Organisms represent a complex arrangement of anatomical structures and individuated parts that must maintain functional associations through development. This integration of variation between functionally related body parts and the modular organization of development are fundamental determinants of their evolvability. This is because integration results in the expression of coordinated variation that can create preferred directions for evolutionary change, while modularity enables variation in a group of traits or regions to accumulate without deleterious effects on other aspects of the organism. Using our own work on both model systems (e.g., lab mice, avians) and natural populations of rodents and primates, we explore in this paper the relationship between patterns of phenotypic covariation and the developmental determinants of integration that those patterns are assumed to reflect. We show that integration cannot be reliably studied through phenotypic covariance patterns alone and argue that the relationship between phenotypic covariation and integration is obscured in two ways. One is the superimposition of multiple determinants of covariance in complex systems and the other is the dependence of covariation structure on variances in covariance-generating processes. As a consequence, we argue that the direct study of the developmental determinants of integration in model systems is necessary to fully interpret patterns of covariation in natural populations, to link covariation patterns to the processes that generate them, and to understand their significance for evolutionary explanation.

Morphological integration theory predicts that sets of phenotypic traits that covary strongly due to developmental and/or functional connections between them eventually co-evolve because of a coordinated response to selection, and accordingly become less independently evolvable. This process is not irreversible, however, and phenotypic traits can become less integrated, and hence more independently evolvable, in the context of selection for divergent functions and morphologies. This study examines the reciprocal relationship between shared function, integration and evolvability by comparing integration patterns among serially homologous skeletal elements in the hands and feet of a functionally diverse sample of catarrhine primates. Two hypotheses are tested:(1) species in which the autopods are functionally more similar (e.g. quadrupedal monkeys) have more strongly integrated autopods than species in which the autopods are functionally divergent (e.g. gibbons, humans) and (2) the latter have autopods that are more evolvable, collectively and independently. Morphometric data from selected hand and foot digital rays were used to derive phenotypic variance/covariance matrices. The strength of integration among autopods was quantified using eigenanalysis and a measure of trait variational autonomy. Evolvability was estimated by subjecting phenotypic variance/covariance matrices to simulated random selection gradients, and comparing evolutionary responses among species. Results indicate that integration decreases as hands and feet become functionally divergent, and that the strongly integrated hand and foot skeletons of quadrupedal monkeys respond to selection in a highly collinear manner, even when simulated selective pressures acting on each autopod are in opposite directions in phenotypic space. Results confirm that the evolvability of morphological traits depends largely on how strongly they covary with other traits, but also with body size. The role of pleiotropy as a developmental mechanism underlying integration and evolvability is also discussed.

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The Dmanisi hominins inhabited a northern temperate habitat in the southern Caucasus, approximately 1.8 million years ago. This is the oldest population of hominins known outside of Africa. Understanding the set of anatomical and behavioral traits that equipped this population to exploit their seasonal habitat successfully may shed light on the selection pressures shaping early members of the genus Homo and the ecological strategies that permitted the expansion of their range outside of the African subtropics. The abundant stone tools at the site, as well as taphonomic evidence for butchery, suggest that the Dmanisi hominins were active hunters or scavengers. In this study, we examine the locomotor mechanics of the Dmanisi hind limb to test the hypothesis that the inclusion of meat in the diet is associated with an increase in walking and running economy and endurance. Using comparative data from modern humans, chimpanzees, and gorillas, as well as other fossil hominins, we show that the Dmanisi hind limb was functionally similar to modern humans, with a longitudinal plantar arch, increased limb length, and human-like ankle morphology. Other aspects of the foot, specifically metatarsal morphology and tibial torsion, are less derived and similar to earlier hominins. These results are consistent with hypotheses linking hunting and scavenging to improved walking and running performance in early Homo. Primitive retentions in the Dmanisi foot suggest that locomotor evolution continued through the early Pleistocene. Dear Author, Any queries or remarks that have arisen during the processing of your manuscript are listed below and highlighted by flags in the proof. Please check your proof carefully and mark all corrections at the appropriate place in the proof (e.g., by using on-screen annotation in the PDF file) or compile them in a separate list.For correction or revision of any artwork, please consult http://www.elsevier.com/artworkinstructions.Articles in Special Issues: Please ensure that the words 'this issue' are added (in the list and text) to any references to other articles in this Special Issue.Uncited references: References that occur in the reference list but not in the text e please position each reference in the text or delete it from the list.Missing references: References listed below were noted in the text but are missing from the reference list e please make the list complete or remove the references from the text. Location in articleQuery / remark Please insert your reply or correction at the corresponding line in the proof Q1Kindly check the affiliations. Q2Please check the hierarchy of section headings. Q3The following references have been changed to match the reference list: " Hermann et al., 1998" to "Hermann and Egund, 1998""Geissman (1986" to "Geissmann (1986)""Harcourt-Smith and Aeillo, 2004" to "Harcourt-Smith and Aiello, 2004". Please verify. Q4Please check the placement of footnotes citations 1e4 have been made at the end of the sentence "Parsing the independent effects." Please c...

SUMMARYThe phalangeal portion of the forefoot is extremely short relative to body mass in humans. This derived pedal proportion is thought to have evolved in the context of committed bipedalism, but the benefits of shorter toes for walking and/or running have not been tested previously. Here, we propose a biomechanical model of toe function in bipedal locomotion that suggests that shorter pedal phalanges improve locomotor performance by decreasing digital flexor force production and mechanical work, which might ultimately reduce the metabolic cost of flexor force production during bipedal locomotion. We tested this model using kinematic, force and plantar pressure data collected from a human sample representing normal variation in toe length (N=25). The effect of toe length on peak digital flexor forces, impulses and work outputs was evaluated during barefoot walking and running using partial correlations and multiple regression analysis, controlling for the effects of body mass, whole-foot and phalangeal contact times and toe-out angle. Our results suggest that there is no significant increase in digital flexor output associated with longer toes in walking. In running, however, multiple regression analyses based on the sample suggest that increasing average relative toe length by as little as 20% doubles peak digital flexor impulses and mechanical work, probably also increasing the metabolic cost of generating these forces. The increased mechanical cost associated with long toes in running suggests that modern human forefoot proportions might have been selected for in the context of the evolution of endurance running.