Behaviour and Speciation in Birds and Lower Vertebrates

Summary Behaviour mechanisms in food, mate and habitat selection in birds and lower vertebrates are reviewed, with special attention to interspecies divergence in the absence 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 in their 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 of interspecies differences in courtship are listed. Since characters of courtship are causally and functionally related to characters used in other situations, they may diverge in isolated populations without selection for divergence. Such changes are especially likely to be related to changes in aggressiveness and timidity, sexual dimorphism 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 important in tiding the species over a period in which courtship displays, etc., have altered without 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 to generalized 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 and the food taken. It is suggested that the structures and feeding patterns available influence the course of individual learning, and are thus instrumental in controlling the food objects taken. In this way isolated populations could become structurally and behaviourally adapted to local food sources; and on re‐meeting, the structural differences acquired would lead to differences in food selection, even though the young 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 this suggestion. 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 characters of the environment. Occasional cases of selection of an abnormal habitat are probably 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 adapted to the new habitat, and later come to select it through a genetically determined preference.