Speciation In Birds Research Articles

Bird speciation in the tropics

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 given.

Bird speciation on the Australian Continent

Bird speciation on the Australian Continent

Behaviour and Speciation in Birds and Lower Vertebrates

SummaryBehaviour mechanisms in food, mate and habitat selection in birds and lowervertebrates are reviewed, with special attention to interspecies divergence in theabsence of selection for divergence as such.5 11. The nature of sign stimuli and fixed action patterns are discussed.Learning sometimes plays a part in the ontogeny of the latter, and they vary intheir ontogenetic stability.5 111, I. Evidence for the efficacy of ethological reproductive isolating mechan‐isms is summarized.§ 111, z (a). Mechanisms of pair formation and courtship in birds are reviewed,and the evolution of the behaviour patterns involved is considered. The types ofinterspecies differences in courtship are listed. Since characters of courtship arecausally and functionally related to characters used in other situations, they maydiverge in isolated populations without selection for divergence. Such changes areespecially likely to be related to changes in aggressiveness and timidity, sexualdimorphism and colour. Divergence in this way is likely to be significant in pro‐viding a basis for, and influencing the direction of, subsequent selection for di‐vergence in sympatric forms. Learning of specific characteristics may be importantin tiding the species over a period in which courtship displays, etc., have alteredwithout corresponding genetic changes in responsiveness.§111, 2 (b‐d). Some similar data for lower vertebrate groups are reviewed.§ IV, I. Competition between sympatric forms is reduced by differences in diet.9 IV, 2. Most birds take a wide range of food objects, initially responding togeneralized stimuli characteristic of diverse objects. Interspecies differences are related to differences in habitat, location of feeding within a habitat, size of food,or nature of food. Learning is important in the ontogeny of feeding behaviour,especially in limiting the initially wide range of objects which elicit feeding.There is a close correlation between structure, feeding behaviour patterns andthe food taken. It is suggested that the structures and feeding patterns availableinfluence the course of individual learning, and are thus instrumental in controllingthe food objects taken. In this way isolated populations could become structurallyand behaviourally adapted to local food sources; and on re‐meeting, the structuraldifferences acquired would lead to differences in food selection, even though theyoung of both have a similar initial responsiveness to a wide range of objects.5 IV, 3–4. Some data for other vertebrate groups are also compatible with thissuggestion.4 V, I, Differences in habitat selection are often important in reducing competi‐tion and are sometimes conducive to reproductive isolation.4 V, 2. Habitat selection in birds depends on a responsiveness to certain charactersof the environment. Occasional cases of selection of an abnormal habitat areprobably not due to genetic changes, but to the functioning of the normal mechan‐isms in abnormal circumstances. Various mechanisms, including individual learning,may act to perpetuate such divergences. The population may then become adaptedto the new habitat, and later come to select it through a genetically determinedpreference.