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Morphology and Evolution of the Wrists of Burrowing and Nonburrowing Shrews (Soricidae)
Abstract
Forelimb burrowers are expected to have traits in the wrist that restrict freedom of movement relative to nonburrowers, preventing deflection of the hand by obstructions and reducing the energetic cost of digging. Wrist skeleton and ligaments were compared among representatives from several lineages of shrews. Only two traits differ between burrowers and nonburrowers; neither affects wrist function. In addition, all shrews have several traits expected to be present only in burrowers, indicating that wrist evolution was conservative and that the shared traits may have been inherited from a burrowing ancestor. The few wrist traits that differ indicate that functional interactions between anatomical elements may play a role in determining patterns of morphological evolution.
... Unlike previous studies, which have primarily examined the loss of soft tissues around the hinge, condyloid/ellipsoidal, or ball and socket diarthrotic (highly mobile) joints of major long bones (Schwarz et al., 2007;Bonnan et al., 2010;Holliday et al., 2010), this study explored whether soft tissue loss in the predominantly gliding/planar (i.e., more restrictive diarthrotic articulations; Yalden, 1966b;Swiderski, 1991) wrist joints exhibit the same pattern of changing mobility as those of the elbow and shoulder joints (Hutson andHutson, 2012, 2013). We then had the secondary objective of using these results to elucidate the function of automatic wrist folding in crocodilians. ...
... We demonstrated that this conservative method is not applicable to the hinge-like elbow and ball and socket shoulder joints (see also Bennett, 1997). However, the lack of freedom of movement in more firmly interlocking gliding/planar diarthrodial joints, such as in carpal and tarsal joints (Yalden, 1966b;Swiderski, 1991), strongly suggests that ROM in these types of joints (viz. which are not adapted to hyperextend, etc.) is often limited by the extent of articular surfaces in these areas, rather than impaction. ...
... This approach produced ROM4 and ROM5 values that were smaller than ROM1 values for both S. camelus wrist flexion and A. mississippiensis wrist folding. To reiterate, however, this methodology may only apply to firmly interlocking carpal and antebrachiocarpal joints that are not adapted for hyperextension, etc., as has been previously reported in ROM studies for some therian wrist joints (Swiderski, 1991). ...
A recent study hypothesized that avian-like wrist folding in quadrupedal dinosaurs could have aided their distinctive style of locomotion with semi-pronated and therefore medially facing palms. However, soft tissues that automatically guide avian wrist folding rarely fossilize, and automatic wrist folding of unknown function in extant crocodilians has not been used to test this hypothesis. Therefore, an investigation of the relative contributions of soft tissues to wrist range of motion (ROM) in the extant phylogenetic bracket of dinosaurs, and the quadrupedal function of crocodilian wrist folding, could inform these questions. Here, we repeatedly measured wrist ROM in degrees through fully fleshed, skinned, minus muscles/tendons, minus ligaments, and skeletonized stages in the American alligator Alligator mississippiensis and the ostrich Struthio camelus. The effects of dissection treatment and observer were statistically significant for alligator wrist folding and ostrich wrist flexion, but not ostrich wrist folding. Final skeletonized wrist folding ROM was higher than (ostrich) or equivalent to (alligator) initial fully fleshed ROM, while final ROM was lower than initial ROM for ostrich wrist flexion. These findings suggest that, unlike the hinge/ball and socket-type elbow and shoulder joints in these archosaurs, ROM within gliding/planar diarthrotic joints is more restricted to the extent of articular surfaces. The alligator data indicate that the crocodilian wrist mechanism functions to automatically lock their semi-pronated palms into a rigid column, which supports the hypothesis that this palmar orientation necessitated soft tissue stiffening mechanisms in certain dinosaurs, although ROM-restricted articulations argue against the presence of an extensive automatic mechanism. Anat Rec, 2014. © 2014 Wiley Periodicals, Inc.
... Phyletic shape change may therefore be more conservative in the carpus (and tarsus) due to the constraints imposed on a multifaceted performing system. Consequently, carpal variation may often be uncorrelated with behavioural differences (Swiderski, 1991;Szalay, 1994). Because of this functional constraint, bones of the wrist, as well as those of the ankle, are regarded as potentially important for resolving phylogenetic relationships of mammalian taxa (Court, 1994;Szalay, 1994;Luo et al., 2003;Salton & Szalay, 2004). ...
... Relative mediolateral width of the distal articular surface of the radius and ulna is indicative of locomotor behaviour in some primates (Godinot & Beard, 1993), insectivorans (Yalden, 1966), and carnivorans (Hopwood, 1945). Climbers and diggers have a mediolaterally expanded distal antebrachium, which bears shifting ulnar and radial loads associated with climbing and digging (Swiderski, 1991;Godinot & Beard, 1993). Cursors and ecologically generalized quadrupeds have a dorsopalmarly wider distal antebrachium (Hopwood, 1945), which bears the loads associated with movement along the axes of palmar and dorsiflexion. ...
... In several eutherian taxa, the shape of the distal ulna correlates strongly with positional behaviour (Hopwood, 1945;Yalden, 1966;Swiderski, 1991;Hamrick, 1997;Hamrick et al., 2000;Thorington & Darrow, 2000). This is also the case for tenrecoids (Figs 3, 4). ...
The mammalian carpus can be difficult to interpret both phylogenetically and functionally. It is evolutionarily constrained in terms of functional morphology, yet there is considerable variation among many eutherian and metatherian lower and higher level taxa. The ecologically diverse Tenrecoidea (Mammalia) is a useful model for morphological interpretation of the interplay between function and phylogenetic constraint. Elements from the wrist and hand of 13 tenrecoid species, and one species each from Macroscelididae, Solenodontidae, and Erinaceidae, were compared to test form–function hypotheses of specific carpal, metacarpal, and phalangeal characters. Qualitative comparisons illustrate that several aspects of the tenrecoid carpus can be correlated with positional behaviour. Convergences within Tenrecoidea, and between tenrecoids and nontenrecoids with similar locomotor regimes, confirm a small number of carpal characters and a larger number of distal forearm, metacarpal, and phalangeal characters that reliably correspond with functional expectations. In addition, several features of the carpus appear to be phylogenetically constrained and indicate specific affinities within Tenrecoidea. Finally, there are a significant number of carpal features that vary among the studied taxa and remain ambiguous in terms of phylogenetic and/or functional significance. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 93, 267–288.
... as a starting point for the clock rate. We used the independent gamma rates model to estimate relaxed clock rate variation, and a fossilized birth-death prior on branch lengths with a speciation prior ('speciationpr') of exp (10) and flat priors for extinction ('extinctionpr') and fossilization ('fossilizationpr') priors. We set the sampling probability of terminal lineages ('sampleprob') to 0.09, as our sample of 44 extant species comprises 9% of extant lipotyphlan species. ...
Shrews are among the most speciose of mammalian clades, but their evolutionary history is poorly understood. Their fossil record is fragmentary and even the anatomy of living groups is not well documented. Here, we incorporate the oldest, most complete fossil shrew yet known into the first phylogenetic analysis of the group to include molecular, morphological and temporal data. Our study reveals previously unknown diversity among total- and crown-group soricids. This includes a novel element of the mammalian skeleton: a robust, needle-like sesamoid extending cranially from the second thoracic neural arch in Myosoricini, comparable in length to these species' humeri. Additionally, ‘red-toothed’ shrews have an unusually elongate basicranium, and ‘white-toothed’ shrews probably evolved from a common ancestor with dental pigmentation. The fossil Domnina and crown soricids have a double-jaw articulation and incomplete zygomatic arch, but unlike nearly all crown species, Domnina has open vomeronasal canals and a tympanic process of the basisphenoid. Domnina and other heterosoricids are phylogenetically outside crown Soricidae. The oldest, well-supported total-group soricoids are North American, not Asian, and Soricidae probably originated during the Palaeocene or early Eocene. The diverse mammalian genus Crocidura originated and began to diversify during the Miocene.
... Multiple studies have done qualitative examinations of carpal bones to assess their utility in locomotion and phylogeny (Argot, 2001;Lehmann, 1963;Salton & Sargis, 2008;Stafford & Thorington, 1998;Thorington & Darrow, 2000). Many of these studies found morphological correlations between carpal morphology and locomotor mode; however, at least one found no correlation between carpals and locomotor mode (e.g., Swiderski, 1991). Yalden (1970) did a qualitative analysis of carnivore carpals and determined that there are differences in the carpals based on locomotor modes but did not give a quantitative breakdown of these distinctions. ...
The relationship of carpal morphology to ecology and habitat is under studied in carnivorans and more generally in mammals. Here, we use 3D‐scanning techniques to assess the usefulness of a carpal bone, the scapholunar, in carnivorans to reflect ecology and habitat, and to reconstruct the ecology of five extinct carnivorans from two fossil sites: Rancho La Brea and Natural Trap Cave. We 3D‐scanned scapholunars and measured articular surface areas and angles between articular facets using GeoMagic and Rhino 3D‐software. We analyzed the difference in these metrics using multivariate analysis of variance and discriminant function analysis. Results show that the scapholunar reflects ecological signal, with clear groupings of cursorial carnivorans and grappling/climbing carnivorans; however, phylogenetic signal was also present in the results with hyaenids, canids, and large felids in distinct morphospaces. Extinct species Miracinonyx trumani (American cheetah) and Smilodon fatalis (sabertooth cat) showed surprising results with M. trumani grouping with pantherines instead of Acinonyx or Puma, suggesting it runs but still retains the ability to grapple prey. S. fatalis groups with pantherines, but also shows some unique adaptations, suggesting it had a different range of wrist motion than living cats. Overall, the scapholunar is a good indicator of ecology and functional morphology and can be another tool to use in modern and fossil carnivorans to reconstruct extinct ecologies and locomotor behaviors.
... Notably, however, a small number of focused studies suggested that attenuated articular contacts and soft tissue linkages at their ulnocarpal joints do not increase pronation/ supination ROM in primates, as is traditionally assumed ( O ' C on n o r 1 97 5 ; R a r e y 19 7 7 ; Ziemer 1 9 7 8) . Nonetheless, observations here indicated that an ulnocarpal joint for alleviating ulnar locomotor-induced torsion in nontherian tetrapods, which is formed by a mix of proximal post-axial carpal elements, retained this plesiomorphic function to varying degrees in the ulnocarpal joints of therians (i.e., pisotriquetral basin or socket; Yalden 1966Yalden , 1970Yalden , 1971Yalden , 1972Swiderski 1991;cf. Stafford and Thorington 1998). ...
Although investigations of forelimb characteristics are central to therian evolutionary studies, the functional origins of forearm pronation are neglected. However, recent research based on bipedal manipulations strongly suggests that proximal radioulnar joint mobility is highly conserved in tetrapods. This new information calls for a replication of previously published physical simulations of forearm bone movements, to investigate whether active therian pronation/supination evolved from the plesiomorphic mechanism via which locomotor-induced torsion is passively alleviated during forelimb retraction. Preliminary results using representative extant and extinct tetrapod forelimb elements are supportive, and also offer insight into why another overlooked forearm trait, osteological full pronation (mechanically aligned elbow and wrist/finger joints), evolved only in therians and chameleons. During forelimb retraction in tetrapods with unfused radii/ulnae, the radius unexpectedly remains fixed in place as a functional complex with the firmly planted manus/carpus, which the ulnar complex (ulna/humerus) displaces relative to. Therefore, the highly conserved functional morphology of the tetrapod forearm indicates that enhanced therian manual dexterity, which emphasizes isolated radial movements bipedally, was preceded by the locomotor evolution of ulnar supination relative to the radius quadrupedally. This counterintuitive information indicates that the traditional hypothesis, that therian pronation/supination evolved arboreally to amplify radial mobility, requires modification. The authors propose that proximal long-axis rotations of the therian ulnar complex co-evolved with osteological full pronation during a period of arboreal, chameleon-like locomotion, to continue allowing torsion at a reinforced proximal radioulnar joint. These adaptations were later or simultaneously co-opted for object manipulation using active radioulnar pronation/supination.
... Sá nchez-Villagra & Dö ttling, 2003), but the functional interpretation of its morphological variation is still limited (Swiderski, 1991;Salton & Sargis, 2008). In general, scratchdigging mammals have broad stout hands with long strong claws. ...
Among the ecomorphologically diverse Octodontoidea rodents, fossorial habits are prevalent in Ctenomyidae and Octodontidae and occur in some members of Echimyidae. To detect traits linked to scratch-digging, we analyzed morpho-structural variation in the carpus and metacarpus of 27 species of extinct and living octodontoids with epigean, fossorial and subterranean habits. Within a context of relative morphological uniformity, we detected the following specialized traits in the burrowing Clyomys (Echimyidae), Spalacopus (Octodontidae), Ctenomys and †Eucelophorus (Ctenomyidae): broad shortened carpus, robust metacarpals, markedly broad and short metacarpal V, and predominance of ray III (mesaxony, incipient in Spalacopus). In addition, the specialized subterranean Ctenomys presented an enlarged scapholunar in extensive contact with the unciform, and with a complex-shaped proximal articular surface. These features are interpreted as responses to mechanical requirements of scratch-digging, providing greater carpal rigidity and resistance to direct forces exerted during the digging stroke. In Ctenomys, the radius-scapholunar joint restricts movement at wrist level. The phylogenetic distribution of traits shows that the most derived carpal and metacarpal morphologies occur among subterranean octodontoids, also possessing important craniodental adaptations, and supports the hypothesis that the acquisition of digging specializations would have been linked to increasing burrowing frequency in some lineages. Nevertheless, octodontoids with less morphological specializations have metacarpal modifications advantageous for digging, suggesting that scratch-digging specialization preceded the acquisition of tooth-digging traits, in agreement with the general claim that scratch-digging is the primary digging strategy in burrowing mammals.
The mammalian scapula, like many bones, is a single structural element that serves as an attachment site for several muscles. The goal of this study was to determine whether the scapula evolves as an integrated unit, or as a collection of distinct parts. Shape differences among the scapulae of tree squirrels, chipmunks, and ground squirrels were described using thin-plate spline analysis. This technique produces a geometric description of shape differences that can be decomposed into a series of components ranging in scale from features that span the entire form to features that are highly localized. Shape differences among tree squirrel scapulae were found only in large-scale features, indicating spatially integrated shape change. Chipmunks and ground squirrels differ from tree squirrels in several features, but shared differences reflecting divergence of their common ancestor were found only in the small-scale features. Divergence of ground squirrels from the common ancestor involved some large-scale changes but was dominated by small-scale changes. Divergence of chipmunks was dominated by large-scale changes. Thus, the scapula evolved as an integrated unit during some transitions but as a collection of distinct parts during others. These results suggest that evolutionary patterns of the postcranial skeleton may be as complex as the patterns that have been described for skulls and feeding mechanisms.
—The fossorial rodent Geomys bursarius excavates tunnels to find and gain access to belowground plant parts. This is a study of how the foraging path of this animal, as denoted by feeding-tunnel systems constructed within experimental gardens, reflects both adaptive behavior and constraints associated with the fossorial lifestyle. The principal method of tunnel construction involves the end-to-end linking of short, linear segments whose directionalities are bimodal, but symmetrically distributed about 0°. The sequence of construction of left- and right-directed segments is random, and segments tend to be equal in length. The resulting tunnel advances, zigzag-fashion, along a single heading. This linearity, and the tendency for branches to be orthogonal to the originating tunnel, are consistent with the search path predicted for a “harvesting animal” (Pyke, 1978) from optimal-foraging theory. A suite of physical and physiological constraints on the burrowing process, however, may be responsible for this geometric pattern. That is, by excavating in the most energy-efficient manner, G. bursarius automatically creates the basic components to an optimal-search path. The general search pattern was not influenced by habitat quality (plant density). Branch origins are located more often than expected at plants, demonstrating area-restricted search, a tactic commonly noted in aboveground foragers. The potential trade-offs between construction methods that minimize energy cost and those that minimize vulnerability to predators are discussed.
One of the most challenging problems in evolutionary biology is morphological stasis—the maintenance of a standard morphology over vast periods of time during which much environmental change has taken place. We review empirical data for one well studied group of vertebrates, salamanders of the family Plethodontidae. Despite much evolution at the level of allozymes, proteins such as albumin, and DNA, morphological evolution has been slow, and has been concentrated in relatively short spans of time. The dominant theme has been morphological stasis. An important contributing factor appears to be plasticity—behavioral, physiological, and developmental—which allows organisms to compensate environmental, and even genetic, perturbations without having to change morphologically. Organisms are self-produced and self-maintained systems, and we argue that the best measure of evolutionary adaptation is the persistence of this autopoietic system. Internal dynamics of the organism determine what change will occur and how it is to be expressed. Stable systems can be established which transcend species borders, and the fossil record offers no evidence with regard to speciation rates, or the relationship of speciation events to morphological evolution. We urge that more emphasis be placed on the organism in evolutionary studies.
Activity of small mammals in relation to time of day and various weather conditions was studied during April, May, October and November, 1960, in grassy vegetation located one mile south of Haslett, Ingham County, Michigan. The activity was recorded by means of a two-box photographic apparatus. One box, containing a camera with a synchronized flash, was set on one side of a surface runway used by small mammals. The other box, containing climatological instruments, a millimeter rule, and a clock, was set on the opposite side of the runway. This latter box was arranged in such a way that when an animal activated the apparatus by stepping on a treadle set in the runway, a photograph was then taken of the animal against a background of the dials of the instruments.
The photographic records showed that the shorttail shrew, Blarina brevicauda, and the meadow vole, Microtus pennsylvanicus, were the most common mammals in the habitat studied. The records were analyzed statistically with respect to the environmental conditions that were present when each of these species was photographed in the runways. Both species were active in runways chiefly at night, although in autumn the shorttail shrew was more active in daylight than the meadow vole. In general, activity of both species showed poor correlation with changes in such environmental factors as temperature, humidity, barometric pressure and periods of precipitation. Activity, therefore, was influenced principally by an intricate interplay of all factors combined.
The evolutionary relationships of the soricine shrews were examined cladistically and phenetically using allozyme electrophoresis.
Nine species representing three tribes of the Soricinae (sensu Repenning, 1967), plus three crocidurine species, were examined.
UPGMA cluster analysis generated a phenogram from the Rogers' similarity matrix; the soricine species clustered separately
from the crocidurine species. Using the crocidurines as an outgroup, distance- Wagner and qualitative locus-by-locus cladistic
analyses were performed. Results support Repenning's (1967) classification rather than those proposed by Stirton (1930), Gureev
(1971), and Reumer (1984). The Blarinini and Soricini shared a common ancestor after the Neomyini diverged. Comparing the
results of the electrophoretic analyses to the fossil record provided estimated divergence times within the Soricinae of early
Miocene for the initial dichotomy and middle Miocene for the Blarinini-Soricini dichotomy.
There are two types of pectoral girdles in frogs, defined by the fused versus overlapping condition of the epicoracoid cartilages along the ventral midline. The two types show no correlation with different locomotor behaviors or ecology among frogs. Therefore, an hypothesis of 'functional equivalence' between girdle types was proposed and tested by in vitro loading tests. Similar patterns of deformation were found between girdle types under static and dynamic loads, thus supporting the hypothesis. Additionally, two other aspects of variation in frog pectoral morphology, presence or absence of clavicles and presence or absence of epicoracoid horns, were shown to be forced mechanical correlations of a particular girdle type.
Divergent evolution within the superfamily Soricoidea (Mammalia: Insectivora) has produced, among other genera, the fossorial mole (Scapanus), the semi-fossorial, semi-cursorial shrew-mole (Neurotrichus), and the nonfossorial, semi-cursorial shrew (Sorex). Detailed dissections were made of the limbs of these three genera; the dimensions of each element of the appendicular skeleton and of the major parts of the axial skeleton were analyzed statistically with reference to the body length; and the muscles of the fore limb of Scapanus and of Neurotrichus were measured volumetrically. Correlation of these data with observed behavior of living animals has revealed the manner and degree of the locomotor adaptations in each genus. Locomotion in Sorex is of a generalized type; the limbs are delicate, are held beneath the body, and move in a fore-and-aft plane. The appendicular skeleton is of an unspecialized monodelphian pattern and the feet are digitigrade. All moles (Talpidae) are characterized by a shoulder-joint double in nature, the usual joint being present as well as one between the greater tuberosity of the kumerus and the clavicle. This bracing of the humerus by the clavicle allows the animal to burrow by thrusting laterally against the soil, whereas all other burrowing mammals loosen the soil by rapid anteroposterior strokes of the fore limbs. Even in a primitive mole (Neurotrichus), which has longer and more anteriorly directed clavicles than has a more specialized mole (Scapanus), the clavicle can support medial thrusts from the humerus because the posterior pull of the Mm. subclavius and costoscapularis ventralis holds the clavicle in position. Important fossorial adaptations in the fore limbs of moles are as follows: the pectoral girdle has migrated anteriorly and the manubrium has lengthened accordingly; the manubrium has deepened ventrally; the clavicles have shortened and thickened; the scapula has become narrow and rodlike and the infraspinatus fossa is disappearing; the humerus projects laterally (Neurotrichus) or dorsolaterally (Scapanus) and has broadened; all processes of the humerus have increased in size, the tendon of origin of the M. biceps brachii is enclosed in a transverse tunnel in the proximal end of the humerus; pronation and supination are impossible due to the elbow and wrist being hinge-joints but the antebrachium has undergone a 90⚬ supination due to torsion with the result that the plane of the manus coincides with the plane of forearm extension; a flexor check-ligament has developed between the humerus and the digits; and the manus has enlarged into a digging tool with long, stout claws. For all fossorial characters Scapanus is more specialized than is the primitive Neurotrichus. The latter has retained a long antebrachium for rapid running and can depress the humerus until the palm can be placed upon the ground; Scapanus can place only the medial edge of the manus upon the ground and is an awkward runner. Rotation of the true medial side of the humerus posteriorly by the Mm. teres major, latissimus dorsi, subscapularis, and pectoralis superficialis posticus, all greatly hypertrophied, swings the forearm and hand posteriorly through an arc. This rotary movement of the humerus is the basic talpid locomotor action, both for digging and for running. The contraction of the powerful M. teres major would displace the vertebral end of the scapula lateroventrad were it not for the M. rhomboideus posticus, which runs across the back between the two scapulae; in Scapanus a part of this muscle has become tendinous. In Neurotrichus the M. trapezius anticus elevates the skeleton of the shoulder-joint; in Scapanus this joint is more firmly fixed due to the shorter clavicle, and the muscle has degenerated. In the hind limb the parts of the pelvic girdle of Scapanus are more firmly ankylosed, and the tibia, fibula, and pes are shorter and broader than in Neurotrichus or Sorex. These modifications of Scapanus are for more powerful action of the pelvic limb necessitated by the act of bracing while burrowing and the act of pushing accumulated dirt from the burrow. The muscles of the hind limbs of the three genera are similar except that the M. peroneus digiti quarti is lacking in Scapanus. Evidence is presented to show that the Talpidae did not necessarily develop from an aquatic ancestor, as suggested by Campbell (1939).
On the basis of a phenotypic model of R. Lande a nonlinear analysis is performed to investigate the evolutionary dynamics of functionally coupled quantitative traits. The underlying fitness topography has multiple peaks with a ridge and two hills adjacent to a saddle. Evolution of a complex of functionally constrained characters corresponds precisely to moving uphill along a ridge. For modelling the topology of the ridge, I follow ideas of Rechenberg and Wagner and use a so-called corridor model. The analysis reveals certain population-genetic constraints for the evolutionary emergence of a selectively favored complex of functionally constrained characters. Due to the population-genetic structure, as reflected in the pattern of variation and covariation, a population will often not be allowed to become adapted to existing physiological requirements, such as functional coupling of characters. Instead, within the present model where extinction cannot occur, it will evolve in some other direction toward an optimum that may be physiologically rather remote. In particular, there exists an optimal pattern of genetic and phenotypic variances and covariances in the following sense: on the one hand an increasing deviation from this pattern imposes increasing restrictions on the set of initial conditions enabling a population to move uphill along the ridge; on the other hand, an increasing deviation leads to a decreasing rate of adaptation along the ridge. Finally, some consequences of these constraints for possible interpretations of certain empirical results are discussed.
The range of divergence of a character, as measured by the difference between its largest and smallest local sample means, is highly significant and positively correlated with its average local-sample standard deviation in all fish, lizard, snake, and bird case histories studied. More important, the two variables remain significantly correlated when both are divided by the mean of local sample means, which eliminates the contribution from the relationship between mean and standard deviation. We conclude that those characters that exhibit greater variation in local populations show greater divergence of extremes between populations, and that such a relationship is not a statistical artifact. In providing a possible explanation for how within-population variation limits divergence, we postulate that there is an inverse relationship between the variability of a character in a local population and the effect it is likely to have on individual fitness. Furthermore, we argue that the divergence of a character must be thought of as the product of the probability of its responding and the magnitude of that response when the population encounters a new environment. Our success in demonstrating a significant relationship between the divergence of a set of characters and their individual phenotypic variances appears to be due to the fact that we have maximized the probability of a response in all characters. Since not all characters have the same chance of responding to an environmental shift, we have increased our chances that all characters will be affected by considering numerous phyletic lineages in many diverse environments.
A model of multivariate phenotypic evolution is analysed under the assumption that all characters have the same variance or at least constant ratios of variance. The rate of evolution is examined as a function of the amount of phenotypic variance in a variety of adaptive landscapes (fitness functions). It is demonstrated that the effect of variation depends on the type of adaptive landscape. In “well behaved” adaptive landscapes the rate of evolution can theoretically increase without limits, depending on the amount of heritable phenotypic variation. However, in other adaptive landscapes there are upper limits to the rate of evolution which cannot be exceeded if phenotypic variation is developmentally unconstrained, i. e. if it is the same for all characters. Further it is shown that the maximal rate of evolution becomes small if the number of characters becomes large. Fitness functions of this type are called malignant. It is argued that malignant fitness functions are more adequate models for the evolution of typical organismic systems, because they are models of functionally interdependent characters. It is concluded that there are upper limits to the rate of phenotypic evolution if the variation of functionally interdependent characters is developmentally unconstrained. The possible role of developmental constraints in adaptive phenotypic evolution is discussed.
A model for calculating the energy cost of burrowing by fossorial rodents is presented and used to examine the energetics of foraging by burrowing. The pocket gopher Thomomys bottae (Rodentia: Geomyidae) digs burrows for access to food. Feeding tunnels of Thomomys are broken into segments by laterals to the surface that are used to dispose of excavated soil. Energy cost of burrowing depends on both soil type and on burrow structure, defined by the length of burrow segments, angle of ascent of laterals, depth of feeding tunnels, and burrow diameter. In a desert scrub habitat, Thomomys adjust burrow segment length to minimize cost of burrowing. Observed segment lengths (mean=1.33 m) closely approximate the minimum-cost segment length of 1.22 m. Minimizing energy expended per meter of tunnel constructed maximizes efficiency of foraging by burrowing in the desert scrub. Burrow diameter and cost of burrowing increase with body size, while benefits do not, so foraging by burrowing becomes less enconomical as body size increases. Maximum possible body size of fossorial mammals depends on habitat productivity and energy cost of burrowing in local soils.
Vermeij, G. J. (Dept. of Zoology, University of Maryland, College Park, Maryland 20742) 1974. Adaptation, Versatility, and Evolution. Syst. Zool. 22:466–477 .—The potential versatility of form and potential adaptive zone of a higher taxon or body plan depend on the number and range of independent morphogenetic parameters. The actual versatility and adaptive zone of a given higher taxon are determined by interactions of members of that taxon with species belonging to groups with a different body plan. More recently evolved higher taxa tend to exhibit a greater potential versatility of form than related more ancient taxa. In the course of earth history, more potentially versatile taxa have tended to replace less versatile groups in the same or similar adaptive zone. This increase in potential versatility is not the inevitable consequence of the decrease in number of higher taxa through geologic time, but is the result of strong selection favoring some plans of organization over others which may or may not have become extinct. Increasing potential versatility allows for greater homeostasis, efficiency, and exploitation of continuously available resources, and integration of structures and functions. New body plans may arise either from stocks of small adult body size and with relatively simple integration patterns, or else by a process of correlative progression of structures and functions from a relatively specialized ancestral stock.
Liem, Karel F. (Museum of Comparative Zoology, Harvard University, Cambridge, Mass. 02138) 1974. Evolutionary strategies and morphological innovations: Cichlid pharyngeal jaws. Syst. Zool. 22:425–441 .—The percoid fish family Cichlidae possesses a phenomenal ability to colonize lakes and to diversify to an extent unmatched by any other vertebrate family in the presence of predator pressure and strong competition. The invading cichlids successfully occupy contiguous and occasionally overlapping adaptive zones and specialize progressively into diversified subzones, ramifying prodigiously and covering a breadth of total adaptation that would have been entirely unpredictable if we were aware only of the rudiments of the evolutionary process. This evolutionary avalanche can be attributed to the cooccurrence of a wide range of prospective adaptive zones in the lacustrine environment, and the presence of a unique morphological key innovation of maximum versatility. The new adaptive complex has been revealed in this study by electromyographic analysis synchronized with cineradiography of the cichlid pharyngeal jaw apparatus. The morphological novelty characterizing the family Cichlidae involves the development of: a synarthrosis between the lower pharyngeal jaws, a strategic shift of insertion of the two fourth levator externi muscles, and synovial joints between upper pharyngeal jaws and basicranium. This specialized, highly integrated key innovation enables the cichlids not only to transport (deglutination) but also to prepare food, freeing the premaxillary and mandibular jaws to evolve numerous specializations dealing with the collection of dramatically diverse foods. The functional integration of the innovation is so basic and its potential adaptive versatility so rich that it is maintained throughout the adaptive radiation even though numerous nondisruptive evolutionary changes do take place, providing prodigious opportunities for explosive evolution during the exploitation of rich resources of food in the lacustrine environment. The conversion of the preexisting elements into a new and significantly improved cichlid adaptive complex of high selective value may have evolved by rapid steps under influence of strong selection pressure acting on the minor reconstruction of the genotype which is involved in evolutionary changes of the pertinent ontogenetic mechanisms. Such relatively simple evolutionary processes are probably the cause for the general phenomenon that only slight reconstructions of existing structures are necessary for successful and rapid adaptation to drastic shifts of adaptive zones.
Organisms are self-producing and self-maintaining, or "autopoietic" systems. Therefore, the course of evolution and adaptation of an organism is strongly determined by its own internal properties, whatever role "external" selection may play. The internal properties may either act as constraints that preclude certain changes or they open new pathways: the organism canalizes its own evolution. As an example the evolution of feeding mechanisms in salamanders, especially in the lungless salamanders of the family Plethodontidae, is discussed. In this family a large variety of different feeding mechanisms is found. The authors reconstruct this evolutionary process as a series of "bifurcation points" of either constraints or opportunities forming a sequence of preconditions for the formation of a high-speed projectile tongue characteristic of tropical salamanders. Furthermore, it is shown how parallel evolution of seemingly unrelated domains within an organism such as respiratory physiology, life history biology and pattern of ontogeny has rather direct relevance to the feeding biology, thus demonstrating that organisms always evolve as wholes.