The fast life of a dwarfed giant

Convergent evolution toward a slow pace of life predisposes insular endotherms to anthropogenic extinctions

Island vertebrates have evolved a number of morphological, physiological, and life history characteristics that set them apart from their mainland relatives. However, to date, the evolution of metabolism and its impact on the vulnerability to extinction of insular vertebrates remains poorly understood. This study used metabolic data from 2813 species of tetrapod vertebrates, including 695 ectothermic and 2118 endothermic species, to reveal that island mammals and birds evolved convergent metabolic strategies toward a slow pace of life. Insularity was associated with shifts toward slower metabolic rates and greater generation lengths in endotherms, while insularity just drove the evolution of longer generation lengths in ectotherms. Notably, a slow pace of life has exacerbated the extinction of insular endemic species in the face of anthropogenic threats. These findings have important implications for understanding physiological adaptations associated with the island syndrome and formulating conservation strategies across taxonomic groups with different metabolic modes.

Insular giant leporid matured later than predicted by scaling

The island syndrome describes morphological, behavioral, and life history traits that evolve in parallel in endemic insular organisms. A basic axiom of the island syndrome is that insular endemics slow down their pace of life. Although this is already confirmed for insular dwarfs, a slow life history in giants may not be adaptive, but merely a consequence of increasing body size. We tested this question in the fossil insular giant leporid Nuralagus rex. Using bone histology, we constructed both a continental extant taxon model derived from experimentally fluorochrome-labeled Lepus europaeus to calibrate life history events, and a growth model for the insular taxon. N. rex grew extremely slowly and delayed maturity well beyond predictions from continental phylogenetically corrected scaling models. Our results support the life history axiom of the island syndrome as generality for insular mammals, regardless of whether they have evolved into dwarfs or giants.

Strength of the 'island rule' in birds is positively associated with absence of avian predators

The similar characteristics shared by island environments have been shown to lead to common patterns of adaptations in island species, commonly referred to as the 'insularity syndrome'. A well-known example is the 'island rule', where large species become smaller on islands and small species become larger, leading to well-known cases of dwarfism and gigantism. This pattern was recently verified on a global scale, but the mechanisms underlying it have been poorly investigated. Here, we focused on the role of released pressure from predation and competition experienced by island birds. Using 120 pairs of endemic island species and their mainland sister relatives, we first verified that the island rule was detected in our dataset, and then evaluated the effects of the numbers of raptors and interspecific competitors on the evolution of the insular species' body mass. We found a strong effect of predation on body mass evolution, with a stronger island rule for species occurring on islands with no raptors, while the pattern disappears in their presence. However, we did not find an effect of competition on this pattern. Our study shows the importance of considering ecological interactions for understanding patterns of body size evolution, and the exceptions to those patterns.

Extreme Body Size Variation in Pleistocene Dwarf Elephants from the Siculo-Maltese Palaeoarchipelago: Disentangling the Causes in Time and Space

The phenomenon of insular dwarfism in proboscideans is particularly well represented on the Siculo-Maltese Palaeoarchipelago, an island group on which a species complex of palaeoloxodont elephants evolved during the Middle-Late Pleistocene. This likely included three species from Malta, four from Sicily (and possibly its palaeo-islands), and one from Favignana Island, ranging in size from the 1 m-tall Palaeoloxodon falconeri to continental-sized 4m-tall P. antiquus. However, our understanding of the causes for extreme differences in body size among insular samples in such a small geographic region is still limited. Here, I document the full range in size of elephants from the palaeoarchipelago, and discuss the reasons for size differences on the three islands in time and space in relation to predation, competition, resource limitation, accelerated life history, and duration of isolation. Differences in size between larger and smaller Sicilian elephants from Luparello Fissure, as well as possibly also in P. ex gr. P. mnaidriensis from Puntali Cave, San Teodoro Cave, and Favignana Island, may relate to the duration of isolation, or alternatively environmental stressors associated with the Last Glacial Maximum in the case of the Favignana elephant. Additionally, small but significant differences in size observable in Middle Pleistocene P. ex gr. P. falconeri from different localities on Sicily, as well as in Maltese P. ‘melitensis’ may also relate to duration of isolation, highlighting the need for better geochronological data in order to better distinguish macro-ecological causes driving body size change from more subtle effects relating to duration of isolation.

Palaeohistology reveals a slow pace of life for the dwarfed Sicilian elephant

The 1-m-tall dwarf elephant Palaeoloxodon falconeri from the Pleistocene of Sicily (Italy) is an extreme example of insular dwarfism and epitomizes the Island Rule. Based on scaling of life-history (LH) traits with body mass, P. falconeri is widely considered to be ‘r-selected’ by truncation of the growth period, associated with an early onset of reproduction and an abbreviated lifespan. These conjectures are, however, at odds with predictions from LH models for adaptive shifts in body size on islands. To settle the LH strategy of P. falconeri, we used bone, molar, and tusk histology to infer growth rates, age at first reproduction, and longevity. Our results from all approaches are congruent and provide evidence that the insular dwarf elephant grew at very slow rates over an extended period; attained maturity at the age of 15 years; and had a minimum lifespan of 68 years. This surpasses not only the values predicted from body mass but even those of both its giant sister taxon (P. antiquus) and its large mainland cousin (L. africana). The suite of LH traits of P. falconeri is consistent with the LH data hitherto inferred for other dwarfed insular mammals. P. falconeri, thus, not only epitomizes the Island Rule but it can also be viewed as a paradigm of evolutionary change towards a slow LH that accompanies the process of dwarfing in insular mammals.

No evidence for widespread island extinctions after Pleistocene hominin arrival

The arrival of modern humans into previously unoccupied island ecosystems is closely linked to widespread extinction, and a key reason cited for Pleistocene megafauna extinction is anthropogenic overhunting. A common assumption based on late Holocene records is that humans always negatively impact insular biotas, which requires an extrapolation of recent human behavior and technology into the archaeological past. Hominins have been on islands since at least the early Pleistocene and Homo sapiens for at least 50 thousand y (ka). Over such lengthy intervals it is scarcely surprising that significant evolutionary, behavioral, and cultural changes occurred. However, the deep-time link between human arrival and island extinctions has never been explored globally. Here, we examine archaeological and paleontological records of all Pleistocene islands with a documented hominin presence to examine whether humans have always been destructive agents. We show that extinctions at a global level cannot be associated with Pleistocene hominin arrival based on current data and are difficult to disentangle from records of environmental change. It is not until the Holocene that large-scale changes in technology, dispersal, demography, and human behavior visibly affect island ecosystems. The extinction acceleration we are currently experiencing is thus not inherent but rather part of a more recent cultural complex.

Quantitative genetics of body size evolution on islands: an individual-based simulation approach

According to the island rule, small-bodied vertebrates will tend to evolve larger body size on islands, whereas the opposite happens to large-bodied species. This controversial pattern has been studied at the macroecological and biogeographical scales, but new developments in quantitative evolutionary genetics now allow studying the island rule from a mechanistic perspective. Here, we develop a simulation approach based on an individual-based model to model body size change on islands as a progressive adaptation to a moving optimum, determined by density-dependent population dynamics. We applied the model to evaluate body size differentiation in the pigmy extinct hominin Homo floresiensis, showing that dwarfing may have occurred in only about 360 generations (95% CI ranging from 150 to 675 generations). This result agrees with reports suggesting rapid dwarfing of large mammals on islands, as well as with the recent discovery that small-sized hominins lived in Flores as early as 700 kyr ago. Our simulations illustrate the power of analysing ecological and evolutionary patterns from an explicit quantitative genetics perspective.

Effect of isolation on coat colour polymorphism of Polynesian rats in Island Southeast Asia and the Pacific

Populations of vertebrate species introduced onto islands regularly develop similar phenotypic changes, e.g., larger or smaller body size, shortened limbs, duller coats, as well as behavioural changes such as increased tameness and reduced flight-initiation distance. These changes overlap in part with those associated with the 'domestication syndrome', especially tameness and changes in coat patterns, and might indicate a similar neural crest involvement in the concurrent development of multiple phenotypic traits. Here I examine long-term data on free-living populations of wild Polynesian rats from seven mainland countries and 117 islands (n = 3,034), covering the species' native and introduced range. Mainland populations showed no aberrant coat patterns, with the exception of one albino, whereas aberrant coat patterns were found in 12 island populations. Observed coat colour polymorphisms consisted of leucistic (including singular white patches), melanistic (darkly pigmented) and piebald (mixed) coat patterns. After isolation for at least seven centuries, wild Polynesian rat populations on islands seem to exhibit a trend towards a higher incidence of aberrant coat patterns. These phenotypic changes are here explained as a neutral, non-adaptive process, likely part of the 'domestication syndrome' (via the commensal pathway of domestication), in combination with genetic drift, little or no gene flow between the islands and/or the mainland and a relaxed selection (as a result of the weakening or removal of competitor/predator pressure) under commensality.

Genetic diversity and drivers of dwarfism in extinct island emu populations

Australia's iconic emu (Dromaius novaehollandiae novaehollandiae) is the only living representative of its genus, but fossil evidence and reports from early European explorers suggest that three island forms (at least two of which were dwarfs) became extinct during the nineteenth century. While one of these-the King Island emu-has been found to be conspecific with Australian mainland emus, little is known about how the other two forms-Kangaroo Island and Tasmanian emus-relate to the others, or even the size of Tasmanian emus. We present a comprehensive genetic and morphological analysis of Dromaius diversity, including data from one of the few definitively genuine Tasmanian emu specimens known. Our genetic analyses suggest that all the island populations represent sub-populations of mainland Dnovaehollandiae Further, the size of island emus and those on the mainland appears to scale linearly with island size but not time since isolation, suggesting that island size-and presumably concomitant limitations on resource availability-may be a more important driver of dwarfism in island emus, though its precise contribution to emu dwarfism remains to be confirmed.

Mammalian bone palaeohistology: a survey and new data with emphasis on island forms

The interest in mammalian palaeohistology has increased dramatically in the last two decades. Starting in 1849 via descriptive approaches, it has been demonstrated that bone tissue and vascularisation types correlate with several biological variables such as ontogenetic stage, growth rate, and ecology. Mammalian bone displays a large variety of bone tissues and vascularisation patterns reaching from lamellar or parallel-fibred to fibrolamellar or woven-fibred bone, depending on taxon and individual age. Here we systematically review the knowledge and methods on cynodont and mammalian bone microstructure as well as palaeohistology and discuss potential future research fields and techniques. We present new data on the bone microstructure of two extant marsupial species and of several extinct continental and island placental mammals. Extant marsupials display mainly parallel-fibred primary bone with radial and oblique but mainly longitudinal vascular canals. Three juvenile specimens of the dwarf island hippopotamid Hippopotamus minor from the Late Pleistocene of Cyprus show reticular to plexiform fibrolamellar bone. The island murid Mikrotia magna from the Late Miocene of Gargano, Italy displays parallel-fibred primary bone with reticular vascularisation and strong remodelling in the middle part of the cortex. Leithia sp., the dormouse from the Pleistocene of Sicily, is characterised by a primary bone cortex consisting of lamellar bone and a high amount of compact coarse cancellous bone. The bone cortex of the fossil continental lagomorph Prolagus oeningensis and three fossil species of insular Prolagus displays mainly parallel-fibred primary bone and reticular, radial as well as longitudinal vascularisation. Typical for large mammals, secondary bone in the giant rhinocerotoid Paraceratherium sp. from the Late Oligocene of Turkey is represented by dense Haversian bone. The skeletochronological features of Sinomegaceros yabei, a large-sized deer from the Pleistocene of Japan closely related to Megaloceros, indicate a high growth rate. These examples and the synthesis of existing data show the potential of bone microstructure to reveal essential information on life history evolution. The bone tissue and the skeletochronological data of the sampled island species suggest the presence of various modes of bone histological modification and mammalian life history evolution on islands to depend on factors of island evolution such as island size, distance from mainland, climate, phylogeny, and time of evolution.

Growth in fossil and extant deer and implications for body size and life history evolution

Background

Body size variation within clades of mammals is widespread, but the developmental and life-history mechanisms by which this variation is achieved are poorly understood, especially in extinct forms. An illustrative case study is that of the dwarfed morphotypes of Candiacervus from the Pleistocene of Crete versus the giant deer Megaloceros giganteus, both in a clade together with Dama dama among extant species. Histological analyses of long bones and teeth in a phylogenetic context have been shown to provide reliable estimates of growth and life history patterns in extant and extinct mammals.

Results

Similarity of bone tissue types across the eight species examined indicates a comparable mode of growth in deer, with long bones mainly possessing primary plexiform fibrolamellar bone. Low absolute growth rates characterize dwarf Candiacervus sp. II and C. ropalophorus compared to Megaloceros giganteus displaying high rates, whereas Dama dama is characterized by intermediate to low growth rates. The lowest recorded rates are those of the Miocene small stem cervid Procervulus praelucidus. Skeletal maturity estimates indicate late attainment in sampled Candiacervus and Procervulus praelucidus. Tooth cementum analysis of first molars of two senile Megaloceros giganteus specimens revealed ages of 16 and 19 years whereas two old dwarf Candiacervus specimens gave ages of 12 and 18 years.

Conclusions

There is a rich histological record of growth across deer species recorded in long bones and teeth, which can be used to understand ontogenetic patterns within species and phylogenetic ones across species. Growth rates sensu Sander & Tückmantel plotted against the anteroposterior bone diameter as a proxy for body mass indicate three groups: one with high growth rates including Megaloceros, Cervus, Alces, and Dama; an intermediate group with Capreolus and Muntiacus; and a group showing low growth rates, including dwarf Candiacervus and Procervulus. Dwarf Candiacervus, in an allometric context, show an extended lifespan compared to other deer of similar body size such as Mazama which has a maximum longevity of 12 years in the wild. Comparison with other clades of mammals reveals that changes in size and life history in evolution have occurred in parallel, with various modes of skeletal tissue modification.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0295-3) contains supplementary material, which is available to authorized users.

Island life in the Cretaceous - faunal composition, biogeography, evolution, and extinction of land-living vertebrates on the Late Cretaceous European archipelago

The Late Cretaceous was a time of tremendous global change, as the final stages of the Age of Dinosaurs were shaped by climate and sea level fluctuations and witness to marked paleogeographic and faunal changes, before the end-Cretaceous bolide impact. The terrestrial fossil record of Late Cretaceous Europe is becoming increasingly better understood, based largely on intensive fieldwork over the past two decades, promising new insights into latest Cretaceous faunal evolution. We review the terrestrial Late Cretaceous record from Europe and discuss its importance for understanding the paleogeography, ecology, evolution, and extinction of land-dwelling vertebrates. We review the major Late Cretaceous faunas from Austria, Hungary, France, Spain, Portugal, and Romania, as well as more fragmentary records from elsewhere in Europe. We discuss the paleogeographic background and history of assembly of these faunas, and argue that they are comprised of an endemic 'core' supplemented with various immigration waves. These faunas lived on an island archipelago, and we describe how this insular setting led to ecological peculiarities such as low diversity, a preponderance of primitive taxa, and marked changes in morphology (particularly body size dwarfing). We conclude by discussing the importance of the European record in understanding the end-Cretaceous extinction and show that there is no clear evidence that dinosaurs or other groups were undergoing long-term declines in Europe prior to the bolide impact.

The evolutionary ecology of dwarfism in three-spined sticklebacks

1. Body size is a defining phenotypic trait, but the ecological causes of its evolution are poorly understood. Most studies have considered only a single putative causal agent and have failed to recognise that different environmental agents are often correlated. 2. Darwin suggested that although trait variation across populations is often associated with abiotic variation, evolution is more likely to be driven by biotic factors correlated with the abiotic variation. This hypothesis has received little explicit attention. 3. We use structural equation modelling to quantify the relative importance of abiotic (pH, metal concentrations) and biotic (competition, predation) factors in the evolution of body size in three-spined sticklebacks Gasterosteus aculeatus on the island of North Uist, Scotland. We combine phenotypic data from multiple isolated populations, detailed characterisation of their environment and a common garden experiment that establishes the genetic basis of size differences. 4. Three-spined sticklebacks on North Uist show almost unprecedented intraspecific evolution of body size that has taken place rapidly (<16,000 years). The smallest fish mature at only 7% of the mass of ancestral, anadromous fish. Dwarfism is associated with reduced abundance of a smaller competitor species, the nine-spined stickleback Pungitius pungitius, and with low pH indicative of poor resource conditions. Dwarfism also tends to occur where an important predator, the brown trout Salmo trutta, is also small. The abundance of P. pungitius and the size of S. trutta are themselves related to underlying abiotic environmental variation. 5. Despite the close association between abiotic and biotic factors across populations, our results support Darwin's hypothesis that biotic factors, associated with variation in the abiotic environment, are more important in explaining evolution than is abiotic variation per se. This study demonstrates the importance of considering the relationships between environmental variables before conclusions can be drawn about the causes of (body size) evolution on islands.

The tempo and mode of evolution: body sizes of island mammals

The tempo and mode of body size evolution on islands are believed to be well known. It is thought that body size evolves relatively quickly on islands toward the mammalian modal value, thus generating extreme cases of size evolution and the island rule. Here, we tested both theories in a phylogenetically explicit context, by using two different species-level mammalian phylogenetic hypotheses limited to sister clades dichotomizing into an exclusively insular and an exclusively mainland daughter nodes. Taken as a whole, mammals were found to show a largely punctuational mode of size evolution. We found that, accounting for this, and regardless of the phylogeny used, size evolution on islands is no faster than on the continents. We compared different selection regimes using a set of Ornstein-Uhlenbeck models to examine the effects of insularity of the mode of evolution. The models strongly supported clade-specific selection regimes. Under this regime, however, an evolutionary model allowing insular species to evolve differently from their mainland relatives performs worse than a model that ignores insularity as a factor. Thus, insular taxa do not experience statistically different selection from their mainland relatives.

The blue lizard spandrel and the island syndrome

BACKGROUND

Many small vertebrates on islands grow larger, mature later, lay smaller clutches/litters, and are less sexually dimorphic and aggressive than their mainland relatives. This set of observations is referred to as the 'Island Syndrome'. The syndrome is linked to high population density on islands. We predicted that when population density is low and/or fluctuating insular vertebrates may evolve correlated trait shifts running opposite to the Island Syndrome, which we collectively refer to as the 'reversed island syndrome' (RIS) hypothesis. On the proximate level, we hypothesized that RIS is caused by increased activity levels in melanocortin receptors. Melanocortins are postranslational products of the proopiomelanocortin gene, which controls pleiotropically pigmentation, aggressiveness, sexual activity, and food intake in vertebrates.

RESULTS

We tested the RIS hypothesis performing a number of behavioral, genetic, and ontogenetic tests on a blue colored insular variant of the Italian Wall lizard Podarcis sicula, living on a small island off the Southern Italian coast. The population density of this blue-colored variant was generally low and highly fluctuating from one year to the next.In keeping with our predictions, insular lizards were more aggressive and sexually dimorphic than their mainland relatives. Insular males had wide, peramorphic heads. The growth rate of insular females was slower than growth rates of mainland individuals of both sexes, and of insular males. Consequently, size and shape dimorphism are higher on the Island. As predicted, melanocortin receptors were much more active in individuals of the insular population. Insular lizards have a higher food intake rate than mainland individuals, which is consistent with the increased activity of melanocortin receptors. This may be adaptive in an unpredictable environment such as Licosa Island. Insular lizards of both sexes spent less time basking than their mainland relatives. We suspect this is a by-product (spandrel) of the positive selection for increased activity of melanocortins receptors.

CONCLUSIONS

We contend that when population density is either low or fluctuating annually as a result of environmental unpredictability, it may be advantageous to individuals to behave more aggressively, to raise their rate of food intake, and allocate more energy into reproduction.

Physiological and life history strategies of a fossil large mammal in a resource-limited environment

Because of their physiological and life history characteristics, mammals exploit adaptive zones unavailable to ectothermic reptiles. Yet, they perform best in energy-rich environments because their high and constant growth rates and their sustained levels of resting metabolism require continuous resource supply. In resource-limited ecosystems such as islands, therefore, reptiles frequently displace mammals because their slow and flexible growth rates and low metabolic rates permit them to operate effectively with low energy flow. An apparent contradiction of this general principle is the long-term persistence of certain fossil large mammals on energy-poor Mediterranean islands. The purpose of the present study is to uncover the developmental and physiological strategies that allowed fossil large mammals to cope with the low levels of resource supply that characterize insular ecosystems. Long-bone histology of Myotragus, a Plio-Pleistocene bovid from the Balearic Islands, reveals lamellar-zonal tissue throughout the cortex, a trait exclusive to ectothermic reptiles. The bone microstructure indicates that Myotragus grew unlike any other mammal but similar to crocodiles at slow and flexible rates, ceased growth periodically, and attained somatic maturity extremely late by approximately 12 years. This developmental pattern denotes that Myotragus, much like extant reptiles, synchronized its metabolic requirements with fluctuating resource levels. Our results suggest that developmental and physiological plasticity was crucial to the survival of this and, perhaps, other large mammals on resource-limited Mediterranean Islands, yet it eventually led to their extinction through a major predator, Homo sapiens.

The Liang Bua faunal remains: a 95k.yr. sequence from Flores, East Indonesia

Excavations at Liang Bua, a limestone cave on the island of Flores, East Indonesia, have yielded a well-dated archaeological and faunal sequence spanning the last 95k.yr., major climatic fluctuations, and two human species -H. floresiensis from 95 to 17k.yr.(1), and modern humans from 11k.yr. to the present. The faunal assemblage comprises well-preserved mammal, bird, reptile and mollusc remains, including examples of island gigantism in small mammals and the dwarfing of large taxa. Together with evidence from Early-Middle Pleistocene sites in the Soa Basin, it confirms the long-term isolation, impoverishment, and phylogenetic continuity of the Flores faunal community. The accumulation of Stegodon and Komodo dragon remains at the site in the Pleistocene is attributed to Homo floresiensis, while predatory birds, including an extinct species of owl, were largely responsible for the accumulation of the small vertebrates. The disappearance from the sequence of the two large-bodied, endemic mammals, Stegodon florensis insularis and Homo floresiensis, was associated with a volcanic eruption at 17 ka and precedes the earliest evidence for modern humans, who initiated use of mollusc and shell working, and began to introduce a range of exotic animals to the island. Faunal introductions during the Holocene included the Sulawesi warty pig (Sus celebensis) at about 7ka, followed by the Eurasian pig (Sus scrofa), Long-tailed macaque, Javanese porcupine, and Masked palm civet at about 4ka, and cattle, deer, and horse - possibly by the Portuguese within historic times. The Holocene sequence at the site also documents local faunal extinctions - a result of accelerating human population growth, habitat loss, and over-exploitation.

Insular shifts in body size of rice frogs in the Zhoushan Archipelago, China

1. Differences in body size between mainland and island populations have been reported for reptiles, birds and mammals. Despite widespread recognition of insular shifts in body size in these taxa, there have been no reports of such body size shifts in amphibians. 2. We provide the first evidence of an insular shift in body size for an amphibian species, the rice frog Rana limnocharis. We found significant increases in body size of rice frogs on most sampled islands in the Zhoushan archipelago when compared with neighbouring mainland China. 3. Large body size in rice frogs on islands was significantly related to increased population density, in both breeding and non-breeding seasons. Increases in rice frog density were significantly related to higher resource availability on islands. Increased resource availability on islands has led to higher carrying capacities, which has subsequently facilitated higher densities and individual growth rates, resulting in larger body size in rice frogs. We also suggest that large body size has evolved on islands, as larger individuals are competitively superior under conditions of harsh intraspecific competition at high densities. 4. Increases in body size in rice frogs were not related to several factors that have been implicated previously in insular shifts in body size in other taxa. We found no significant relationships between body size of rice frogs and prey size, number of larger or smaller frog species, island area or distance of islands from the mainland. 5. Our findings contribute to the formation of a broad, repeatable ecological generality for insular shifts in body size across a range of terrestrial vertebrate taxa, and provide support for recent theoretical work concerning the importance of resource availability for insular shifts in body size.

Insular Carnivore Biogeography: Island Area and Mammalian Optimal Body Size

Patterns of size variation in insular mammals have been used to support the claim that mammals have a single optimal body size. This hypothesis enjoys wide support, despite having been questioned on both theoretical and empirical grounds. It is claimed that species of optimal size maintain the highest population densities. Therefore these species are thought to inhabit the smallest islands, where larger and smaller species are generally absent. We sought such a pattern by testing how area affects the body sizes of the largest and smallest carnivore species on islands. Using data on carnivores from 322 islands, we found that the sizes of carnivores on small islands tend to be close to the order's mode. Furthermore, we found that the size distribution of carnivore species that inhabit islands resembles that of those whose range is entirely continental. We conclude that insular carnivores provide no support for theories proposing a single optimal size, and we suspect such theories are also flawed on theoretical grounds.

The impact of Quaternary Ice Ages on mammalian evolution

The Quaternary was a time of extensive evolution among mammals. Most living species arose at this time, and many of them show adaptations to peculiarly Quaternary environments. The latter include continental northern steppe and tundra, and the formation of lakes and offshore islands. Although some species evolved fixed adaptations to specialist habitats, others developed flexible adaptations enabling them to inhabit broad niches and to survive major environmental changes. Adaptation to short-term (migratory and seasonal) habitat change probably played a part in pre-adapting mammal species to the longer-term cyclical changes of the Quaternary. Fossil evidence indicates that environmental changes of the order of thousands of years have been sufficient to produce subspeciation, but speciation has typically required one hundred thousand to a few hundred thousand years, although there are both shorter and longer exceptions. The persistence of taxa in environments imposing strong selective regimes may have been important in forcing major adaptive change. Individual Milankovitch cycles are not necessarily implicated in this process, but nor did they generally inhibit evolutionary change among mammals: many evolutionary divergences built over multiple climatic cycles. Deduction of speciation timing requires input from fossils and modern phenotypic and breeding data, to complement and constrain mitochondrial DNA coalescence dates which appear commonly to overestimate taxic divergence dates and durations of speciation. Migrational and evolutionary responses to climate change are not mutually exclusive but, on the contrary, may be synergistic. Finally, preliminary analysis suggests that faunal turnover, including an important element of speciation, was elevated in the Quaternary compared with the Neogene, at least in some biomes. Macroevolutionary species selection or sorting has apparently resulted in a modern mammalian fauna enriched with fast-reproducing and/or adaptively generalist species.