Convergence on the ground above the ground under the ground

Convergence is pervasive, and moving from the animals let us take a particularly striking example from the plants. Consider two desert plants, one a cactus (Peniocereus striatus) that grows in the Sonoran Desert of Mexico, the other a type of spurge (Euphorbia cryptospinosa) from east Africa. Both are rather unprepossessing plants, straggly, and to the uninitiated, which the plant presumably hopes includes ravenous herbivores, look as if they are dead. Not only are the plants quite similar in general habit, but the stems are astonishingly convergent,137 with a characteristic cross-section that shows flat ribs and intervening recesses (Fig. 6.7). This arrangement represents a clever adaptation, because in times of severe drought and water stress the ribs fold tightly together, thereby shielding the furrows where the pores (stomata) for gas exchange are located, but from which water can also readily escape. The interiors of the stems are also very similar, with the interior cells storing water and a central pith rich in food (starch). The similarities between these plants even extend to the reddish pigment, located just beneath the surface, which confers the moribund appearance to the plant. Of course, there are differences such as the details of the stomata, which in the cactus can be sealed with some sort of water-soluble substance, whereas in the spurge there are short spines that, it is hypothesized, restrict air circulation and thus water loss.

Cacti and spurges are only distantly related, and their common convergence is because of the rigours of living in an arid environment, although the degree of similarity is, to put it mildly, noteworthy. To a considerable extent evolutionary convergences have been placed in the context of adaptation to extreme environments. Not surprisingly, this specific and striking correspondence between the stems of a cactus and a spurge is only one of a whole swath of similarities that almost inevitably unite plants living in conditions of extreme aridity, the so-called xerophytes.138 Similar remarks apply to the various Mediterranean-style floras in areas as remote from the Mediterranean itself as West Australia, South Africa, and Chile,139 and extend to plant-animal interactions such as seed dispersal by ants.140 The floras are not, it needs to be stressed, identical and the various differences also have evolutionary significance. Even so, in a survey of how plants function from the tropics to the tundra it is clear that convergence, for example in leaf structure,141 is a pervasive phenomenon.142

Defying gravity and exposed to wind, not to mention such loads as a fruit crop or a passing arboreal animal, it is not surprising that the anatomy of plants reveals many structural constraints, nor is it odd that the number of solutions to a particular biological problem may be severely limited. One such example is the convergence seen among the girder-like structures, known as petioles, which support leaf systems.143 In itself this is to be expected: buildings are not made of blotting paper and neither are plants. There are, however, more specific organizations seen in plants that would seem to be unique. What could be a better example of evolutionary uniqueness, of the sheer quirkiness of life, than a flower? Other worlds we may presume will have their equivalents to plants. Most probably they will build their cell walls with a carbohydrate similar to cellulose, and they will almost certainly power their photosynthesis by using chlorophyll (p. 110). So, too, in form they will range from ground-creeping herbs to lianas and trees.144 All these are rather generalized predictions, based either on the universal nature of biochemistry (see p. 77) or reflecting the broad constraints of life.

On the other hand the specifics, such as flowers, let alone anything remotely similar to say a tulip or a daisy, are quite out of the question. They are surely a contingent happenstance of life on Earth, not life on Threga IX. From a historical perspective this must in one sense be true given that the flowering plants (the angiosperms) evidently evolved from a single ancestor, probably at some time in the Jurassic. Matters may, however, be rather more complicated. Whether the smiling biped on Threga IX offers us a tulip is not really relevant; remember we are in pursuit of general biological properties. Thus, in the case of the flowers, they display several specific characters. Most obvious, and appreciated, are the two whorls of leaves modified to form the often brightly coloured petals and the associated bract-like sepals. The reproductive arrangement follows the well-trodden path of male (pollen from the stamen) goes to female (via the stigma and ultimately the contained ovule), the happy conclusion of which is the fertilized seed.145 To be sure, the structure of the flower is specialized, sometimes remarkably so as in the orchids, but the basic principle of plant sex (pollen meets ovule) is from an evolutionary perspective much more ancient than the flowering plants themselves. Yet no other group of plants has a reproductive organ exactly like those of the flowering plants. Not only is the flower highly distinctive, but its evolutionary origins are still rather mysterious: seldom can a review paper avoid parroting Darwin's remark to the effect that the origin of the flowering plants remains 'an abominable mystery'.146

Yet, as is well known to botanists, there exists an otherwise rather obscure assemblage of plants known as the Gnetales.147 They in turn belong to a larger group, known as the gymnosperms, a group that is likely to be more familiar in the form of the pine tree and other conifers. As with other gymnosperms, the reproductive organs of the Gnetales are located on cones, but in this case they are remarkably flower-like.148 Even more intriguing is a rather strange phenomenon known as double fertilization. In both the flowering plants and the Gnetales the pollen grain carries two nuclei, which having landed on the female stigma are delivered to the ovule by the agency of the pollen tube. One of the male nuclei has, of course, the heroic task of fertilization, which leads to the formation of the seed. The second nucleus, however, is not the runner-up because it fuses with another female nucleus. Hence the term double fertilization. In the case of the Gnetales naturally enough this second fertilization provides an extra, so-called supernumerary, embryro.149 In the flowering plants, however, there is a further twist to the story because typically yet another female nucleus joins in to produce a triploid cell (i.e. three haploid sets of chromosomes combine, one set from each of the three nuclei). This cell, equivalent to the supernumerary embryo, then divides to form a mass of tissue known as the endosperm. Triple sex is not just a baroque ornamentation: the endosperm has a specific function, that is, to provide nourishment to the adjacent and growing seed. This is widely regarded as one of the key innovations to explain the overwhelming success of the flowering plants, whose evolutionary radiation has many interesting parallels to the Cambrian 'explosion' of marine animals.

So distinctive is this arrangement of double fertilization that, when it is taken together with the flower-like structures of the Gnetales, a strong supposition has arisen that the origins of the flowering plants are to be found in a gnetalean-like group that flourished perhaps about 150 Ma ago.150 Nor are flowers and double fertilization the only similarities that exist between the two groups. Among the long list of features in common between the flowering plants and Gnetales, particular mention has been made of the similarities in vessels (that is the xylem, where the water-conducting tubes are open rather than consisting of a series of discrete cells (tracheids))151 used to transport water through the plant, and in the genus Gnetum the venation of the leaf. Taken together, these similarities have been used to support a close evolutionary connection between the two groups and thereby explain the unique origin of flowers.

Yet all this turns out to be convergent. Despite some early research from molecular biology supporting a link between flowering plants and Gnetales, more extensive analyses both decisively reject this relationship and propose that Gnetales is surprisingly close to the conifers.152 So on this basis it is argued that the cardinal features of flower-like structures and double fertilization153 have been arrived at independently, as indeed have a number of other characters.154 So, too, other supposed similarities, such as the structure of the xylem, are now shown to be exaggerated.155 All this, of course, presupposes that the phylogenetic conclusions are correct. It needs to be pointed out that there are a number of extinct plants, notably a group of interest to the herbivorous dinosaurs and known as the bennetti-taleans, which also have some flowering plant-like characteristics. Accordingly, given current phylogenetic uncertainties, it remains possible that some features actually evolved earlier in the history of plants and so were respectively inherited by both the Gnetales and the flowering plants.156 Most of the latest research (note 152), however, continues to place the Gnetales very close to the pine trees and their relatives,157 so making the argument for convergence the more likely. That such features as specialized sex organs (a.k.a. flowers) open to insect pollination,158 not to mention the nutrition of the embryo,159 should emerge in more than one lineage of advanced plants is surely not that surprising.160 Perhaps flowers are an evolutionary inevitability; and, moreover, even here the options are not unlimited. Once again, convergence of floral anatomies is widespread,161 reflecting such constraints as methods of pollination by both insects and birds.162 So we might speculate that not only does Threga IX have plants and flowers, but perhaps something not so different from a bird pollinating a flower.

Birds themselves provide further compelling examples of evolutionary convergence. There are, for example, repeated trends: towards dark plumage in tropical seabirds;163 in the many independent examples of long-distance migrants the development of more pointed wings164 as well as parallel changes in the skeletal structure;165 a cluster of characters (for example, in wing and leg structure) that converge in at least some grain- and insect-eating birds living at very high altitudes;166 convergences in the aerially feeding insectivorous birds167 and nectar feeders;168 and a striking convergence between certain aquatic birds, specifically the grebe and the loon.169 Indeed, birds as a whole are a rich source of insights into the prevalence of evolutionary convergence,170 as well as having some striking similarities with other groups. A well-known example is the parallels between humming birds and the sphinx moths, both hovering above flowers as they draw out the nectar.171 As we shall see in Chapter 8, the birds are even more instructive in a wider context, as when we encounter that 'honorary mammal' the kiwi, consider the physiology of warm-bloodedness, or compare their powers of vocalization: once again striking convergences emerge.

There are, moreover, other sorts of convergence that are equally intriguing. Birds form a popular part of many peoples' diet, but in New Guinea one bird, known as Pítohuí, is avoided because of its unpleasant taste. The bird's repellent nature stems from a chemical (a type of steroidal alkaloid) that interferes with the sodium channels of the nervous system. The chemical itself is most probably sequestered from the bird's diet. Yet this chemical defence mechanism has evolved independently in the Neotropical poison-dart frogs,172 and has also been found in another New Guinea bird, probably unrelated to Pitohui.173 Nor is this the only example of chemical convergence; a repellent is also found in the crested auklet. This bird, a denizen of the subarctic Pacific and Bering Sea, has evolved a chemical conver-gency with a group of insects known as the heteropterans, in which it occurs in repugnatorial glands.174 So, too, in plants themselves the extensive employment of defensive alkaloids, cyanogenic glycosides, and proteinase inhibitors also appears to be rampantly convergent.175 Herbivores, of course, take steps to avoid noxious plants, and in the case of the lizards, which are predominantly insectivorous carnivores, the several independent shifts to plant-eating have led to a correlated evolution of chemosensory abilities that represents a type of convergence in response to a common selective pressure.176

Within the mammals also there are compelling examples of convergence. One of the best-documented case histories concerns the so-called fossorial species, that is, animals such as the mole which dig their way through light soils. Just as there is a parallel between the sabre-tooth cats and marsupial thylacosmilids, so the example of convergence of the Australian marsupial mole to its placental equivalent is a well-known and striking example. Somewhat less widely appreciated is that among the extinct marsupial faunas of South America (home of course to the sabre-toothed thylacosmilids) there is another striking parallel to the placental moles, this time in the form of a remarkable group known as the necrolestids.177 But the convergences do not stop there, and among the extant burrowing mammals the similarities extend across a much wider range of groups than just the mole and its various marsupial equivalents. In total this evolutionary convergence encompasses no less than three orders, 11 separate families, and at least 150 genera (Fig. 6.8).178 Convergences are extensive, involving not only anatomy, but extending to physiology, behaviour, and even aspects of genetics. Some of the most obvious similarities, such as powerful digging forelimbs with prominent claws,179 rudimentary eyes and small size (including small testicles, sensibly enough for the males dragging their way along tunnels), are rather predictable. Living in burrows, where levels of CO2 from soil respiration are often

Geomys

Scalopus

Scapanus Talpa

Spalax Ellobius

Rhizomys, Tachyoryctes

Ctenomys

Clyomys

Amblysomus Chrysochloris

Cryptomys Georychus

Ctenomys

Clyomys

Amblysomus Chrysochloris

Cryptomys Georychus

Geomys

Scalopus

Scapanus Talpa

Spalax Ellobius

Rhizomys, Tachyoryctes

Urotrichus Scaptonyx figure 6.8 Convergent evolution in the burrowing (fossorial) mammals, including the familiar mole {Talpa), mole-rat {Spalax), and marsupial mole (Notoiyctes). (Reproduced from fig. 1 of E. Nevo (1995) Mammalian evolution underground. The ecological-genetic-phenetic interfaces, Acta Theriologica, Supplement 3 (Ecological genetics in mammals II, eds. G. B. Hartl and J. Markowski), pp. 9-31, with permission of author and Acta Theriologica.)

elevated, places physiological burdens on these digging mammals, and here too we see convergences. In the following chapter I explore some of the other fascinating examples of convergence in terms of sensory systems. The subterranean mammals provide some outstanding examples, most notably in the case of hearing (pp. 190-191). Unsurprisingly, visual systems are correspondingly usually highly reduced, but as we shall see (pp. 175-177) in the case of the bizarre star-nosed mole there is another even more remarkable convergence between its nose and the eye.

As I shall argue, these convergences involving sensory systems are of considerable importance in assessing the likelihood of recurrent emergence of such biological properties as intelligence. Linked to this is the generally rather neglected evidence for convergence in behaviour (see also Chapter 10). Study of the burrowing mammals also throws up apparently quirky examples of behavioural convergence, such as their response when confronted with a carrot.180 In a survey of more than 20 small mammals, it was found that without exception the subterranean species much preferred to start eating at the lower tip. Rather remarkably, one type, the Zambian common mole rat, can identify the polarity of the carrot even if both ends of the root have been cut off. This uniform preference for attacking the carrot at one end makes sense, of course, because a burrowing mammal is much more likely to encounter the lower end of a plant root. In the case of the burrowing animals here, too, there are other striking parallels in behaviour; typically they are solitary, communicate seismically,181 and are generally highly aggressive.182 Such convergences of fossorial activity are, of course, easiest to document in extant faunas, but it is also worth drawing attention to some striking convergences in fossil taxa. The controversial status of the possible Cretaceous bird Mononykus is returned to below, but equally remarkable are highly specialized palaeanodont mammals (Epoicotherium and Xenocranium) from the Oligocene of North America. These animals, equipped with powerful frontal claws, a prominent snout, and lacking sight, are strikingly convergent in many respects on other fossorial mammals, especially the group of insectivores known as the African golden moles (or chrysochlorids).183

Perhaps the most remarkable example of behavioural convergence involves the evolution of eusociality, most famously in

figure 6.9 The naked mole-rat, Heterocephalus. (Photograph courtesy of H. Burda, University of Essen.)

Heterocephalus, better known as the naked mole-rat (Fig. 6.9). The preface of the main book184 on naked mole rats has a telling story:

In the mid-1970s R. D. Alexander [one of the book's editors] ... lectured on the evolution of eusociality ... In an effort to explain why vertebrates had apparently not evolved eusociality, he hypothesized a fictitious mammal that, if it existed, would be eusocial. This hypothetical creature had certain features that patterned its social evolution after that of termites (e.g., the potential for heroic acts that assisted collateral relatives, the existence of an ultrasafe but expansible nest, and an ample supply of food requiring minimal risk to obtain it). Alexander hypothesized that this mythical beast would probably be a completely subterranean rodent that fed on large tubers and lived in burrows inaccessible to most but not all predators, in a xeric [dry] tropical region with heavy clay soil (p. viii).

The mythical beast emerged as the naked mole rat.

Eusociality refers to a colonial system whereby typically only one female is reproductive, and the remaining individuals are divided into several castes, e.g. workers, that operate in a cohesive and coordinated way, notably in food collection and care of the young. Despite

figure 6.10 Another social mole-rat, a bathyergid. (Photograph courtesy of H. Burda, University of Essen.)

its complexity, eusociality is patently convergent. One notable feature of this convergence is not only the social structure, but also the fact that the colony makes strenuous efforts to protect the breeding female/queen from danger. Eusociality has evolved several times in the mammals, including another group of mole-rats (the bathyergids, Fig. 6.10)185 as well as some social voles.186 The overall convergence, however, extends much further because there are striking parallels with various groups of insects (most famously in the hymenopter-ans (ants, bees, and wasps)), but also the termites and various other groups such as the gall thrips187 and certain beetles.188 In the insects, where of course it is best known, not only is eusociality rampantly convergent, but also remarkably instructive in terms of parallels with human organization and activities (agriculture, warfare, and competitive exclusion; see Chapter 8). The often striking differences between the various castes of a eusocial insect, most familiar in the contrast between the bloated and near-immobile queen versus beweaponed and agile soldiers, has another parallel in the naked mole rats because rather remarkably (for a vertebrate) the colony also shows morphological castes.189 Nor are insects the only eusocial arthropods; this characteristic has also evolved in some crustaceans, in the form of a coral-reef shrimp (Synalpheus),,190 where eusociality has evolved independently three times.191

A recurrent theme of this book is not only the implications of convergence for evolution, but also the problems it can pose for its resolution. Not, I hasten to add, in terms of the reality of evolution. This is emphatically not in question; rather the reverse: the details of convergence actually reveal many of the twists and turns of evolutionary change as different starting points are transformed towards common solutions via a variety of well-trodden paths. Rather, convergence, if not identified, may lead to blatantly erroneous phylogenetic reconstructions. The constraints imposed by the burrowing mode of life offer a potentially telling example of this problem. There is an extraordinary fossil reptile, Mononykus, from the Upper Cretaceous of Mongolia. It has attracted considerable attention, because of both its strange morphology and its proposed relationships. Bipedal, and with a fore-limb equipped with a powerful claw, Mononykus has been interpreted as a flightless bird on the basis of various skeletal characteristics.192 As such it is regarded as a key discovery in the evolution of early birds, even though Mononykus certainly remained firmly on (or in) the ground, and its age means that it post-dates a number of more advanced birds that have been unearthed in strata of Lower Cretaceous age in north China.193 When originally described the strange arm of this animal, with its powerful claw, was recognized as similar to that of digging mammals, although other features of Mononykus appeared to preclude this mode of life.194 Zhonghe Zhou, however, has suggested that the adaptation to burrowing is the correct interpretation,195 and by neglecting this and other convergent features the role of Mononykus as a very strange bird needs rethinking.

Moles, other fossorial mammals, and possibly even Mononykus offer, therefore, an excellent example of numerous separate evolution trajectories converging towards a common solution, best adapted for occupation of this demanding habitat. It is hardly surprising, therefore, that if we turn our attention to a somewhat analogous ecology, that of burrowing frogs196 and reptiles, especially the limbless snakes and the sand-dwelling (and sand-diving) lizards, we again find instructive examples. Among the snakes, the convergences include those of brain structure, reflecting the various adaptations to a fos-sorial existence, including diminished vision and the ability to communicate seismically. So far as the burrowing reptiles are concerned, once again there are certainly differences but also convergences.197 Among the lizards such a fossorial mode of life has evidently been adopted independently at least eight times, and the mechanical problems imposed by entering and moving in the aeolian dunes have led to many parallels.198 Even so, historical antecedents play their part. As Nick Arnold cogently remarks, with specific respect to ear reduction in iguanian lizards, 'Different lineages of organisms often evolve a number of similar traits independently, but the order in which these are assembled may often be different, even in ecological analogues, especially if the taxa concerned are not closely related ... This [is the] phenomenon of equipotentiality, where more or less the same overall condition is reached by different routes'.199 These comments are of key importance to the overall theme of this book: the evolutionary routes are many, but the destinations are limited. In a related context it is also worth remarking that many lizards have toes equipped with fringes, which assist not only with movement across loose sand, but also other substrates, including running across water.200 Not surprisingly, the evolution of toe fringes is both convergent and correlated to the nature of the substrate upon which the lizard moves.201 In a related fashion the evolutionary trend in lizards that become limbless, and thereby adopt a snake-like habit, has manifested itself several times. Significantly, despite this convergence, there are only two effective destinations represented by distinct types of ecomorph: small and short-tailed forms that burrow and larger 'grass-swimmers' with long tails.202

If we reconsider deserts, but this time in the context of the mammals living largely above the surface, again we find a richness of convergences.203 This is especially true among the rodents,204 many of which are bipedal hoppers, nocturnal and possessing enormous ears: the desert kangaroo rats are well named. Here, too, there are some interesting details. Deserts in South America, notably the Monte of Argentina, and Australia are effectively unoccupied by these specialized rodents, yet, as Michael Mares notes, we can be sufficiently confident from the general prevalence of convergence that in these deserts the same story so far as it concerns rodents will probably unfold.205 If evolution is not inevitable, it is at the very least highly predictable. South America is of particular interest because it appears that an extinct group of marsupials, known as the argyrolagids,206 were strongly convergent on the placental desert rodents, especially the kangaroo rats and a convergent equivalent known as the jerboas. G. G. Simpson simply remarks, 'the argyrolagids present one of the most striking known examples of evolutionary convergence'.207 Mares208 proposes an interesting explanation for the relative failure of the incoming placentals to diversify in this desert habitat in South America. He suggests that when they flooded south from North America as the Panamanian isthmus emerged to separate the Atlantic from the Pacific (see p. 130) this niche had already been occupied by the argyrolagids, which had then only recently become extinct. One of the immigrants, a rodent known as Eligmodontia, does, however, seem to be evolving in the direction of turning into something like a kangaroo rat.

To conclude: it does indeed appear to be the case that on the extremities of life the options narrow, choices diminish, and convergence is the norm. Let us imagine some suitably remote planet, inhospitable, with vast deserts, and a thin atmosphere. Life, if it existed, might be strangely reminiscent of similar regions on Earth. Tough plants, burrowing animals, and in the sky flying organisms with remarkably pointed wings: a sort of mirror image of our world when the going gets tough. But surely, so the argument continues, in the more benign areas where the mainstream of life enjoys an exuberance of diversity, matters are far less constrained. In this riot of forms, we might suppose, nearly anything is possible. Evolution is little more than a chaos of contingency, free of trends and with a myriad of equally likely destinations. Here the problems of convergence melt away. Nothing could be further from the truth.

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