BEHAVIOUR, SYSTEMATICS, AND NATURAL SELECTION

Abstract

Summary. 1 When behaviour characters are used in systematics, extra caution is needed to distinguish between true differences and immediate environmental effects. 2 Apart from small mutational steps and extremely fragmentary evidence on the effect of selection, no direct evidence about evolutionary change in time is available, and the argument is based on the assumption that differences between related contemporary forms, and similarities between non-related contemporary forms, reflect changes in time. 3 Behaviour characters are in principle neither more nor less useful than morphological or other characters; they merely add characters to the total by which overall likeness is judged. 4 Comparative data can be used for two purposes: (a) a description of the behaviour changes which have happened in evolution, with the ultimate aim of describing how the behaviour machinery has changed; and (b) an assessment of the relative importance of selection versus random change. 5 The description of behaviour changes aims at reducing “qualitative” differences between taxa to accumulations of small quantitative steps. 6 Critical studies of survival value, applied to total behaviour patterns, are needed to assess the effects of selection. Where this has been done, either by studying convergence or by comparing divergent related species, it has been found that many “characters” are interrelated components of adapted systems. These systems are complexes of morphological, physiological and ethological features. 7 There are numerous relationships between the various functional systems of an animal, and many characters therefore have been subject to indirect effects of selection pressure. 8 There is no reason to suppose that behaviour characters have been subject to selection to a greater extent than any other character; the ethologist's high regard for the influence of selection is due to his characteristics rather than to those of the material.