine if sperm precedence occurred with multiple matings of E. editha. It should be emphasized at the start that Euphydryas editha is a difficult animal with which to work in the laboratory. Although much effort has been expended, techniques of continuous culture have yet to be devised. Until such efforts are successful, animals can be obtained only once a year, and then only for a short period. The work reported in this paper extended over two years. Techniques were refined during the spring of the first year, and during the spring of the second year, the experiments described below were performed. The results obtained are in some ways unsatisfactory. The number of animals involved was of necessity small, and the definitiveness of the results clearly could be improved by additional experimentation. On the other hand, the results do provide persuasive evidence of the operation of sperm precedence in Euphydryas editha. Since this information directs attention to a ph'enomenon which is probably of considerable importance in shaping reproductive strategies in Lepidoptera, and which also provides an explanation for some of the peculiarities of the reproductive biology of Euphydryas editha, presentation of these preliminary results seems justified. 1 This work was supported in part by Grant GB-1430 from the National Science Foundation and Grant 5T1-GM-365 from the Public Health Service.

The temporal component of diversity among species of birds.

Abstract: Interspecific competition in birds may be reduced as a result of (1) morphological diversity, (2) partitioning of the environment by means of specialization and habitat selection, and (3) staggered nesting seasons. The increased species diversity in the tropics, relative to temperate latitudes, requires that one or more of these factors be operating more intensively. Klopfer (1962, fig. 9) has demonstrated similarity of bill size diversity between the birds of Panama and those of Massachusetts. Also, a comparable analysis of the weights of tropical and temperate "passerines" shows similar body size diversity in the two groups (Ricklefs, in preparation). Recently, MacArthur (1965) has reviewed the roles of specialization and habitat selection in reducing competition and has pointed out that a large component of increased bird species diversity in the tropics may be accounted for by increased turnover of species between habitats in addition to increases within habitats. Klopfer (1962, pp. 76-77 and fig. 7) noted that "in the tropics many related species have staggered their breeding cycles," but few data were presented and no temperate comparison could be made. MacArthur (1964) also investigated the role of staggered nesting seasons, analyzing data from England (Lack, 1950) and Costa Rica (Skutch, 1950, 1954, 1960). A quantitative estimate of temporal diversity was made by converting raw data (the number of nests begun during each month) to equally probable months for nesting (hereafter, months) using the expression e-1plOg,pi,

Zoogeography and evolution.

Abstract: Many biologists have recognized that most of the evolutionary experiments made by nature are failures in the sense that they do not survive long and do not give rise to continuing phyletic lines. It has been estimated that, of all species that have lived on the earth, less than one per cent are now living, and it has been stated: "There is no way to predict, as far as the incipient species is concerned, whether the new niche it enters is a dead end or the entrance into a large new adaptive zone" (Mayr, 1963: 621). From a zoogeographic standpoint, however, it can be said that the chances of success for a newly formed species are liable to be greatly influenced by its place of origin.

Ecological and cytogenetic studies of drosophila flavopilosa, a neotropical species living in cestrum flowers.

Abstract: The flavopilosa group of species of Drosophila, subgenus Drosophila, was established for D. flavopilosa Frey and 13 other species from the Neotropical region (Wheeler et al., 1962). In Chile, the exclusive natural breeding and feeding sites of D. flavopilosa are the flowers of the solanaceous plant, Cestrum parqui L'Her. Recently, Dr. A. Hunter from the Los Andes University (Colombia) and the present author, found inside the flowers of another species of Cestrum (C. tomentosum Sandwith) near Bogota, larvae of two other species of the group (D. acroria and another form not yet determined). These observations suggest that the members of the group are mainly pollen feeders, and probably are associated with different species of Cestrun. Although up to now it has not been possible to breed D. flavopilosa under laboratory conditions, the analysis of larval samples collected directly in their natural breeding sites, has allowed the author to study the chromosomal structure of some Chilean populations of the fly (Brncic, 1962). These studies have demonstrated that the populations of D. flavopilosa are polymorphic for the gene arrangements in their chromosomes, and that the frequencies of certain heterokaryotypes vary according to the geographic regions. These studies have also shown that there is an altitudinal gradient in the distribution of some of these gene arrangements. Such an analysis was interesting for many reasons.

The adaptive significance of gastropod torsion

Abstract: Unique among mollusks, the Gastropoda are characterized by a curious asymmetry, resulting from a rotation of the visceral hump and shell. Thus in the earlier stages of their embryonic development, gastropods have the same arrangement of parts as other mollusks, with the rudiments of the mantle cavity, gills, coelomic ducts, and associated structures at the posterior end of the body. But eventually, the entire shell, and along with it the mantle cavity and its contained organs, undergoes a counterclockwise rotation, relative to the head and foot, of about 180 degrees. As a result, the mantle cavity, gills, etc., come to lie directly above the head, and the internal parts undergo a rearrangement (Figs. 1, 2). This displacement, referred to as torsion, should not be confused with the spiral coiling of the shell and visceral hump, which is a separate disturbance of symmetry. Common garden snails have both torsion and coiling; the shells of limpets are no longer coiled, but torsion may be seen in the twisted condition of the internal structures. The questions of how torsion originated and what, if anything, is its adaptive significance, are important for several reasons. Torsion is the diagnostic character of the second-largest class in the animal kingdom (in number of described species); an understanding of the process which gave rise to such a taxonomically important property may cast some light on major problems of macroevolution. In the historical analysis of the theoretical literature, one may also gain valuable insights into the validity of various kinds of phylogenetic arguments. It is especially revealing to consider the weakness of some arguments which are widely accepted. Finally, one theory explaining torsion invokes both paedomorphosis and what almost amounts to a macromutation. Not only is this theory widely held, but its truth is used as a premise in theoretical arguments (e.g., de Beer, 1958); the broad implications of such theoretical discussions necessitates rigorous testing of their fundamental premises. Even if one cannot discover which explanation is correct, the elaboration of alternatives removes the support for theories derived from one of them. The following discussion attempts a critical review and synthesis of the problem of torsion, and suggests a new explanatory theory.

Selection for and against ethological isolation between drosophila pseudoobscura and drosophila persimilis.

Abstract: In many groups of animals the ability to recognize mates of one's own species is perhaps the most important factor preventing interspecific hybridization and gene transfer between related species. The behavior upon which such discriminatory ability rests is known as ethological or sexual isolation. In populations of Drosophila, ethological isolation has been demonstrated experimentally by means of sexual isolation tests. Wallace (1954) and Knight et al. (1956) have shown that genetic variability exists within species upon which incipient ethological isolation can be built; Koopman (1950) demonstrated that ethological isolation between a pair of species can be strengthened by selection. In backcross experiments, Tan (1946) and Ehrman (1961) found that ethological isolation in Drosophila is controlled by polygenic systems. Ethological isolation, then, like other polygenic characters, should be susceptible to modification by artifical selection techniques. The present study aims to ascertain if this is indeed so. It differs from previous studies chiefly in two ways: 1) an attempt is made to weaken as well as to strengthen the ethological isolation between two closely related species; and 2) progress in selection is measured in the two sexes of each species separately, so that differential effects become apparent.

Distribution, coexistence, and competition of whirligig beetles.

Abstract: At present we know very little about the degree to which similar species, occurring together, may overlap in their utilization of a given array of resources. Larkin (1956) has suggested that this overlap may be great among freshwater organisms, particularly fish. MacArthur (1958) concluded that the opposite was true among birds of the north temperate zone. Restriction of ecological overlap would presumably arise from interspecific competition and it has been claimed that interspecific competition is a frequent consequence of the manner in which natural communities are organized (Hairston et al., 1960). Therefore, attempts to generalize about the diversity and relative abundance of species in a community or in a single trophic level might logically proceed from reasoning about how resources are apportioned through competition (MacArthur, 1957, 1960; Hairston, 1959). The effectiveness of this approach would be enhanced if definite statements about the relationship of interspecific competition and the division of resources among species could be derived from nature. The fact that some ecologists (Andrewartha and Birch, 1954; Thompson, 1956) have been unable to accept competition as an integral feature of community organization further indicates a need for specific field studies. The concept of interspecific competition has been derived in four, rather distinct ways: (1) theoretical investigations (Lotka, 1925, 1932; Volterra, 1926; Gause, 1934; Gause and Witt, 1935; Winsor, 1934; Hutchinson and Deevey, 1949); (2) laboratory experimentation (Gause, 1934; Gause et al., 1934; Park, 1948, 1954, 1957; Crombie, 1945, 1946; Birch, 1953; Utida, 1953; Frank, 1952, 1957; Mucibabic, 1957a, 1957b); (3) observation of local species distributions (Tansley, 1917; Broadhead, 1958; Williams, 1963); and (4) studies concerning the evolution of modern species (Lack, 1947; Bowman, 1961; Ross, 1962; Mayr, 1963). A clear understanding of what is meant by the term interspecific competition has been achieved as a result of the variety of endeavors just summarized, and Birch (1957) has composed a verbal definition. Birch correctly insisted on a strict meaning for the term and offered the following definition: "Competition between animals occurs when a number of animals (of the same or of different species) utilize common resources the supply of which is short. . . Interspecific competition then, refers to the interactions occurring between two or more species populations when they attempt to persist in the same place despite an overlap in their utilization of the same, limiting (insufficient) environmental resources. The ideas of "overlap" and "limiting" are crucial. However transient the periods of ecological overlap and resource limitation may be, they constitute competitive situations. A striking feature emerging from the whole gamut of studies involving interspecific competition is the elusiveness of this interaction in nature. It is very difficult to recognize and even more difficult to demonstrate competition. While theoretical 1 This paper is part of a thesis submitted to the University of Michigan for the degree of Doctor of Philosophy. The complete field and laboratory data are available in the thesis appendix or on microfilm from University Microfilms, Ann Arbor, Michigan. 2 Present address: Department of Biology, University of Rochester, Rochester, New York.

Studies on the genome constitution of triticum timopheevi zhuk. ii. the t. timopheevi complex and its origin.

Abstract: The evolution of wheat is, broadly speaking, well understood today; it originated in the Middle East, where species formation occurred primarily through spontaneous hybridization and polyploidization. Three diploid species, namely T. monococcum (Kihara, 1924), Aegilops speltoides (Sarkar and Stebbins, 1956; Riley et al., 1958) and Aegilops squarrosa (McFadden and Sears, 1946; Kihara, 1944) all with n = 7 chromosomes, have contributed genomes to the formation of the allotetraploids and an allohexaploid. The genomes of each of these ancestral diploid species are structurally different and have been classified as A, B, and D, respectively (Kihara, 1924; Sears, 1948, 1956). All tetraploid wheats are now known to possess the AB genome, whereas the hexaploid has the ABD genomes. When we consider the evolutionary history of the wheats in more detail, however, many gaps in our knowledge are evident. One such gap is the origin of T. timopheevi (n = 14), a species that until recently has held an enigmatic position in the evolution of the wheat genus. The problems that surround this species can be appreciated if we consider first some aspects of the whole group of the tetraploid wheats, to which this species belongs. The tetraploid wheats can be divided into two groups, the Emmer group and the Timopheevi group. The Emmer group is morphologically polymorphic as is indicated by the numerous species that taxonomists have recognized (Table 1, Fig. 1B). Of these species T. dicoccoides represents the wild member of the Emmer group (Sears,

Immunology and rates of enzyme evolution in the amphibia in relation to the origins of certain taxa.

Abstract: Along with an increased interest in biological evolution on the part of biochemists there has been a development of biochemical methods suitable for comparative studies. The existence of such techniques has in turn interested evolutionary biologists in their possible applications. Such methods, aimed mainly at dealing with proteins, include electrophoresis, study of the kinetics of biologically active molecules such as enzymes, ultracentrifugal properties, amino acid compositions, fingerprint patterns, amino acid sequencing, and various immunological methods (Wilson and Kaplan, 1964; Wilson et al., 1964). Perhaps the ideal method is the determination of the amino acid sequences of homologous proteins, as has been begun in cytochrome c (Margoliash, 1963; Smith and Margoliash, 1964) and in hemoglobin (Braunitzer et al., 1964; Hill et al., 1963; Hill and Buettner-Janusch, 1964), but this involves a very great deal of time and work and might fairly be characterized as cumbersome, and therefore not a choice method for extensive comparisons and surveys. In our search for more convenient methods of corm-paring biological macromolecules we have investigated in particular the possibilities of quantitative immunological procedures as methods that would measure structural differences over larger portions of a molecule than would any other available method except sequencing. In this paper we will present our findings bearing on certain evolutionary problems in the Amphibia.

The relationships of the tree shrews: the evidence of the nervous system.

Abstract: In a recent paper Van Valen (1965) re-evaluates the evidence for inclusion of the Tupaiidae in the Order Primates. He not only examines previous work but adds new data from his own studies. As a result of his re-evaluation, he concludes that \". . . a special tupaiid-primate relationship is possible but unlikely and that the similarities between recent tupaiids and primates are probably convergences and primitive retentions.\" The only body of evidence which he did not consider was that derived from studies of the central nervous system by Le Gros Clark (1924, 1926, 1929), and summarized in his general works on primate evolution ( 1934, 1959) . To my knowledge there has been no further original work published on the neuroanatomy of tupaiids until recently (Jane et al., 1965; Campbell, 1965; Tigges, 1966; Campbell et al., 1966; Glickstein et al., 1966). In addition, it appears that no one has critically re-examined Le Gros Clark's arguments based on the anatomy of the nervous system. I will attempt here to review Le Gros Clark's major arguments as well as describe some recent findings derived from my own work, some of which has been done in collaboration with Drs. John A. Jane and David Yashon. Le Gros Clark ( 1959) summarizes those characters in which the tupaiid brain contrasts with the brain of insectivores. 1. The relative size of the brain as a whole. 2. The expansion of the neopallium, accompanied by a displacement downwards of the rhinal sulcus. 3. The formation of a distinct temporal pole of the neopallium. 4. The backward projection of the occipital pole. 5. The presence of a calcarine sulcus.