摘要:

The high number of species in the tropics permits two interpretations, an ecological and a genetic one. According to the genetic one, certain processes are active in the tropics (e.g. increased mutation rates or a speeding up of the number of generations) which accelerate the process of the multiplication of species. According to the ecological one, the tropical environment favours the accumulation and continued coexistence of an exceptionally high number of species, produced by the orthodox speciation process.The analysis of bird speciation favours a largely ecological interpretation. There is no evidence that speciation in the tropics differs in principle from that in the temperate zone. An alternation of dry and pluvial periods during the Pleistocene, together with continued if not accelerated mountain building in South America, south‐east Asia, and New Guinea, resulted in the steady production of numerous geographical isolates some of which completed the speciation process.Ecological factors may accelerate the rate of speciation in the tropics. This includes the possibility of a greater sedentariness of populations and a correspondingly greater efficiency of geographic barriers. It includes perhaps a greater narrowness of ecological niches and consequently a better chance for a new species to carve out a niche in an already rich fauna.Among populations coexisting in a geographical isolate, only a small percentage seem to complete speciation. The others either fuse again with the parental species when the geographical isolation ends or become extinct. Size of the speciating population and its ability to find a new niche seem to be crucial for success.All guesses as to which recent species owe their origin to what particular past climatic events, must be considered as highly speculative. The unambiguous establishment of such correlations has so far been unsuccessful, at least so far as birds are concerned. Absolute figures for rates of speciation cannot be give

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01808.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Tropical countries have many times more species of most taxa than temperate ones, and small areas in the tropics have a smaller multiple of the number of species of small temperate areas. Where many species are present, abundances tend to be more equal and geographic distributions more spotty. Most tropical environments are less seasonal and more productive, and the dry areas and mountains which are relatively more seasonal and less productive have fewer species. The species which have reached offshore islands are often much commoner there and occupy expanded habitats.To account for these relations, the following general hypothesis seems necessary:species interactions are important and the tropics have a head start on speciation. The head start, or greater rate, allows extra species to pile up in the tropics, but because of the importance of competition, no single area becomes as greatly enriched. Rather, faunal differences between areas increase. The lesser excess of tropical species in small areas is largely due to greater productivity and reduced seasonality which make marginal ways of life profitable. With more overlap in resources, the closely packed tropical species have more uniform abundances and the coexistence of these species is more precarious, causing the spotty geographic distributions.Neither the species diversity of the food supply nor the longer breeding season (supposedly allowing staggered nesting seasons with an early shift and a later shift) is relevant to bird species diversity.

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01809.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Land snails of the genusPartulaFérussac offer several advantages for the study of micro‐evolutionary change. They are highly variable both within and between populations. Their mobility is low, so that there are genetic differences between natural populations only short distances apart (20 m or less). They are ovoviviparous, and easy to rear in the laboratory, so that these genetic differences can be investigated experimentally. Finally, they show, at least in some places, a very curious pattern of speciation.Since 1962 we have been studying the population genetics ofPartulaspecies on the island of Moorea in French Polynesia.The late Professor H. E. Crampton, in a classic monograph, recorded ten species from the island. Later, he added an eleventh. One of the species (P.dendroicaCrampton) is an allopatric replacement ofP. suturalisPfeiffer and probably deserves only the rank of geographical race, since the two forms cross freely in the laboratory. We have evidence thatP. tohiveanaCrampton,P. olympiaCrampton, andP. mooreanaHartman will all, in some localities, hybridize withP. suturalis.We also have evidence of natural hydridization betweenP. aurantiaCrampton andP. suturalis, and betweenP. exiguaCrampton andP. taeniataMörch.There are thus two major‐species groups on Moorea (thesuturaliscomplex and thetaeniatacomplex). The status of the other ‘species’ (P.mirabilisCrampton, P.solitariaCrampton and P.diaphanaCrampton and Cooke) remains uncertain, although they are probably members of thetaeniatacomplex.Two members of a species‐group may behave as distinct species at one locality, but hybridize freely or intergrade at another. Such changes can take place over distances of 200 m or less. Some of the changes seem to be related to geographical barriers, There is, for instance, an apparent ring‐species with a diameter of about 5 km. Other changes are more difficult to interpret, since they occur without obvious relation to geographical features.Ecological and genetic studies on P.taeniatahave led to the suggestion that striking divergence between adjacent populations can take place even in the absence of geographical barriers. Whether this can continue to the point of speciation is still uncertain, but an explanation in these terms would clarify many of the puzzling phenomena we have observed in populati

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01810.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

The simuliid fauna of the Ethiopian region is notably isolated, only two of its species occurring elsewhere; the region has 124 described species, and others still undescribed. Simuliid larvae and pupae are adapted to attachment in moving water; adult females may disperse over considerable distances. This paper considers three examples of evident speciation in an ecological context:(1) speciation on afromontane ‘islands’ (2) theSimulium naeveigroup and the adaptive significance of the association between the early stages and freshwater crabs; (3) theS. damnosumcomplex, which includes the widespread vectors of human onchocerciasis. Maps show the distribution of species in these three gro

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01811.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Among freshwater fishes greater diversity is shown, both within taxa and in communities, at low than at high latitudes.This paper discusses contributions that ecological studies on tropical fishes make towards understanding this diversity, the mechanisms of speciation, and the ability of so many species to coexist in these complex communities.In Africa studies have centred around the explosive speciation of cichlids in the Great Lakes; in the rivers non‐cichlids have speciated more abundantly. In South America catfishes and characoids exhibit the most spectacular adaptive radiations, evolving mainly under riverine conditions. Rates of evolution are indicated by the presence of five endemicHaplo‐chromiscichlids in L. Nabugabo, a small lake cut off from L. Victoria 4000 years ago (Greenwood, 1965), and of eighteen cyprinids in the 10,000 year old L. Lanao in the Philippines (Myers, 1960).Tropical fish communities are diverse wherever no extreme environmental conditions restrict fish distribution. No one explanation can account for this diversity in all cases. In rivers it seems mainly due to allopatrically evolved species coming together in the course of time. Species flocks are characteristically found in lakes, and here microgeographical isolation can allow ‘allopatric’ speciation within the lake, as amongst the ‘Mbuna’ cichlids of Nyasa, but not among NyasaTilapia.Isolating mechanisms include physical, chemical and biotic barriers causing spatial isolation; temporal isolation; and behavioural barriers, many of these fishes having complicated courtship displays, and some ‘homing’ to particular places to spawn.The numbers of habitat niches are enhanced by the complexities of the tropical environment. Nevertheless there is a good deal of overlap–in Victoria thirtyHaplochromisspecies share a major habitat (Greenwood, 1964). Specializations to use a particular food have developed several times so that even in a single habitat several species may tap the same food source; ten species of Mbuna share theAufwuchsof Nyasa's rocky shores (Fryer, 1959). This appears to contradict Gause's exclusion hypothesis, but no two species are likely to overlap in all their living requirements throughout life. Recent work on variations in oxygen tolerances of young cichlids (Welcomme, 1967) indicates how many as yet unstudied factors must affect the dynamic balance of numbers between co‐habiting species.Devices that reduce competition, and the role of predators, are both discussed.Predators abound, and predator pressure appears to have been a key factor in moulding evolution in these communities. Many fish species have developed some form of parental care, with associated decrease in fecundity. Biotic pressures are of far greater importance than climatic factors in the evolution of th

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01812.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Nous nous sommes efforcés depuis 1957, date de nos premières recherches à Madagascar, de preciser les caractères systematiques des différentes espèces de Lémuriens.Sans minimiser l'importance des critères morphologiques, nous nous sommes jusqu'à maintenant appliqués, par des études éco‐éthologiques de mieux connaître les différentes formes existantes et de préciser leurs caractères distinctifs. Les études ont éxé menées sur le terrain au cours de plusieurs missions et en captivité dans la station de recherche de Tananarive et au Laboratoire de Brunoy, en France.C'est ainsi que nos études nous ont amenéà revenir, par exemple, pour les Lémurs, à la classification de Schwarz groupant en une seule espèce toute une série de formes de Lemur et à les considérer comme des sous‐espèces de l'espèceLemur macaco.Les recherches sur les chromosomes faites par plusieurs chercheurs ces dernières années soul‐évent cependant quelques problèmes car on a trouvé des formules chromosomiques différentes pour des animaux fort semblables à beaucoup de points de vue. Nous avons churché tous les cas d'hybrides connus et tenté nous‐mêmes, et avec R. Albignac chercheur de l'O.R.S.T.O.M. a Tananarive de réaliser toutes les hybridations possibles en captivité.C'est un aperçu de ces recherches en cours que nous avons voulu exposer, ainsi que quelques observations nouvelles sur les hybrides obtenus.L'évolution des espèces le plus souvent sociales et territoriales posent des problèmes très inter‐essants et Madagascar nous offre encore à ce point de vue un champ d'étude privilége qu'il faut utiliser avant qu'il ne soit trop tard à cause des destructions de plus en plus rapides du milieu forestier. Nous avons ainsi pu faire quelques observations dans la nature, notamment sur la variabilité au sein des groupes, qui nous permettent à faire quelques hypothèses sur les mécanismes de l'évolution des Lémuriens à Madagascar. II semble que le territorialisme de la plupart des Lémuriens favorise la formation d'homozygotes ce qui a peut‐être pour conséquence d'accélérer la spéciation. Beaucoup de questions concernant de nombreuses espèces restent cependant encore sans réponse et il faut se garder de trop vouloir déduire des observations fragmentaires que nous possèdons. Des études plus poussées sur le terrain et particulièrement dans certaines régions qui servent de limites à des sous‐espèces nous paraissent interés

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01813.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

The value of structural characters should be assessed by their role in local reproductive processes. Just as in genetics the abstract species has been replaced by the population, the functional contacts between animals and plants should not be considered on the species level but in relation to the whole local ecosystem. Incidental cross‐links therein are described as leading to ‘concurrent evolution’ as a corollary. Inside the rain forest homeostasis is provided in the vegetative sphere by pest‐pressure, all‐year germination and accidental replacement, but it is especially strong in the reproductive sphere, by mutual collaboration in time, this via a feedback by means of internal, permanent animal life. These circumstances explain the lack of vicariance and the polymorphy, maintaining nevertheless normal interspecific selection. A comparison with seeds, seedlings, flowers, etc., from temperate vegetation is illustrative. There, abiotic factors, edaphic and climatic, are dominant and the whole is more open to external influences. Their anemochory (wind dispersal) and autochory (self dispersal) prove to be not fundamental for plant‐life.In the tropics archaic dispersal methods (by fish and reptiles) of archaic, juicy, large seeds (sarcotesta) persist in a conservative, homeostatic environment alongside modern methods. Both lead to typically tropical traits and may serve to explain phylogenetic trends. These are discussed for Leguminosae. The aril is a compromise suited for dryer regions, but maintaining archaic attractivity of the seed–not yet the pericarp‐fruit.‘Bat‐fruits’ provide a typical modern touch.Archaic pollination‐methods (for example by beetles and simple flies) are bound to the ecosystem as a whole, which provides subsistence. Deceptive cross‐links (short‐circuits) are here more important than food for such unadapted visitors. The flowers probe instincts chemically. Some flowers provide a brood‐place, as inFicus, which breeds its own wasps in an ancient relationship, allowingFicusto escape from the forest. Monophily (deceptive or not) is not necessarily late.The bond with other hymenopterans (wasps) arose also as an incidental cross‐connection. Later this developed into a fixed relationship with flower‐insects. Orchids as active ‘ecological parasites’ demonstrate the exploitation of pre‐existing animal life.The more modern impact of birds and bats is described in a synecological context, which makes some parts of syndromes understandable. Case‐histories are given to illustrate the interaction between dispersal and pollination, between functions and structures in seed (fruit) and flowers. We see their clashes and the solution, always found, leading to permanently necessary changes. Dispersal may be anticipated in apparently functionless (or supposedly phylogenetical) floral characters.Points from a pollination‐syndrome may form preadaptations for a dispersal‐syndrome and vice versa. The calyces of orientalTrifoliumspecies illustrate monospermy, and their capitula the possible origin of these charac

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01814.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Tropical genera are frequently old, several going back to at least the Upper Cretaceous. Extinction and subsequent disjunction of range is a rather common phenomenon, the reasons for which are obscure.Tropical rain‐forest groups have carried important potentialities and this has led to the development from these ancestral groups towards many specialized microtherm offshoots peopling the colder parts of the globe. This has sometimes also led to north‐south bi‐hemis‐pherical different counterpart families, affinities, or lineages.Evolution among tropical plants has worked in an erratic way and there is an astonishing variation in the rate of potentialities. This cannot be correlated with degree of versatility in adaptation.The generous tropical rain‐forest conditions offer the lowest possible survival‐value level to plant development, i.e. the lowest competition intensity. For plants of the undergrowth there may be a minor limitation set by pollination deficiency.There are roughly three main kinds of species in the tropical rain forest, (i) the coenospecies, with a variant the complex species, (ii) the reticulately allied species forming ‘solid blocks’, and (iii) the isolated species of genera of remote alliance.The coenospecies and complex species came into being through development by a very gradual process of differentiation by small changes during a long period of dispersion. They have probably not led to the origin of the solid blocks of which the species are often sympatric. Both categories belong to micro‐evolutionary change, but did not contribute to macro‐evolution.The tropical rain forest harbours an unusually large number of isolated species and an impressive number of unusual structures. Many of these are taxonomic parallels of terata and all can be derived from growth phenomena. Similar abnormal phenotypes, with considerable change in high‐ranking structures, may be caused by sudden genetic changes, often single gene mutations. Also synthesogenic changes are sudden steps (amphi‐ and aneuploids).The low survival level of the tropical rain forest has offered the best opportunities for the survival, preservation, and further development of such saltatory evolution. Terata can be arranged in a few main groups according to growth phenomena and there is a striking parallel between terata and taxonomical monstrosities. From this correlation it appears plausible that the ‘hopeful’ monsters have been an, or may be even the most, important means of macro‐evolution by single large steps. With the length of geological time and the infinite numbers of individual tropical plants, an infinitesimally small number of nature's experiments succeeding could well have led to attaining new supraspecific structural levels by saltation. After all the number of structural plans is definitely restricted.Data on hybridization are hitherto scarce and polyploids are scant in the ligneous families; both impressions may be premature throug

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01815.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Few ecosystems provide better opportunities for the study of evolution and speciation than those inhabiting the uppermost parts of the high east African mountains. These mountains are mainly of volcanic origin and lie widely scattered across the wide plateaux of east Africa, several of them reaching altitudes between 3500 and 6000 m. Their vegetation deviates very much from that of the intervening lower country, displaying a marked zonation with a montane forest belt, a (subalpine) ericaceous belt, and an afroalpine belt. The flora of the latter, the afroalpine flora, is of exceptional interest in this connection.The afroalpine flora is famous for its large numbers of geographically vicarious taxa–its Giant Senecios and Giant Lobelias are as renowned as the finches of the Galapagos Islands. Ecologically, the afroalpine biota is indeed also an island biota–the high mountain summits protrude as isolated temperate islands above the warm surrounding plains. These mountains have evidently stood isolated from each other since their origin. Pleistocene climatic changes have certainly modified their vegetation zonation to a considerable extent, but direct contacts between their afroalpine enclaves during the Pleistocene or earlier seem most improbable. These enclaves must therefore have been isolated from each other and from other high mountain areas for a very long time, and dispersal of plants between them must presumably have occurred mainly by long distance transport, possibly facilitated by cyclones.Some 80% of the afroalpine species of vascular plants are endemic to the high mountains of tropical east Africa and Ethiopia. Vicarious taxa occur of different status. In some cases one species occurs on all or most of the east African mountains with a vicariad in other parts of the world, as exemplified bySubulariamonticola(afroalpine) andS. aquatica(circumboreal). In other cases each of two species is confined to one group of mountains, as in the species pairLobelia wollastonii(Virunga Volcanoes and Ruwenzori) andL. telekii(Elgon, Aberdare, Mt Kenya). Finally, there are several groups of vicarious taxa where each taxon is confined as a rule to a single mountain, as in theLobelia deckeniigroup with six cognate species. Numerous similar cases occur among spiders and insects.The differentiation of these vicarious species is assumed to have occurred through natural selection in connection with genetic drift, acting upon geographically isolated and originally very small random samples of the gene pools concerned. The amount of differentiation between different mountain populations differs considerably between different groups–numerous intermediate stages exist between morphologically indistinguishable populations and full‐fledged vicarious species. The rate of evolutionary change seems to differ considerably between different genera and families. Whether internal barriers to interbreeding exist between these vicarious taxa is in most cases unknown.No less interesting than the geographically vicarious taxa mentioned above are some cases of altitudinal vicariism. Some afroalpine species appear to have evolved from afromontane forest species through progressive adaptations favouring survival in the inhospitable afroalpine climate. The most remarkable examples are provided by the strangely specialized Giant Senecios and Giant L

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01816.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Two statements about the tropical rain forest are constantly reiterated:(1) it is the richest in species of all plant communities, (2) it is unusual among species‐rich communities in the frequency of series of closely related (or at least congeneric) apparently sympatric species (cf. Fedorov, 1966).The comparison of species diversity in plant communities of very different physiognomy, e.g. forest and grassland, presents difficulties. On a species/area basis it is possible that some non‐forest communities, e.g. the fynbosch (sclerophyll scrub) of the Cape and sclerophyll communities in Australia and New Caledonia, may be richer than the tropical rain forest, but the latter is certainly much richer than any otherforestcommunity.The total species‐richness of the rain‐forest flora depends in part on the large number of synusiae present, e.g. of lianes, epiphytes, ground herbs etc., but only trees will be discussed here.The species diversity of rain‐forest trees varies regionally (e.g. richest in south‐east Asia, poorer than elsewhere in most parts of tropical Africa) and according to site conditions (e.g. poorer on podsols and on sites with impeded drainage than on well drained latosols in the same area).The frequency of groups of closely related species varies in different regions and in different storeys in the same region. The very large number ofShoreaspp. and other nearly related groups of Dipterocarps in the A storey in south‐east Asia is very striking, but in Africa and America, the numerous congeneric species in the B and C storeys is equally remarkable, e.g.DiospyrosandRinoreain Africa,Miconiain tropical America. Some families tend to be represented in all storeys, but not by many species in any one, others in only one or two storeys, but by many species in the same storey.The common occurrence of groups of morphologically similar and nearly related tree species in the rain forest seems in contradiction to Gause's competitive exclusion principle.These questions suggest certain reflections on the concept of the niche as applied to plants. If it applies in any exact sense to rain forest trees it implies that species which occupy niches must stand in a different relation to the resources (light, water, mineral nutrients) of the environment and to other organisms of the ecosystem.It is well known that even unrelated rain‐forest trees of the same storey tend to be very similar in such features as crown form, size and shape of leaves etc., and are often hard to distinguish in the sterile condition. Characters of possible ecological importance in which they differ include the following:usual mature height (determines the place of the tree in the stratification), growth rate, life span, shade tolerance, type of dispersal mechanism, reproductive strategy, flowering season etc. These tend to be correlated together and not combined at random, e.g. shade‐intolerant species tend to be fast‐growing and to have efficient (wind or bird) dispersal mechanisms, shade‐tolerant to be slow‐growing and to be gravity‐dispersed. But even if allowance is made for all these known differences of possible ecological importance it is difficult to believe that they are enough to fit as many species to occupy different niches as seems to be required by the competitive exclusion principle. Other differences between species probably exist:possibilities are in the way different species exploit the mineral resources of the soil (root activity, differences in recycling of nutrients depending on different length of life of leaves) and defence mechanisms against herbivores and pathogens. The latter may be biochemical or biological as in myrmecophytes.We can only conclude that we know very little about the autecology of rain‐forest trees. Comparative studies of selected species from different storeys in the same

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1969.tb01817.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY